R

Ma

MM

a

ARRAA

KSMMP

C

0d

Journal of Alloys and Compounds 494 (2010) 175–189

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

eview

icrowave versus conventional sintering: A review of fundamentals,dvantages and applications

orteza Oghbaei ∗, Omid Mirzaeeaterials Engineering Department, Semnan University, Iran

r t i c l e i n f o

rticle history:eceived 9 September 2009eceived in revised form 12 January 2010

a b s t r a c t

Microwave sintering has emerged in recent years as a new method for sintering a variety of materialsthat has shown significant advantages against conventional sintering procedures. This review article firstprovides a summary of fundamental theoretical aspects of microwave and microwave hybrid sintering,

ccepted 13 January 2010vailable online 25 January 2010

eywords:interingicrowave

and then advantages of microwave sintering against conventional methods are described. At the end,some applications of microwave sintering are mentioned which so far have manifested the advantagesof this novel method.

© 2010 Elsevier B.V. All rights reserved.

icrowave sinteringowder metallurgy

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1762. Theoretical aspects of microwave sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

2.1. Basics of microwave sintering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1772.1.1. Two directional microwave heating (microwave hybrid heating) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1782.1.2. Energy consumption in microwave sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1782.1.3. Reduced sintering time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1792.1.4. Increased density and more uniform grain size distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1792.1.5. Improved properties at higher heating rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

2.2. Interaction of microwaves with materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793. Application of microwave sintering in engineering materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

3.1. Metals and metal matrix composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1803.1.1. Austenitic (316L) and ferritic (434L) stainless steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1803.1.2. W–Ni–Fe alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1803.1.3. Tungsten powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.1.4. WC/Co composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.1.5. Metastable (Al/Ti) and hybrid (Al/Ti + SiC) composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

3.2. Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1833.2.1. Transparent alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1843.2.2. Zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

3.2.3. Alumina–zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2.4. ZrB2–B4C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2.5. High dielectric constant CaCu3Ti4O12 powder . . . . . . . . . . .3.2.6. High-permeability (Ni0.20Zn0.60Cu0.20)Fe1.98O4 ferrite . . . .

∗ Corresponding author. Tel.: +98 9122396951.E-mail address: m78 oghbaei@yahoo.com (M. Oghbaei).

925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2010.01.068

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

176 M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189

3.2.7. Porcelain bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1873.2.8. Uranium dioxide (UO2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nomenclature

� material electrical conductivity (�−1 m−1)f microwave frequency (Hz)E magnitude of the internal electric field (V/m)ε′′ loss factorε′ permittivityε0 permittivity of free space (F/m)ε′

r relative dielectric constantε′′

eff relative effective dielectric loss factor�a material magnetic permeability (N/A2)C speed of light (m/s)�0 incident wavelength (m)T temperature (K)t time (s)

1

ctfwcb

fpeTfafitHfinpatpa[

fppaosbMtT

� density (kg/m3)CP heat capacity (J/kg K)

. Introduction

Powder metallurgy is a process whereby a material powder isompacted as a green body and sintered to a net shape at elevatedemperatures about 0.6–0.8Tm. There are challenging demandsrom the PM industry for new and improved sintering processith finer microstructures and enhanced physical and mechani-

al properties. This is where the microwave technology is found toe advantageous.

Microwave energy is a form of electromagnetic energy with therequency range of 300 MHz to 300 GHz. Microwave heating is arocess in which the materials couple with microwaves, absorb thelectromagnetic energy volumetrically, and transform into heat.his is different from conventional methods where heat is trans-erred between objects by the mechanisms of conduction, radiationnd convection. In conventional heating, the material’s surface isrst heated followed by the heat moving inward. This means thathere is a temperature gradient from the surface to the inside.owever, microwave heating generates heat within the materialrst and then heats the entire volume [1]. This heating mecha-ism is advantageous due to the following facts: enhanced diffusionrocesses, reduced energy consumption, very rapid heating ratesnd considerably reduced processing times, decreased sinteringemperatures, improved physical and mechanical properties, sim-licity, unique properties, and lower environmental hazards. Thesere features that have not been observed in conventional processes1–5].

Microwave energy has been in use for variety of applicationsor over 50 years. These applications include communication, foodrocessing, wood drying, rubber vulcanization, medical therapy,olymers, etc. Use of microwave technology in material sciencend processing is not rather new. The areas where this technol-gy has been applied include: process control, drying of ceramic

anitary wares, calcination, and decomposition of gaseous speciesy microwave plasma, powder synthesis, and sintering [6,7].icrowave processing of materials was mostly limited until 2000

o ceramics, semiconductors, inorganic and polymeric materials.here was a misconception between researchers that all metals

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

reflect microwave or cause plasma formation, and hence can-not be heated, except exhibiting surface heating due to limitedpenetration of the microwave radiation. The researchers did notnotice that this relation is valid only for sintered or bulk metals atroom temperature, and not for powdered metals and/or at highertemperatures [6]. Now it has been found that the microwave sinter-ing can also be applied as efficiently and effectively to powderedmetals as to many ceramics. This paper compares advantages ofmicrowave sintering against conventional sintering and presentssome applications confirming its advantages.

2. Theoretical aspects of microwave sintering

Microwave energy is a form of electromagnetic energy withthe frequency range of 300 MHz to 300 GHz and the correspond-ing wavelengths are between 1 mm and 1 m. The frequency andwavelength range of microwaves are shown in Fig. 1. Microwaveshave longer wavelengths and lower available energy quanta thanother forms of electromagnetic energy such as visible, ultraviolet orinfrared light. The first microwaves application came to the exten-sive use in communication such as radar, television and satelliteapplications [8]. The second application is microwave heating ofdifferent materials. The most commonly used frequencies for heat-ing purposes are 915 MHz and 2.45 GHz, which are derived fromelectrical energy with the transformation efficiency of about 85%and 50%, respectively [9].

The dielectric interaction of materials with microwaves can bedescribed by two important parameters: absorbed power (P) anddepth of microwave penetration (D). They will determine the uni-formity of heating throughout the material. The average absorbedpower, P, which is volumetric absorption of microwave energy(W/m3) in material, is expressed as Eq. (1) [3,10]:

P = �|E|2 = 2�fε0ε′′eff|E|2 = 2�fε0ε′

r tan ı|E|2 (1)

tan ı is the loss tangent, which indicate the ability of the materialto be polarized and heated. It can be expressed as Eq. (2):

tan ı = ε′′

ε′ (2)

The loss factor measures the ability of the material to transfermicrowave energy into heat and the dielectric constant measuresthe ability of the material to be polarized [10]. The absorbedmicrowave power in material is converted to heat and lead toincrease in its temperature. Temperature increase is shown as Eq.(3) [3]:

T

t= 2�fε0ε′′

eff|E|2�Cp

(3)

The penetration depth D (m) is another important parameterthat determines the depth of penetration at which the incidentpower is reduced by one half exhibiting the uniformity of heatingthroughout the material. The penetration depth can be expressedas in Eq. (4) [3,10,11]:

D = 3�0

8.686� tan ı(ε′r/ε0)1/2

= C

2�f√

2ε′(√

1 + tan2 ı − 1)1/2

(4)

As seen in Eq. (4), the higher the values of tan ı and ε′r, the

smaller the depth of penetration for a specific wavelength. High fre-

M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189 177

frequ

qsp

rtftthtd

d

liMeuip

(amae

2

tpf

Fig. 1. Electromagnetic spectrum and

uencies and large values of the dielectric properties will result inurface heating, while low frequencies and small values of dielectricroperties will result in more volumetric heating [3].

