Tripole university

Material and Metalorgical Departement

Glass-Ceramic "MME660"

Home work "4"

Properties of Glass Geramics

Prepared by :Hana Jamhour

D. Nibile Abed alwahab

Date:- 23/6/2014

INTRODUCTION

The bulk composition of glass-ceramics also directly determines the potential crystalline assemblage, and this in turn determines the general physical and chemical characteristics, for example, hardness, density, thermal expansion coefficient, and acid resistance (1). And a Microstructure of glass-ceramic is equal of importance to composition; because This feature is the key to most mechanical and optical properties, and it can promote or diminish the characteristics of key crystals in glass-ceramics. It is clear that microstructure is not an independent variable. It obviously depends on the bulk composition and crystalline phase assemblage, and it also can be modified, often dramatically, by varying the thermal treatment. The Thermal, Mechanical, and Optical Propertiesof glass-ceramic unusual and in some cases unique contributed much to their commercial success. Almost all of these properties relate to the characteristics of the crystalline phases, and to their microstructural arrangement(2). Some These unique properties is listed below compared the glass:-

Exhibit better thermal stability

Superior electrical insulation

Higher mechanical strength

Thermal expansion coefficient tunability

Physical and chemical properties

1- Microstructure and porosity

One of the notable characteristics of glass ceramics is the extremely

fine grain size and it is likely that this feature is responsible in large

measure for valuable properties of the materiel. And it is have an

almost polycrystalline structure and fine texture, the crystal are fairly

uniform in size and they are randomly oriented where the average

crystal size in glass-ceramic would usually less than 1micron and

materials with average crystal size down to 200 to 30oA are possible.

The very fine texture of glass- ceramic is means that examination by

optical microscope has only limited application since details of

structure are beyond the limit of resolution of microscope and higher

magnification of the electron microscope has to be utilized. Figuer.1. is

an electron micrograph of a typical titania nucleated magnesium-

aluminosilicate glass-ceramic in which the major phase are corderite

and cristobalite together with smaller propertion of rutile, have an

average size of 0.5to 1micron and maximum crystal size is in the region

of 2micron.