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Journal of Alloys and Compounds 654 (2016) 140e145

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Journal of Alloys and Compounds

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Low temperature sintering and ferromagnetic properties ofLi0.43Zn0.27Ti0.13Fe2.17O4 ferrites doped with BaOeZnOeB2O3eSiO2glass

Dainan Zhang a, b, Xiaoyi Wang a, Fang Xu a, Jie Li a, Tinchuan Zhou a, Lijun Jia a,Huaiwu Zhang a, Yulong Liao a, *

a State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology, Chengdu 610054, Chinab Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, USA

a r t i c l e i n f o

Article history:Received 18 June 2015Received in revised form29 August 2015Accepted 8 September 2015Available online 10 September 2015

Keywords:LieZneTi ferritesBZBS glassLTCC

* Corresponding author.E-mail address: yulong.liao@uestc.edu.cn (Y. Liao)

http://dx.doi.org/10.1016/j.jallcom.2015.09.0710925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

In this study, effects of a BaOeZnOeB2O3eSiO2 (BZBS) glass on the ferromagnetic properties of Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites were systematically investigated. Through the solid-state reaction process, itwas observed that a pure spinel phase was obtained with the sintering temperature raging from 880 �Cto 920 �C, indicating the compatibility of co-firing with silver. Results revealed that the addition of BZBSglass significantly promoted grain growth and enhanced ferromagnetic properties of the Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites. With an optimized addition of BZBS glass (2.0 wt.%), the saturation inductionwas increased from ~100 to 285 mT and the FMR line width at 9.3 GHz was dramatically reduced from~800 to 275 Oe. This study indicates that BZBS glass is a promising candidate for low temperature co-fired ceramics (LTCC).

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Low temperature co-fired ceramics (LTCC) have been widelyresearched in recent years on account of their multi-functionalitiesand high performances, which are crucial for the development ofminiaturizing microwave modules and devices [1e4]. It isacknowledged that ceramics co-firing with silver at low tempera-ture (<950 �C) is the key process of manufacturing LTCC devices[5,6]. As one kind of important gyromagnetic material, Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites, were found have superior ferromagneticproperties in our earlier study, such as high saturation inductionand relative low ferromagnetic resonance linewidth at high fre-quency. Nevertheless, they need to be sintered above 1000 �C bytraditional methods [7e10]. Apparently, it is hardly to makeLieZneTi ferrites co-firingwith silver by traditional methods due tothe high sintering temperature. In general, there are two commonsintering agents to reduce the sintering temperature, namelyadding glass and low melting point oxides (B2O3, Bi2O3 etc.)[11e14]. It was believed that adding glass is an easier and effective

.

way to realize excellent electrical properties together withacceptable densification at the low temperature [15].

In this study, BaOeZnOeB2O3eSiO2 (BZBS) glass was chosen asthe sintering agent to reduce the sintering temperature of LieZneTiferrites, because the BZBS glass has a relatively low melting tem-perature (575 �C) [16]. Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites doped with0.0 wt.% to 4.0 wt.% BZBS glass were prepared using a low-temperature ceramic sintering process (from 880 �C to 920 �C).The addition of BZBS glass is expected to facilitate grain growth ofthe ferrites and form a more compact structure under a relativelylow temperature (below 950 �C). Structural and ferromagneticproperties of the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites were discussedand investigated (see Table 1).

2. Experimental procedure

LieZneTi ferrites with chemical composition of Li0.43Z-n0.27Ti0.13Fe2.17O4, and BZBS glass with mass proportion x wt.%(x ¼ 0.0, 0.5, 1.0, 2.0 3.0 and 4.0) were synthesized by a solid-statereaction method. Firstly, High purity raw materials (Li2CO3, ZnO,TiO2, and Fe2O3) were weighed according to the required stoi-chiometric formulation of Li0.43Zn0.27Ti0.13Fe2.17O4. The batchedpowders were mixed and milled for 4 h using a planetary mill with

Table 1Influence of the additive content of BZBS glass to the magnetic properties anddensities of the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites sintered at 920 �C.