For a conductive metallic material, an incident wave is mainlyeflected, and the rest cannot pass through the superficial layer ofhe metal itself. The penetration depth of the microwaves at a givenrequency “f” depends on the electrical and magnetic properties ofhe material and is a very important parameter, because it consti-utes an upper limit to the thickness of the material which can beeated directly by microwaves. The skin depth “d” (m) is defined ashe depth into the conductor from the surface at which the currentensity is 1/e of its value at the surface [4,12], given by Eq. (5):√

1�f��a

(5)

Materials with high conductivity and permeability present aower penetration depth, for a given frequency, but there is alsomplicit temperature dependence due to the changes of � and �a.

ost metals generally have a penetration depth of the microm-ter order, so the direct heating tends to remain superficial, butsing powders with particle size of the penetration depth order, it

s possible to heat them directly and use microwave in the sinteringrocess [4].

As seen from the above equations, the dielectric propertiesε0, ε′ and tan ı) act as an important role in the extent of powerbsorbed by a material. There are no structural parameters (atomic,icrostructural or otherwise) in the equations. Structural features

re assumed to be accounted for by changes in the dielectric prop-rties (ε′

r, ε′′eff and tan ı) [3].

.1. Basics of microwave sintering

The particular requirements of sintering material powders makehis process one of the most challenging applications for microwaverocessing. These requirements often include some or all of theollowing: high temperature, high heating rates, uniform temper-

Fig. 2. Temperature profile within the sample in: (a) conventional hea

encies used in microwave processing.

ature, and equivalent thermal history throughout the specimen.The process of sintering materials in the conventional methodsis incorporated with mixing of material powder with additive,milling, and pressing into green parts followed by sintering withindirect heating of green pellets at about 0.6–0.8Tm in a refractory-type electrical resistance furnace, induction furnace or fossil fuelfurnace. These furnaces use a large number of expensive heatingelements, fuel and refractory materials to achieve and maintainthe high temperature for a long time. Moreover, it consumes muchelectrical energy, much fuel, and longer time. These kind of furnaceswith indirect heating are called conventional sintering furnaceand the heating mechanism is called conventional heating versusmicrowave heating which provide direct heating of the green part.The fundamental difference between microwave sintering and con-ventional sintering is in the heating mechanism. The temperatureprofile for both methods is shown in Fig. 2. For conventional sin-tering, heat is generated by heating elements and transferred tosamples via radiation, conduction, and convection. In microwavesintering, however, the materials themselves absorb microwaveenergy, and then transform it into heat within their bodies [13,14].The microwave heating presents a potential economical sinter-ing process with shortened processing time for the materials. Thismethod is expected to overcome many of the shortcomings of theconventional sintering process [15]. Thus, there has been consider-able interest in microwave heating for the synthesis and processingof materials. Microwave processing has gained worldwide accep-tance as a novel method for heating and sintering a variety ofmaterials, as it offers many advantages in terms of enhanced diffu-sion processes, reduced energy consumption and processing cost,very rapid heating rates and significantly reduced processing times,decreased sintering temperatures, improved physical and mechan-

ical properties, simplicity, unique properties, new materials andproducts and lower environmental hazards, which are not observedin conventional processes [1–5].

There is also another sintering technique that is called sparkplasma sintering (SPS), also known as plasma activated sintering or

ting, (b) microwave heating and (c) microwave hybrid heating.

1 lloys a

fitaveaadhiosto

fdacot[tmtmpIMautwt

2h

ombcemcls

78 M. Oghbaei, O. Mirzaee / Journal of A

eld-assisted sintering technique (FAST) which is an effective sin-ering technique that allows densification of powders or green bodyt a relatively lower temperature with short holding time owing toery rapid heating and cooling rates, several hundred K/min, andnables full densification within minutes [46,47]. In this method,s-received powders are placed in a graphite die, pressed uniaxially,nd then heated by passing a high pulsed current through the pow-ers and/or the die [48]. Both microwave and plasma arc sinteringold forth the promise of rapid processing as well as refinements

n microstructure and enhanced properties. Of course, the causesf enhancement in two techniques are unrelated [45]. Microwaveintering appears to be an attractive alternative to plasma arc sin-ering for large specimens, since the energy is deposited in the bulkf the material by dielectric loss mechanisms [45].

Many researchers have reported unexpected effects resultingrom the use of microwave radiation as an alternative energy sourceuring the processing of materials. These results have includedpparent evidence of accelerated kinetics for a range of processes ineramic, polymeric, and organic systems, and enhanced sinteringf ceramic powder compacts including lower sintering tempera-ures. These unexpected effects are called the “microwave effect”3,16]. As a general summary, the kinetics of synthesis and sin-ering reactions are reportedly augmented by 2 or 3 orders of

agnitude or even more when conventional heating is substi-uted for microwave radiation [17,18]. In sintering processes, the

icrowave effect is quantified by the difference between the tem-eratures of two treatments that lead to similar microstructures.

t is now widely accepted that a microwave effect exists [19,20].icrowave assisted post-treatment of green parts proved to benew method, suitable for near-net-shape and net-shape man-

facturing. The basic idea of using microwave energy for the heatreatment of materials is well known: deposit the energy directlyithin the product and possibly create a uniform temperature dis-

ribution [4].

.1.1. Two directional microwave heating (microwave hybrideating)

During sintering materials by direct microwave heating, vari-us fundamental problems are usually encountered [21]. Firstly,ost of the research on material processing by microwaves is

ased on conventional low-frequency (2.45 GHz) microwave appli-ators; however, such applicators do not couple microwave powerfficiently to many materials at room temperature and poor

icrowave absorption characteristics make initial heating diffi-

ult [21]. Secondly, thermal instabilities may occur, which canead to the phenomenon of temperature runaway [21]; i.e., thepecimen overheats catastrophically. This occurs for a wide vari-

Fig. 3. Direction of two heat sources in microwave hybrid sintering [28].

nd Compounds 494 (2010) 175–189

ety of ceramic materials including Al2O3, SiO2, Fe3O4, �-alumina,ZrO2, etc. [10,21]. The inherent temperature gradients presentduring volumetric heating can lead to severe temperature non-uniformities, which at high heat-up rates may cause non-uniformproperties and cracking [19,22]. In many sintering experiments,insulation has been used to minimize these gradients. However,the use of insulation can seriously aggravate temperature runaway[21]. Finally, in a conventional heating, as shown in Fig. 2, the direc-tion of heating is from outside to inside of the powder compactresulting in higher temperature of the sample surface than the core,while for microwaves, the direction of heating is from inside to out-side of the powder compact resulting in higher temperature of thesample core than the surface. The former mode of heating results inpoor microstructural characteristics of the core of the powder com-pact while the latter results in poor microstructural characteristicsof the surface [1].

These problems have led researchers [23] to develop hybridheating techniques which combine direct microwave heating withinfrared heat sources. Microwave hybrid heating is the most impor-tant example of mixed-absorbed heating that is used to sinter amaterial which has low dielectric loss at low temperature and highdielectric loss at high temperature. The microwaves are absorbedby the component that has high dielectric loss while passingthrough the low-loss material with little drop in energy [3]. Thiscan be performed by using a material, which is called susceptor, andhas high dielectric loss at low temperatures around the green part.At low temperatures, susceptor material absorbs microwave andreaches high temperatures. Then, it can transfer heat to the sam-ple via conventional heating mechanisms. Thus, the sample whichhas high dielectric loss at high temperatures will be able to absorbmicrowaves per se.

A combined action of microwaves and microwave-coupledexternal heating source (microwave hybrid heating) can be utilizedto realize rapid sintering from both inside and outside of the pow-der compact [24]. Two directions of heating of the powder compactin microwave hybrid heating are shown in Fig. 3. The hybrid heat-ing system will heat the sample more readily at low temperaturesand at high temperatures will flatten out the temperature profileinside the sample body [10,21,25].

2.1.2. Energy consumption in microwave sinteringMicrowave heating has been used successfully to sinter a wide

variety of oxide and non-oxide ceramic, metal and composite pow-der. It differs from the conventional heating processes because theuse of microwaves facilitates the transfer of energy directly intothe materials, providing volumetric heating [7]. The direct deposi-tion of energy in the bulk of a material eliminates wasting energyby simultaneous heating of furnace or reactor walls or other mas-sive components and heat carriers and also offers the potential ofapplying high heat-up rates. Hence, microwave methods drasticallyreduce the energy consumption, particularly when compared withhigh temperature processes where heat losses increase drasticallywith rising process temperatures, and it is possible to reduce thetime required to complete a process [5].

Achieving much lower power consumption can be attributed tothe fact that in microwaves it is the sample per se that heats up andacts as the source of heat. Consequently, the effective thermal massreduction lowers the required power input [26].

Microwaves can interact directly with the material, also in thecase of conductive powder compacts and thus avoiding heating

other parts that are not directly involved in the process, such asair or the oven’s walls. A remarkable energy and time saving canbe obtained and the resulting products present often enhancedmechanical properties compared to conventional treated ones[27,28], because the heating is more uniform and volumetric [4].

lloys a

2

iscptrptidrhrmogertcbtn[

2

wrIsdgfcEiiocfioifja

M. Oghbaei, O. Mirzaee / Journal of A

.1.3. Reduced sintering timeIn conventional furnaces, compacts get radiatively heated dur-

ng sintering. Consequently, to prevent thermal gradient at highintering temperatures within the compact, a slower heating rateoupled with isothermal holding at intermittent temperatures isrovided, which increases the process time, thereby contributingo coarsening. Also, high heating rates in conventional furnacesesult in a thermal gradient within the compacts and lead to com-act distortion and inhomogeneous microstructure. One of theechniques to achieve relatively homogeneous as well as fast sinter-ng in compacts is through microwave heating [7,10]. Microwavesirectly interact with the particulates within the pressed compactsather than being conducted into the specimen from an externaleat source, thereby provide rapid volumetric heating [29]. Thiseduces processing times and results in energy saving. The use oficrowave processing reduces typical sintering times by a factor

f 10 or more in many cases [6]. Microwave heating has been sug-ested as a means of reducing the time of preparation and thenergy consumption [30] as its effect originates from the natu-al ability of certain substances to efficiently absorb, and then toransform electromagnetic (radiation) energy into heat. If suffi-ient heat is generated at a local level, chemical reactions maye initiated at a very rapid rate. In addition, the higher sinteringemperature as can be prepared by microwave sintering, can sig-ificantly shorten the sintering time leading to efficient processing31].

.1.4. Increased density and more uniform grain size distributionMicrowave sintering technique can effectively promote the for-

ard diffusion of ions and thus accelerate the sintering process,esulting in the grain growth and the densification of matrix.t is well known that sintering process consists of densificationtage and grain growth stage [32]. The densification rate stronglyepends on the diffusion of ions between sample particles, and therain growth rate is mostly determined by the grain boundary dif-usion. Dube et al. have found that the intense microwave fieldoncentrates around samples during microwave sintering [33].specially, the power of microwave field between sample particless almost 30 times larger than the external field, giving rise to ion-zation at the surface of sample particles. As a result, the diffusionf ions between sample particles is accelerated and the densifi-ation stage is promoted. Moreover, surrounding electromagneticeld can intensely couple with ions at grain boundaries. Under drive

f microwave field, the kinetic energy of ions at grain boundariesncreases, which results in decreasing the activation energy for aorward jump of ions and increasing the barrier height for a reverseump [1]. So the forward diffusion of intergrain ions is promotednd thus accelerates the grain growth during sintering [34]. In many

Fig. 4. Three kinds of materials according to the interaction with micro

nd Compounds 494 (2010) 175–189 179

cases, the microwave sintering has shown to produce samples withmore uniform grain size distribution and higher density [15].

2.1.5. Improved properties at higher heating ratesThe properties of the materials are determined by their

microstructures. It is generally accepted that a critical issue inmicrostructural development is the interplay between densifica-tion and coarsening. To control this microstructural development,parameters such as temperature, pressure, sintering time, and heat-ing rate must be optimized. Of all these parameters, rapid heatinghas reportedly produced beneficial effects such as high final sin-tered density for a given grain size or fine microstructure comparedwith slow heating for similar densities, but conventional fast firingpossesses some difficulties. Differential sintering that causes differ-ential densification is one of the problems most often encounteredin conventional fast firing. In this context, microwave sinteringis an alternative technique to overcome the problems of conven-tional fast firing. Because it is a non-contact technique and theheat is transferred to the product via electromagnetic waves, largeamount of heat can be transferred to material’s interior minimizingthe effects of differential sintering [20]. Microstructural develop-ment, much finer average grain size and higher density achieved inproducts by microwave sintering, result in enhanced mechanicalproperties with respect to conventional treated ones.