BZBS glasscontent

Hc(A/m)

Bs (mT) Br/Bs DH (Oe) Density(g/cm3)

Grain size(mm)

0.0 wt.% 619.9 175.7 0.7433 765 3.623 0.940.5 wt.% 315.42 268.74 0.812 388 4.145 2.831.0 wt.% 271.36 279.65 0.8427 309 4.19 3.942.0 wt.% 255.11 284.98 0.8373 275 4.32 4.203.0 wt.% 261.9 259.01 0.8065 375 4.356 2.324.0 wt.% 270.75 251.67 0.7802 399 4.295 2.48

D. Zhang et al. / Journal of Alloys and Compounds 654 (2016) 140e145 141

steel balls as milling media and then pre-sintered at 800 �C for 2 h.As for the synthesis of BZBS glass, 10 wt.% BaCO3, 40 wt.% ZnO,40 wt.% B2O3 and 10 wt.% SiO2 were mixed and milled for 6 h usingzirconia balls and then oven-dried at 90 �C for 24 h; after dryingand sieving, the powders were then melted in an alumina crucibleat 1300 �C for 1 h, followed by quenching to room temperature.Subsequently, the pre-sintered ferrite powders were mixed withvarious amount of BZBS powders and then wet-milled for 6 h. Thedried mixtures were granulated with 8 wt.% polyvinyl alcohol as abinder, sieved through a mesh of 100 mm, and then pressed intotoroidal samples (∅18 mm � 8 mm) at 10 MPa. Finally, sampleswere sintered in air at 880 �C, 900 �C, and 920 �C for 2 h.

The phase formation was characterized by X-ray diffraction(XRD) using CuKa radiation (D/max 2400; Rigaku, Tokyo, Japan),and the scanning speed was 5�/min at a step of 0.02�. The micro-structure properties of the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites dopedwith various amount of BZBS glass were observed using a scanningelectron microscope (SEM; JSM6490LV, JEOL, Tokyo, Japan). Thevolume densities of the samples weremeasured by the Archimedesmethod. The saturation induction and coercivity were tested by anIwatsu BH analyzer (SY8232) in an alternating magnetic field of1600 A/m at 1 kHz. As for FMR line width (DH), the sample shouldbe ground into a single sphere with diameter of about 1.0 mmfirstly and thenwasmeasured in TE106 perturbationmethod cavityat 9.3 GHz.

Fig. 1. XRD patterns of samples sintered under different temperatures: (a) 880 �C, (b)900 �C, and (c) 920 �C with various BZBS glass content from 0.0 wt.% to 4.0 wt.%.

3. Results and discussion

Fig. 1 shows X-ray diffraction (XRD) patterns of the Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites sintered under different temperatures(880 �C, 900 �C, and 920 �C) with various proportions of BZBS glassfrom 0.0 wt.% to 4.0 wt.%. Almost all of the samples exhibit char-acteristic peaks of spinel structure, except a weak impurity peak ataround 2q ¼ 33� (possibly a-Fe2O3) could be detected due to Lisegregation [9]. It should be noted that as the BZBS glass contentincreased above 0.5 wt.%, the impurity peaks disappeared. Thecorresponding X-ray diffraction peaks can be indexed to (220),(311), (222), (400), (422), (511), and (440) of spinel structure,indicating the spinel structure of Li0.43Zn0.27Ti0.13Fe2.17O4 ferriteswas well preserved during the sintering process after BZBS glasswas added. The XRD results suggest that spinel phase was suc-cessfully formed when the sintering temperature ranged from880 �C to 920 �C, and BZBS glass is an applicable sintering aid forlow temperature co-fired LieZneTi ferrites.

SEM micrographs of the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites dopedwith x wt.% (x ¼ 0.0, 0.5, 1.0, 2.0 3.0 and 4.0) of BZBS glass arepresented in Fig. 2 (sintered at 920 �C). It can be seen that withincreasing doping content, the grain size of the LieZneTi ferritessignificantly increased from less than 1 mm to almost 7 mm (seeFig. 2(a) and (d)). It could be contributed to a rapid grain growthresulted from the formation of a thin layer of glass-rich liquidphase. Meanwhile, intragranular pores can be easily discerned

Fig. 2. SEM micrographs of Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites sintered at 920 �C for 2 h with BZBS glass addition of (a) 0.0 wt.%, (b) 0.5 wt.%, (c) 1.0 wt.%, (d) 2.0 wt.%, (e) 3.0 wt.%, and(f) 4.0 wt.%.