2.2. Interaction of microwaves with materials

In terms of interaction with microwaves, materials can becategorized into three principal groups [3] as shown in Fig. 4:(a) transparent, which are low dielectric loss materials wheremicrowaves pass through without any losses; (b) opaque (conduc-tors), where microwaves are reflected and cannot penetrate (thisproperty is used in radar detection [2]); and (c) absorbing, whichare high-loss materials where microwaves are absorbed dependingon the value of the dielectric loss factor. There can be also a fourthtype of interaction that is a mixed absorb. This type of interactionis observed in composite or multi-phase materials where one ofthe phases is a high-loss material while the other is a low-lossmaterial. Mixed absorbers take advantage of one of the signifi-cant characteristics of microwave processing, which is selectiveheating. Microwave hybrid heating is the most important exampleof mixed-absorb heating [3]. The microwave field can be concen-trated at the neck region of two contacted particles, so that the

ionic mass transport in the grain boundary or neck region is greatlyenhanced in the microwave field. Microwave heating in materialsis closely related to their dielectric properties and defect structure.The sintering atmosphere and the addition of additives change thedefect concentration and dielectric properties, which may affect the

waves: (a) transparent, (b) opaque (conductor) and (c) absorber.

1 lloys and Compounds 494 (2010) 175–189

d[

ph

ofi(rtvppgat

3m

fpppitp

2rcaamgimi

3

aothldmhsdtmpcpaaavms

80 M. Oghbaei, O. Mirzaee / Journal of A

iffusion behavior of green part when it is under microwave fields4].

Microwaves interact with individual particulates within theressed compacts directly and thereby provide rapid volumetriceating [3].

A complex phenomenon intervenes during microwave sinteringf metallic powders, in particular local concentrations of electriceld can occur in excess of the dielectric strength of the mediumair, polymer or glass), leading to arcing, localized melting or veryapid phenomenon of evaporation. Consequently, microwave sin-ering can proceed at greater speed than conventional heating inirtue of the presence of more efficient mechanism of mass trans-ort in presence of liquid phase. Moreover, the microwave sinteringrocess can be self-regulating because the necks formation pro-ressively generates longer conductive paths, thus reducing thereas of electromagnetic field concentration in comparison withhe initial particles dispersion [4].

. Application of microwave sintering in engineeringaterials

Microwave energy has been in use for variety of applicationsor over 50 years. These applications include communication, foodrocessing, wood drying, rubber vulcanization, medical therapy,olymers, etc. Use of microwave technology in material science androcessing is not rather new. The areas where it has been applied

nclude: process control, drying of ceramic sanitary wares, calcina-ion, and decomposition of gaseous species by microwave plasma,owder synthesis, and sintering [6,7].

Microwave processing of materials was mostly limited until000 to ceramics, semiconductors, inorganic and polymeric mate-ials. However, now it has been shown that the microwave energyan be used to sinter virtually all powdered metals as efficientlynd effectively as in the ceramic systems. This has opened upn entirely new research area to investigate the advantages oficrowaves for metallic materials to meet the challenging and

rowing needs in many metallurgical applications [6]. The mostmportant application of microwave sintering can be divided to two

ain kind of engineering materials: metals/metal matrix compos-tes and ceramics.

.1. Metals and metal matrix composites

Metal powders are used in industry for diversity of productsnd applications. Before the first attempt at microwave sinteringf powder metals, there was a misconception between researchershat all metals reflect microwave or cause plasma formation, andence cannot be heated except exhibiting surface heating due to

imited penetration of the microwave radiation. The researchersid not notice that this relation is valid only for sintered or bulketals at room temperature and not for powdered metals and/or at

igher temperatures [6]. Now it has been found that the microwaveintering can also be applied as efficiently and effectively to pow-ered metals as to many ceramics. At 2.45 GHz, it is observed thathe skin depth in the bulk metals is very low (of the order of a few

icrons), and hence very little penetration of microwaves takeslace. However, in the case of fine metal powders rapid heatingan occur. A theoretical model developed predicted that if the metalowder particle size is less than 100 �m, it will absorb microwavest 2.45 GHz. It was further observed that the degree of microwave

bsorption depends upon the electrical conductivity, temperaturend frequency. Many commercial powder-metal components ofarious alloy compositions including iron and steel, copper, alu-inum, nickel, Mo, Co, Ti, W, Sn, etc., and their alloys have been

intered in microwaves producing better properties than their con-

Fig. 5. Effect of heating mode on the thermal profiles of 316L and 434L stainlesssteel compacts [35].

ventional counterparts by using a 2.45 GHz multimode microwavesystem [6]. The following section demonstrates practical advan-tages of microwave sintering for some important metals and metalmatrix composites.

3.1.1. Austenitic (316L) and ferritic (434L) stainless steelPanda et al. compared the effect of heating mode on the sin-

tering time, densification, microstructure, strength and hardnessof 316L (austenitic) and 434L (ferritic) stainless steel [35]. Theheating and time profile of both methods of heating are shown inFig. 5. Taking into consideration the slow heating rate (5 ◦C/min)and the isothermal holds at intermittent temperatures in a con-ventional furnace, there is about a 90% reduction in the processtime for the sintering of stainless compacts in a microwave fur-nace. In the case of microwave sintering, a slight difference in theheating rates for the 316L and 434L compacts is observed, which isattributed to the dependence of microwave-metal coupling on thecomposition. It is evident from Fig. 6 that for both types of stainlesssteel, microwave sintering restricts microstructural coarsening. Forboth the sintering conditions, grain coarsening occurred at 1400 ◦C.For both stainless steels, microwave sintering results in a smallerand narrower pore size distribution. This can be attributed to thelower propensity for pore coarsening due to the lesser sinteringtime in the case of compacts consolidated in microwave furnace[35].

3.1.2. W–Ni–Fe alloyUpadhyaya et al. compared the effect of heating mode on the

microstructure and mechanical properties of 92.5W–6.4Ni–1.1Fealloy [26]. Green compacts were sintered at 1500 ◦C in a radiativelyheated (conventional) furnace and in a 2.45 GHz microwave fur-nace. Fig. 7 compares the heating profiles and power consumptionduring microwave and conventional sintering. The overall heat-ing rate in microwave furnace was 20 ◦C/min. Taking into accountthe slow heating rate (5 ◦C/min) in a conventional furnace, there isabout a 75% reduction in the process time of W–Ni–Fe compacts

in the microwave furnace, which leads to less W grain coarsening.Besides resulting in grain growth, a long sintering time also resultsin precipitation of brittle intermetallic phases in W–Ni–Fe alloysthat microwave fast heating rate mitigated this problem. Despitesuch a fast heating rate, no micro- or macro-cracking was observed

M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189 181

at 1400 ◦C for 1 h in conventional (left) and microwave (right) furnaces [35].

io

agasiamafs

Fc

Fig. 6. Optical micrographs of (a) 316L and (b) 434L compacts sintered

n the microwave-sintered samples. This underscores the efficacyf volumetric heating associated with microwaves.

Fig. 8 compares the microstructures of the differently sinteredlloys. Clearly, microwave sintering results in significantly lower Wrain coarsening. Fig. 9 compares the XRD patterns of conventionalnd microwave-sintered alloys. Phase analysis of conventionallyintered 92.5W–6.4Ni–1.1Fe alloy shows the presence of brittlentermetallic phases (NiW, Fe7W6). Remarkably, these phases arebsent in microwave-sintered compacts. Table 1 compares the

echanical properties of microwave and conventionally sintered

lloys. Better grain distribution and lack of intermetallic phaseormation results in better mechanical properties in microwave-intered compacts [26].

ig. 7. Comparison of heating profile and power consumption of microwave andonventionally sintered 92.5W–6.4Ni–1.1Fe alloy [26].

Fig. 8. SEM micrographs of (a) microwave and (b) conventionally sintered92.5W–6.4Ni–1.1Fe alloy [26].

182 M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189

Fa

3

saosahbmi

ipmact

3

tw

wt

TE[

TS

ig. 9. XRD pattern of microwave and conventionally sintered 92.5W–6.4Ni–1.1Felloy [26].

.1.3. Tungsten powderConventional sintering of the highest melting point metal, tung-

ten powder, requires high sintering temperatures (T > 2773 K)nd relatively long soaking times to achieve densities above 90%f theoretical density. Microwave sintering of as-received tung-ten powder of 99.95% purity and average particle size of 5–7 �mnd activated tungsten powder by reducing its particle size in aigh-energy planetary mill with milling time of 5 h and plate andowl speed of 240 and 670 rpm is carried out in a 3 kW, 2.45 GHzicrowave furnace. Both the powder compacts are sintered under

dentical conditions. Maximum sintering temperature was 2073 K.While the as-received powder got densified to 85% of theoret-

cal density (TD), the density of the compact made from activatedowder is calculated to be 93% of TD. Vickers Hardness measure-ents showed higher hardness of 303 VHN for activated tungsten

gainst 265 VHN for as-received tungsten. The maximum poweronsumption for the whole process was 20 kW and the processingime was well within 6–7 h [36].

.1.4. WC/Co compositeWC/Co samples sintered in a microwave field differ radically in

erms of phases, chemistry, and microstructure when comparedith conventionally sintered samples [37].

Table 2 shows a rough estimate of the typical conditions thatere used in microwave sintering process versus conventional sin-

ering. Fig. 10 exhibits typical microstructures of sintered sample

able 1ffect of sintering mode on the mechanical properties of 92.5W–6.4Ni–1.1Fe alloy26].

Attributes Heating mode

Conventional Microwave

Avg. W grain size (�m) 17.3 ± 0.8 9.4 ± 0.5Bulk hardness, HV2 210 ± 15 295 ± 10W grains microhardness, HV0.05 398 ± 8 407 ± 5Matrix microhardness, HV0.005 120 ± 11 95 ± 6Tensile strength (MPa) 642 ± 23 805 ± 14Elongation (%) 3.5 ± 0.8 11.2 ± 1.1

able 2urvey of all investigated samples based on heating profiles [37].

Microwavesintering (min)

Conventionalsintering

Heating cycle, 850–1250 ◦C 10–30 150 minSoaking time at peak temperature 10 1 hCooling cycle 30 5–10 hEstimated duration of cobalt in the liquid 15 4 h

Fig. 10. SEM images of WC/Co (Specimen E) (a) conventionally and (b) microwavesintered [37].

from the same raw material (Specimen E in Table 3). The angu-lar WC grains appear light and the Co as dark matrix between theWC grains. One significant difference between microwave and con-ventionally sintered samples is that in the former there is hardlyany WC grain growth, which is observed in the conventionally sin-tered WC/Co samples. Therefore the distribution of Cobalt is finerin the microwave-sintered sample suggesting better mechanicalproperties. It has been shown that for conventional sintering ofvery fine grained raw materials, a large amount of grain growthinhibitor is necessary but such large amount would also degrade themechanical properties in the final product. A fine grained sinteredWC/Co product can be obtained in the microwave without usingany grain growth inhibitors, therefore obtaining better mechanicalproperties. The most striking difference observed is that microwavesintering produces a totally different material from the conven-tionally sintered material. When using the conventional sinteringmethod, a large amount (∼20 wt.%) of tungsten dissolves into thecobalt binder. A cobalt binder phase without significant amounts oftungsten is found in a microwave-sintered product. It seems thatmicrowave method prevents dissolution of tungsten into cobaltbinder. The TEM image of the microwave-sintered sample B thatis shown in Fig. 11 agrees with above results. Microwave sinteringresults in sintered dimensions with near three-dimensional uni-form shrinkage, especially when submicron sized WC is used. Theshape of conventionally sintered samples is distorted by gravity.Table 3 lists density, diameter shrinkage and hardness of cor-

responding conventional and microwave-sintered samples. Largediameter shrinkage indicates that capillary forces dominate overgravity forces and this results in a three-dimensional uniformshrinkage in microwave sintering. The microwave-sintered sam-ples have 1–5 GPa better hardness than the conventionally sintered

M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189 183

Table 3Raw materials, sintering conditions, density, shrinkage, Rockwell A hardness, corrosion (loss in HNO3) and erosion of WC/Co samples [37].

Specimennumber

Co (wt.%) Raw WCgrain size(�m)

Sinteringmethod

Sinteringtemperature (◦C)

Soakingtime (min)

Density (%of theoreti-cal)

Diametershrinkage(%)

Rockwell Ahardness(GPa)

Corrosion (lossin HNO3) (48 h)(wt.%)

Erosion weightloss/area(kg/m)

A 12 0.1–1 Conventional 1300 60 88.3 15.6 – – –A 12 0.1–1 Microwave 1300 10 98.4 20.3 – – –B 6 3–5 Conventional 1450 60B 6 3–5 Microwave 1450 10E 6 0.1–1 Conventional 1450 100E 6 0.1–1 Microwave 1350–1400 12

Fig. 11. Bright field TEM image of (a) conventionally and (b) microwave sinteredWC/Co (Specimen B) [37].

98.1 8.8 – – –98.1 12.3 – – –99.7 – 175.5 1.16 60.699.6 – 177.6 0.20 37.1

samples. Also it can be seen that the microwave-sintered sampleshave about six times more resistance against corrosion and abouttwo times more resistance against erosion than conventionally sin-tered samples [37].

3.1.5. Metastable (Al/Ti) and hybrid (Al/Ti + SiC) compositesTwo-directional microwave assisted rapid sintering was car-

ried out to fabricate Al/Ti metastable composites and Al/(Ti + SiC)hybrid composites by Thakur et al. [31]. The length scale of Ti wasin microns (20 �m) and of SiC in nanometers (50 nm). As SiC is awell known microwave susceptor material, incorporation of nano-SiC as a reinforcing material assisted in heating and sintering thepowder compacts. The results of the different properties measure-ments are shown in Table 4. It can be seen that coefficient of thermalexpansion of the Al matrix reduced due to the presence of Ti andTi + SiC reinforcements. Addition of SiC nanoparticulates to Al–Tiformulations assisted in increasing microhardness, macrohardness,Al–Ti interfacial hardness, 0.2% YS and UTS while the ductility wasmarginally affected.

The nano-SiC particles, having high surface area, present in theAl/Ti compact as shown in Fig. 12 act like a susceptor materialproviding extra source of heat to the surrounding Al powders andTi particles and helping them in more effective bonding (due toenhanced diffusion) and hence reducing the porosity of the hybridcomposite [31].