D. Zhang et al. / Journal of Alloys and Compounds 654 (2016) 140e145142

when there was not sufficient BZBS glass (Fig. 2(b) and (c)). How-ever, we can hardly see the pores when dopedwith 2.0 wt.% of BZBSglass (Fig. 2(d)). When the BZBS glass content exceeded 2.0 wt.%,further grain growth is restricted and some small grains could notcombine with large grains, see Fig. 2(e) and (f). It is considered thatexcessive liquid phase presented on grain boundaries would bring

Fig. 3. The saturation induction (Bs) of the samples sintered at 880 �C, 900 �C and920 �C with BZBS glass content from 0.0 wt.% to 4.0 wt.%.

in an additional resistance for sintering and competitive graingrowth. It should be noted that grain edges of the samples dopedwith 0.0 wt.% ~ 1.0 wt.% BZBS glass are hackly, nevertheless thegrain edges are quite smooth when doped with 2.0 wt.% ~ 4.0 wt.%,which could be explained by that the hackly edge embossmentsdissolved in glass liquid during the sintering process. In a word, the

Fig. 4. The coercivity (Hc) of the samples sintered at different temperatures with BZBSglass content from 0.0 wt.% to 4.0 wt.%.

Fig. 5. The remanence square of the samples sintered at 880 �C, 900 �C and 920 �Cwith BZBS glass content from 0.0 wt.% to 4.0 wt.%.

D. Zhang et al. / Journal of Alloys and Compounds 654 (2016) 140e145 143

SEM results reveal that the grain growth of the Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites was intensely influenced by addition ofBZBS glass.

Fig. 3 shows the saturation induction (Bs) value of the Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites with various amounts of BZBS glass sin-tered under different temperatures. Firstly, it was observed that theBs values for all the samples sintered at different temperature(880 �C, 900 �C, and 920 �C) showed a similar tendency. Bs valuerapidly increased with the adding of BZBS glass and achieved itsmaximum when the addition amount was 2.0 wt.%. Furtherincreasing the BZBS glass had no benefit on the enhancement of Bsvalue; on the contrary, it decreased the Bs value. For the initial Bsincrease, it can be explained that a moderate amount of BZBS glasspromoted the grain growth and the grain size could reach about7 mm. Therefore with the proportion of large size grains increased,the degree of crystallizationwas promoted and subsequently the Bsvalue was enhanced. For the thereafter Bs decrease, it can beexplained that toomuch nonmagnetic liquid phasewas formed andthen diluted the LieZneTi ferrites and finally decreased the Bsvalue. It can be concluded that an optimal amount of BZBS glasscould strongly enhanced the Bs value, indicating the successful

Fig. 6. The bulk density of LieZneTi ferrites samples sintered at different tempera-tures (880 �C, 900 �C and 920 �C) with various BZBS glass addition from 0.0 wt.% to4.0 wt.%.

synthesis of Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites at low temperature.The coercivity (Hc) of the samples sintered at different tem-

peratures was shown in Fig. 4. Hc decreased rapidly and achievedits lowest value (from ~800 to 275 A/m) when 2.0 wt.% of the BZBSglass was added. It was reported that the Hc value is inverselyproportional to grain size [17]. For this work, it could be confirmedfrom the SEM results that the sample doped with 2.0 wt.% BZBSglass possessed the maximum size of grains (Fig. 2(d)), and thelowest Hc value (275 A/m) was observed consequently. The sampledoped with 0.0 wt.% BZBS glass possessed the minimum size ofgrains (Fig. 2(a)) and the highest Hc value (~800 A/m). On the otherhand, when doped with 2.0 wt.% BZBS glass, the solid phase reac-tion could be accelerated and reacted more thoroughly. Subse-quently, the densification degree of the sample was promoted(Fig. 2(d)) and finally leaded to high Bs value. Moreover, for thesame BZBS glass doping amount, Hc of the samples was founddecreased with the elevation of sintering temperature from 880 to920 �C. This could be attribute to the bigger grains were formedwith the increased sintering temperature, which finally decreasedthe coercive force. It should be focused on the fact that Hc increasedslightly when excessive amount of BZBS glass was added. This couldbe contributed to the fact that too much liquid phase at the grainboundaries, resulting in the increased hindrance force.

Fig. 5 shows the remanence square ratio of the Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites. It can be seen that the inflection point ofeach curve is the point where the amount of BZBS glass was2.0 wt.%, and the tendency of remanence square ratio is quitesimilar to the tendency of Bs (Fig. 3). In addition, the remanencesquare ratio increased slowly when the sintering temperatureincreased. It should be noted that the optimal value (about 0.85)was quite close to the samples which were sintered at a relativelyhigh temperature (above 950 �C) such as Ref. [18]. What's more,other ferromagnetic properties presented a similar rule in com-parisonwith the high temperature sintering process. However, thisstudy shows a novel formula of glass (BZBS) which could realize thelow-temperature sintering.