3.2. Ceramics

Research on the heating effects of microwaves on materials isvery active in the field of ceramics [38]. In recent years, microwaveheating has been well employed in the field of sintering and joiningof ceramics as a result of its advantages against conventional sinter-

Fig. 12. Schematic diagram showing the presence of nano-SiC particulates in thepowder compact of Al, Ti and SiC ready for microwave sintering in which SiC particlesact as susceptor material enhancing the sintering efficiency [31].

184 M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189

Table 4Result of porosity values of the extruded samples, coefficient of thermal expansion measurements, hardness and tensile testing [31].

Sample Porosity Coefficient ofthermalexpansion(×10−6 ◦C−1)

Macrohardness(HR15T)

Microhardness 0.2% YS (MPa) UTS (MPa)

Matrix Reinforcement Ti Interfacematrix-Ti

Pure Al 1.018 ± 0.11 26.325 ± 0.2 55.84 ± 0.55 44.79 ± 0.89 – – 92.8 ± 16.5 122.0 ± 1545.6947.4948.8650.39

ia

3

pesapFtsgd(As

3

nshhthdhbmt

F4

Al + 3% Ti 1.498 ± 0.11 25.151 ± 0.4 57.36 ± 0.85Al + 5% Ti 1.568 ± 0.04 23.390 ± 0.4 60.15 ± 0.52Al + 3% Ti + 0.5% SiC 1.004 ± 0.11 23.075 ± 0.5 59.09 ± 0.59Al + 5% Ti + 0.5% SiC 1.297 ± 0.11 22.042 ± 0.3 65.46 ± 0.58

ng. The following demonstrates microwave sintering advantagesgainst conventional sintering for some ceramics.

.2.1. Transparent aluminaCheng at al. successfully sintered transparent alumina sam-

les by microwaves [39]. In the conventional sintering processes,xtremely high sintering temperatures (up to 1900 ◦C) and longoaking times (several hours) under high vacuum or pure hydrogentmosphere are applied in the fabrication of transparent aluminaroducts to achieve the highest density and minimum porosity.ig. 13 shows the microstructure of the sample microwave sin-ered at 1750 ◦C with dwelling time of 45 min. As can be seen, theample exhibits very neat grain boundary structure and uniformrain growth with no porosity. Microwave-sintered samples withwelling time of 15–45 min had the same density of 3.97 g/cm3

∼100% of theoretical density). A typical polycrystalline transparentl2O3 sample that was microwave sintered at 1750 ◦C for 45 min ishown in Fig. 14 [39].

.2.2. ZirconiaThe processing of 3 mol% yttria partially stabilised zirconia

anopowder into components was investigated by pressurelessintering with both pure conventional heating and microwaveybrid heating by Binner et al. [40]. The use of microwave hybrideating had resulted in a much finer average grain size at all densi-ies. Due to the volumetric nature of the microwave hybrid heating,igh heating rate (20 ◦C/min) could be used without risking any

amage to the samples from thermal stresses, whilst the radianteating was limited to only 7 ◦C/min. At higher heating rates theodies were found to crack as a result of thermal expansion mis-atch between the surface and centre of the bodies due to the low

hermal conductivity in these materials.

ig. 13. Microstructures of the Al2O3 sample microwave sintered at 1750 ◦C for5 min [39].

± 0.96 225.38 ± 22 75.14 ± 1.6 98.0 ± 18.9 130.3 ± 16± 0.87 230.97 ± 28 77.35 ± 1.0 103.2 ± 6.3 143.7 ± 5± 0.54 225.05 ± 17 85.81 ± 5.7 111.4 ± 27.1 142.9 ± 25± 0.54 232.54 ± 17 94.51 ± 9.7 150.9 ± 15.3 183.9 ± 18

3.2.3. Alumina–zirconiaThe nanocomposites of alumina–zirconia containing 1, 3 and

5 volumetric percent of zirconia were microwave hybrid fast sin-tered by Menezes and Kiminami [20]. After sintering for 35 min, thesamples presented density more than 99.0% of theoretical density.Microstructural observations indicated uniform grain size distribu-tion without any observable cracks or pores and no abnormal graingrowth. The obtained results, indicated that microwave hybridheating is a great potential method for fabricating alumina–zirconiananocomposites with uniform microstructures, despite the shortsintering cycle, and for suppressing grain growth in ceramicnanocomposites [20].

3.2.4. ZrB2–B4CA two phase particulate ceramic composite containing ZrB2 and

B4C was densified by microwave and conventional sintering a mix-ture of ZrB2 and 4 wt.% B4C powders by Zhu et al. [41]. ZrB2 iselectrically conductive and has limited coupling with microwaveenergy, hence the presence of 4 wt.% B4C (∼8.7 vol%) in the ZrB2matrix was thought to facilitate the microwave heating of ZrB2,because B4C has been reported to be a microwave absorber [42].For microwave sintering, the heating and cooling rates were 50and 100 ◦C/min, respectively that are higher than those for con-ventional sintering (10 and 30 ◦C/min, respectively). Based on thedensity measurements shown in Fig. 15, microwave sintering wasmore effective in promoting densification of ZrB2–4 wt.% B4C par-ticulate composites from 88% to >98% at lower temperature range

of 1630–1720 ◦C without promoting grain growth compared toconventional sintering. The measured hardness of the microwave-sintered materials was nearly identical to that of conventionallysintered materials with the same relative density. For example,

Fig. 14. Appearance of a highly transparent Al2O3 sample microwave sintered at1750 ◦C for 45 min [39].

M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189 185

Fm

t1t

3

uuaaobaceihaCdr

Fb8

ig. 15. Relative density of ZrB2–4 wt.% B4C ceramic composites densified byicrowave and conventional sintering at different temperatures [41].

he hardness of microwave-sintered specimen at temperature of720 ◦C was 17.5 GPa compared to 16.6 GPa for conventional sin-ered specimen at temperature of 1870 ◦C [41].

.2.5. High dielectric constant CaCu3Ti4O12 powderIn general, the conventional ceramic (solid-state) processing is

sed as synthesis of CaCu3Ti4O12 (CCTO). This synthesis techniquesually needs long reaction time at elevated temperature. Therere some cases reported after heating for several days at temper-tures up to 1000 ◦C with some intermittent regrinding stages forbtaining single phase CCTO powder [43]. Microwave heating haseen suggested as a means of reducing the time of preparationnd the energy consumption by Yu et al. [44]. The green pelletsalcining were performed in both microwave and conventionallectric furnace at 1000 ◦C for comparison. For microwave heat-ng, the samples were heated for periods of 30 min. After each

eating step, the samples were allowed to cool for about 15 minnd an aliquot was taken for X-ray analysis till the single phaseCTO formed. Figs. 16 and 17 show the XRD patterns of CCTO pow-er calcined by microwave treatment and conventional heating,espectively. For the microwave heating, the single phase CCTO has

ig. 16. XRD patterns of CCTO powder calcined at different temperature for 30 miny microwave heating: (a) 750 ◦C; (b) first, 800 ◦C; (c) second, 800 ◦C; and (d) third,00 ◦C [44].

Fig. 17. XRD patterns of CCTO calcined (a) first at 1000 ◦C for 24 h, (b) second at1000 ◦C for 24 h and (c) third at 1000 ◦C for 24 h by conventional solid-state reactionsynthesis [44].

obtained after fourth 30 min heating and for the conventional heat-ing after third 24 h, so the total microwave heating time is about 2 h,which is much shorter than that used in the conventional synthe-sis (3 × 24 h) with a relatively low temperature. After the desiredsingle phase was synthesised, these two kinds of powder were sin-tered at 1100 ◦C for 3 h in an electric furnace in air. After sinteringat 1100 ◦C for 3 h, the relative dielectric constant from microwavesynthesised powder (∼21400, at 1 KHz) is higher than that fromconventional synthesised powder (∼10240, at 1 KHz) at room tem-perature as shown in Fig. 18. The sintered pellets from microwavesynthesised powders possess a higher dielectric constant than thatfrom conventional synthesised powders [44].

3.2.6. High-permeability (Ni0.20Zn0.60Cu0.20)Fe1.98O4 ferriteHigh-permeability NiZn ferrites have been widely applied in

various electronic devices. In order to obtain high relative initialpermeability, a coarse and compact microstructure is required;a high-permeability NiZn ferrite, (Ni0.20Zn0.60Cu0.20)Fe1.98O4, wasprepared by Yan and Hu using microwave sintering and conven-

tional sintering [34]. Obviously, it can be seen from Fig. 19 that thedensity and relative initial permeability of the microwave-sinteredsamples are drastically higher than those of the conventionallysintered samples. Also it can be found that tan ı/�i of microwave-

Fig. 18. Dielectric properties of sintered pellets from microwave synthesis (MS) andconventional synthesis (CS) powder [44].

186 M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189

Frs

stceo

isalocb

Fig. 20. SEM micrographs of samples sintered at 980 ◦C by: (a) conventional sinter-ing technique and (b) microwave sintering technique [34].

ig. 19. The dependence of (a) densities, (b) relative initial permeability and (c)elative loss factor tan ı/�i of microwave and conventionally sintered samples onintering temperature [34].

intered samples is greatly improved in comparison with that ofhe conventionally sintered ones. For both microwave-sintered andonventionally sintered samples, the tan ı/�i values reach the low-st at 980 ◦C sintering temperature, indicating that 980 ◦C is theptimum sintering temperature.

For (Ni0.20Zn0.60Cu0.20)Fe1.98O4 ferrite using microwave sinter-ng, the optimum sintering temperature is as low as 980 ◦C and theintering time is only 0.5 h, the relative initial permeability reachesbout 2000 and tan ı/�i reaches about 8.7 × 10−6 at 100 kHz and

eads to bigger grains and more compact matrix. Microstructuresf the samples sintered at 980 ◦C using microwave sintering andonventional sintering techniques are presented in Fig. 20. It cane clearly seen that there is presence of large number of pores at

Fig. 21. Sintering curves of microwave and conventional sintered porcelain bodies(electrical porcelain had sintering curve similar to that of the sanitary ware but with10 min of soaking time) [5].

M. Oghbaei, O. Mirzaee / Journal of Alloys and Compounds 494 (2010) 175–189 187

F(

twb

3

ts

ig. 22. Water absorption versus the sintering temperature of: (a) sanitary ware,b) dental and (c) electrical samples [5].

he intergrain boundaries in the conventionally sintered sample,hereas the matrix of the microwave-sintered sample is found to

e more uniform and compact, and its grain size is bigger [34].

.2.7. Porcelain bodiesBodies of sanitary ware, dental and electrical porcelain were sin-

ered quickly by Menezes et al. aimed at microwave hybrid fastintering [5]. The raw materials were sintered in a conventional fur-

Fig. 23. Modulus of rupture versus the sintering temperature of: (a) sanitary ware,(b) dental and (c) electrical samples [5].

nace and in a microwave furnace (multimode cavity) at 2.45 GHz.Fig. 21 illustrates the microwave sintering cycle, including the mon-

itored cooling step in comparison with the conventional cycle. Aheating rate of 5 ◦C/min and soaking times of 30 and 60 min wereapplied for the conventional sintering. Sintering cycles with twoheating rates were used for the microwave sintering, the initialheating of approximately 65 ◦C/min up to 1000 ◦C, and the second

188 M. Oghbaei, O. Mirzaee / Journal of Alloys a

Fig. 24. Density of sintered UO2 pellets as a function of sintering temperature.Sintering time: 1 h [15].

FS

rSsr

csaFvhsvas

3

fi[pibb

[[[

[

[

[

[

[

[

[

[

[

[

[75 (1992) 341–346.

ig. 25. Grain size of sintered UO2 pellets as a function of sintering temperature.intering time: 1 h [15].

ate of approximately 25 ◦C/min up to the sintering temperature.oaking times of 6, 8 and 10 min were used in the microwave hybridintering of the dental, sanitary ware and electrical formulation,espectively.

Fig. 22 shows the water absorption of the microwave andonventionally sintered samples. As can be seen, despite thehort heating cycle the microwave-heated samples reached waterbsorptions similar to those of the conventional fired samples.ig. 23 shows the modulus of rupture of the microwave and con-entionally sintered samples. It is seen that despite the shorteating cycle the sanitary ware and electrical microwave-heatedamples attained modulus of rupture similar to those of the con-entional fired samples. The dental microwave-heated samplesttained modulus higher than those of the conventionally firedamples [5].

.2.8. Uranium dioxide (UO2)The microwave sintering of UO2 pellets can enhance the densi-

cation and grain growth compared to the conventional sintering15]. The variation of density with temperature for pellets pre-

ared by both method of conventional and microwave sintering

s depicted in Fig. 24. It can be seen that the UO2 pellet formedy the microwave sintering has a higher density than that formedy the conventional sintering method at the same sintering tem-

[

[

[

nd Compounds 494 (2010) 175–189

perature. The variation of grain size with temperature is shown inFig. 25. The grain size of microwave-sintered pellets is seen to belarger than that of the sintered pellets by the conventional methodat the same sintering temperature [15].

4. Conclusion

Microwave sintering has achieved worldwide acceptance due toits significant advantages against conventional sintering methods.A summary of these are:

(1) Sintering materials with microwave consumes much lowerenergy than conventional sintering.

(2) Diffusion process intensifies by using microwave due to itsenhanced mechanism.

(3) Higher heating rates can be attained and thus the sintering timereduces by using microwave sintering.

(4) Generally higher density and better grain distribution can beachieved through microwave sintering.

(5) Better physical and mechanical properties can be obtainedusing microwave sintering.

Acknowledgement

The authors would like to thank the office of gifted students atSemnan University for its financial support.