The densities of the samples dopedwith various amount of BZBSglass sintered under different temperatures are presented in Fig. 6.It can be seen that both the sintering temperature and the pro-portion of BZBS glass are influencing factors. As for the formerfactor, the elevated sintering temperature promoted the graingrowth and accelerated the process of small size grains combiningtogether into larger grains, which decreased porosity factor andfinally increased the bulk density. As for the latter factor, the den-sities was dramatically increased when doped with just a smallamount of BZBS glass (0.5 wt.%). Nevertheless, when the BZBS glassamount exceeded 2.0 wt.%, the rising trend slows down its step oreven turns into downtrend. The above phenomenon could beexplained by that too much BZBS glass with lower densitydecreased the average density of the sample. In short, these resultsindicate that the BZBS glass can effectively improve the densifica-tion of the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites.

Fig. 7 shows the FMR spectra of the Li0.43Zn0.27Ti0.13Fe2.17O4ferrites dopedwith various amount of BZBS glass sintered at 920 �C.It can be seen for almost all the samples, Lorentz-Fit lines weremore anastomotic than Gauss-Fit lines. At first, without the BZBSglass, both the Lorentz-Fit and the Gauss-Fit fitted poorly.When theaddition of BZBS glass was 2.0 wt.%, not only the Lorentz-Fit line butalso the Gauss-Fit line was fitted best with experimental datacompared with the rest of the samples. The FMR line width (DH)value calculated from the experimental data is presented in Fig. 8. Itwas observed that the DH value decreased significantly whendoped with just 0.5 wt.% BZBS glass (from ~800 Oe to 388 Oe) andthe curve shows a downtrend when the doping content was lessthan 2.0 wt.%. In addition, DH value reached its minimum (275 Oe)

Fig. 7. FMR spectra with fitted Lorentz and Gauss curves of the LieZneTi ferrites sintered under 920 �C with (a) 0.0 wt.%, (b) 0.5 wt.%, (c) 1.0 wt.%, (d) 2.0 wt.%, (e) 3.0 wt.%, and (f)4.0 wt.% BZBS glass addition.

D. Zhang et al. / Journal of Alloys and Compounds 654 (2016) 140e145144

when doped with 2.0 wt.% BZBS glass and increased slightly withfurther increasing doping content. On the basis of the formula,

DHpolycrystal ¼ DHsinglecrystal þ 2:07

H2a

4pMs

!þ 1:5ð4pMsÞP

in polycrystalline garnets [19,20], where Ha represents the anisot-ropy field and P represents the porosity. When adding an appro-priate amount of BZBS glass, the Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites

Fig. 8. FMR line width (DH) calculated from the experimental data of FMR spectra ofthe LieZneTi ferrites sintered under 920 �C with various BZBS glass content.

were composed of relatively large-size grains, resulting in lowporosity and the P value decreased subsequently, which finallycaused the reduction of DH. However, when the doping contentamount exceeded 2.0 wt.%, small grains and large grains coexisted(Fig. 2(e) and (f)), resulting in the increase of anisotropy field.Therefore, the above-mentioned factors finally caused the decreaseof DH when further doping with BZBS glass.

4. Conclusion

In summary, Li0.43Zn0.27Ti0.13Fe2.17O4 ferrites were successfullysynthesized under a relatively low temperature (from 880 to920 �C). The BZBS glass doping content plays a vital role in the lowtemperature co-firing process of the LieZneTi ferrites. All thesamples doped with various amount of BZBS glass sintered atdifferent temperatures showed pure spinel phase, indicating thesuccessful synthesis of the ferrites. Moreover, 2.0 wt.% BZBS glass isthe optimal doping amount with which the LieZneTi ferritespossess enhanced ferromagnetic properties such as saturation in-duction (from ~100 to 285 mT), coercivity (from 620 to 255 A/m)and FMR line width (from ~800 to 275 Oe). It can be concluded thatBZBS glass is a promising candidate for synthesis of Li0.43Z-n0.27Ti0.13Fe2.17O4 ferrites via low-temperature co-fired technology.

Acknowledgments

This work was financially supported by the National NatureScience Foundation of China under Grant No. 51502033 and No.61571079, National Basic Research Program of China under GrantNo. 2012CB933104, 111 Project No. B13042.

D. Zhang et al. / Journal of Alloys and Compounds 654 (2016) 140e145 145

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