References

[1] P. Yadoji, R. Peelamedu, D. Agrawal, R. Roy, Materials Science and EngineeringB 98 (2003) 269–278.

[2] D. Agrawal, Journal of Materials Education 19 (1999) 49–58.[3] D.E. Clark, D.C. Folz, J.K. West, Materials Science and Engineering A 287 (2000)

153–158.[4] C. Leonelli, P. Veronesi, L. Denti, A. Gatto, L. Iuliano, Journal of Materials Pro-

cessing Technology 205 (2008) 489–496.[5] R.R. Menezes, P.M. Souto, R.H.G.A. Kiminami, Journal of Materials Processing

Technology 190 (2007) 223–229.[6] D. Agrawal, Sohn International Symposium Advanced Processing of Metals and

Materials, vol. 4, 2006, pp. 183–192.[7] D.E. Clark, W.H. Sutton, Annual Review of Materials Science 26 (1996) 299–331.[8] E.T. Thostenson, T.W. Chou, Composites: Part A 30 (1999) 1055–1071.[9] K.E. Haque, International Journal of Mineral Processing 57 (1) (1999) 1–24.10] W.H. Sutton, American Ceramic Society Bulletin 68 (2) (1989) 376–386.11] Y. Zhao, Y. Chen, Progress in Nuclear Energy 50 (2008) 1–6.12] R.J. Meredith, Engineers’ Handbook of Industrial Microwave Heating. IEE Power

Series 25, ISBN 0852969163, The Institution of Electrical Engineers, London, UK,1998, pp. 1–55.

13] A. Goldstein, N. Travitzky, A. Singurindi, M. Kravchik, Journal of the EuropeanCeramic Society 19 (12) (1999) 2067–2072.

14] V. Tsakaloudi, E. Papazoglou, V.T. Zaspalis, Materials Science and EngineeringB 106 (2004) 289–294.

15] J.H. Yang, K.W. Song, Y.W. Lee, J.H. Kim, K.W. Kang, K.S. Kim, Y.H. Jung, Journalof Nuclear Materials 325 (2004) 210–216.

16] J. Wang, J. Binner, B. Vaidhyanathan, N. Joomun, J. Kilner, G. Dimitrakis, T.E.Cross, Journal of the American Ceramic Society 89 (6) (2006) 1977–1984.

17] R. Roy, R. Peelamedu, L. Hurtt, J. Cheng, D. Agrawal, Materials Research Inno-vations 6 (3) (2002) 128–140.

18] B. Vaidhyanathan, A.P. Singh, D.K. Agrawal, T.R. Shrout, R. Roy, S. Ganguly,Journal of the American Ceramic Society 84 (6) (2001) 1197–1202.

19] A. Birnboim, D. Gershon, J. Calame, A. Birman, Y. Carmel, J. Rodgers, B. Levush,Y.V. Bykov, A.G. Eremeev, V.V. Holoptsev, V.E. Semenov, D. Dadon, P.L. Martin,M. Rosen, R. Hutcheon, Journal of the American Ceramic Society 81 (6) (1998)1493–1501.

20] R.R. Menezes, R.H.G.A. Kiminami, Journal of Materials Processing Technology203 (2008) 513–517.

21] M.S. Spotz, D.J. Skamser, D.L. Johnson, Journal of the American Ceramic Society78 (4) (1995) 1041–1048.

22] A.W. Fliflet, R.W. Bruce, R.P. Fischer, IEEE Transactions on Plasma Science 28(3) (2000) 924–935.

23] M.A. Janney, C.L. Calhoun, H.D. Kimrey, Journal of the American Ceramic Society

24] W.L.E. Wong, S. Karthik, M. Gupta, Materials Science and Technology 21 (9)(2005) 1063–1070.

25] K.H. Brosnan, G.L. Messing, D.K. Agrawal, Journal of the American CeramicSociety 86 (8) (2003) 1307–1312.

26] A. Upadhyaya, S.K. Tiwari, P. Mishra, Scripta Materialia 56 (2007) 5–8.

lloys a

[[[

[[

[

[

[

[

[

[

[[

[

[

[

[

[

M. Oghbaei, O. Mirzaee / Journal of A

27] R.M. Anklekar, D.K. Agrawal, R. Roy, Powder Metallurgy 44 (4) (2001) 355–362.28] M. Gupta, W.L.E. Wong, Scripta Materialia 52 (2005) 479–483.29] Z. Xie, J. Yang, X. Huang, Y. Huang, Journal of the European Ceramic Society 19

(1999) 381–387.30] D.R. Baghurst, A.M. Chippindale, D.M.P. Mingos, Nature 332 (6162) (1988) 311.31] S.K. Thakur, T.S. Kong, M. Gupta, Materials Science and Engineering A 452–453

(2007) 61–69.32] A. Barba, C. Clausell, C. Feliu, M. Monzo, Journal of the American Ceramic Society

87 (4) (2004) 571–577.33] D.C. Dube, P.D. Ramesh, J. Cheng, M.T. Lanagan, D. Agrawal, R. Roy, Applied

Physics Letters 85 (16) (2004) 3632–3634.34] M. Yan, J. Hu, Journal of Magnetism and Magnetic Materials 305 (2006)

171–176.

35] S.S. Panda, V. Singh, A. Upadhyaya, D. Agrawal, Scripta Materialia 54 (2006)

2179–2183.36] G. Prabhu, A. Chakraborty, B. Sarma, International Journal of Refractory Metals

& Hard Materials 27 (2009) 545–548.37] E. Breval, J.P. Cheng, D.K. Agrawal, P. Gigl, M. Dennis, R. Roy, A.J. Papworth,

Materials Science and Engineering A 391 (2005) 285–295.

[[[

[

nd Compounds 494 (2010) 175–189 189

38] K. Saitou, Scripta Materialia 54 (2006) 875–879.39] J. Cheng, D. Agrawal, Y. Zhang, R. Roy, Materials Letters 56 (2002) 587–

592.40] J. Binner, K. Annapoorani, A. Paul, I. Santacruz, B. Vaidhyanathan, Journal of the

European Ceramic Society 28 (2008) 973–977.41] S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, E.J. Yadlowsky, M.D. Keitz,

Composites: Part A 39 (2008) 449–453.42] J.D. Katz, R.D. Blake, J.J. Petrovic, H. Sheinberg, Metal Powder Report 43 (12)

(1988) 835–837.43] R.N.P. Choudhary, U. Bhunia, Journal of Material Science 37 (24) (2002)

5177–5182.44] H. Yu, H. Liu, D. Luo, M. Cao, Journal of Materials Processing Technology 208

(2008) 145–148.

45] D. Lynn Johnson, Ceramics International 17 (1991) 295–300.46] B. Yaman, H. Mandal, Materials Letters 63 (2009) 1041–1043.47] D. Salamon, Z. Shen, Materials Science and Engineering A 475 (2008) 105–

107.48] N.F. Gao, J.T. Li, D. Zhang, Y. Miyamoto, Journal of the European Ceramic Society

22 (2002) 2365–2370.