Unit 6 Ceramic Fabrication Process: Conventional Routes to Ceramics
Solid ceramic bodies are generally produced by using the process of powder compaction followed by firing at high temperature. Sintering or densification occurs during this heat treatment and is associated with joining together of particles, volume reduction, decrease in porosity and increase in grain size. The phase distribution or microstructure within the ceramic is developed during sintering and fabrication technniques used for shaping ceramics are described here. The aim of these techniques is to produce microstructures suitable for particular applications.
(a) Precipitation from solution
Alumina occurs as the mineral bauxite and is refined in the Bayer process whereby ore is initially dissolved under pressure in sodium hydroxide so that solid impurities (SiO2, TiO2, Fe2O3) separate from sodium aluminate solution. This solution is either seeded with gibbsite crystals (α-Al2O3·3H2O) after its neutralisation with CO2 gas. Temperature, alumina supersaturation and amount of seed affect particle size during crystallisation.
Problems can arise when two or more components are coprecipitated. Thus, different species do not always deposit from solution at the reaction pH, while washing procedures can selectively remove a precipitated component as well as dissolve entrained electrolyte. The difficulty in maintaining chemical homogeneity is serious as inhomogeneities have a deleterious effect on the mechchanical and electrical properties of ceramics. Because precipitation results in agglomerated powders, grinding, drymilling or wetmilling with water or a nonaqueous liquid are used for particle size reduction so that powder compacts will sinter to near theoretical density.
Precipitation reactions are not restricted to oxides and hydroxides. Hence, for the high Tc oxide superconductor La1.85Ba0.15CuO4, La, Ba and Cu oxalates were deposited from electrolyte solutions and sintered in air at 1373K. Because these materials reversibly intercalate O2, the annealing temperature and rate of cooling, which affect their superconducting properties and the Cu[3+]/Cu[2+] ratio must be carefully controlled.
(b) Powder mixing techniques
Multicomponent oxide powders are synthesised from conventional mixing techniques by initially blending together starting materials, usually metal oxides and carbonates, after which the mixtures are ground or milled. Comminuted powders are then calcined, sometimes after compaction, and the firing sequence may be repeated several times with intermediate grinding stages. As for coprecipitation, impurities can be introduced into the ceramic from the grinding operation; grinding also results in angular shaped powders.
Several problems are associated with mixing powders. High temperatures required for reaction between components can result in loss of volatile oxides, while milling may not comminute powders sufficiently for complete reaction to occur on calcination. It is difficult to obtain reproducible uniform distributions of material in ball-milled powders especially when one fraction is present in small amounts as occurs in electroceramics whose properties are often controlled by grain boundary phases containing minor quantities of additives. The YBa2Cu3O7-δ superconductor was synthesised by mixing Y2O3, BaCO3 and CuO, grinding and heating at 1223K in air. Powder was then pressed into pellets, sintered in flowing O2, cooled to 473K in O2 and removed from the furnace.
(c) Uniaxial pressing
In uniaxial pressing a hard steel die is filled with either dry powder, or a powder containing up to several weight percent of H2O, and a hard metal punch is driven into the die to form a coherent compact. Van der Waals forces cause aggregation of fine powders so that binders such as polyvinyl alcohol and lubricants are incorporated into them by, for example, spray drying in order to improve their flow properties and homogeneity of the product. It is important that the unfired or green body has adequate strength for handling before the firing operation, during which organic additives are decomposed. Uniaxial pressing can be readily automated and is particularly suited for forming components with a simple shape such as flat discs and rings that can be produced to close dimensional tolerances, thus avoiding postfiring diamond maching operations.
(d) Hot uniaxial pressing
Hot uniaxial pressing or hotpressing involves simultaneous application of heat and pressure during sintering. A refractory die, usually graphite, is filled with powder, which, after compaction, is heated in an inert atmosphere. Hotpressing produces higher density and smaller grain sizes at lower temperatures compared with uniaxial pressing and is particularly suited for fabrication of flat plates, blocks and cylinders. Stresses set up by the applied pressure on contacts between particles increase the driving force for sintering and remove the need for very fine particle sizes. Additives such as magnesium oxide and yttrium oxide, which are often used for Si3N4, allow achievement of theoretical density at lower temperatures. These sintering aids result in formation of a liquid phase and particle rearrangement because of capillary forces arising from the Laplace equation and by dissolution recrystallisation processes. However, advantages brought about by additives have to be offset by degradation in mechanical behaviour of sintered components especially at high temperature because glassy and crystalline grain boundary phases derived from them often have inferior properties compared with the matrix.
(e) Solid state sintering
The driving force for sinterinig is reduction in surface free energy associated with a decrease of surface area in powder compacts due to removal of solid vapour interfaces. Vapour phase nucleation is described by using the Kelvin equation, which is also applicable to mass transport process in a consolidated powder. The vapour pressure difference across a curved interface can enhance evaporation from particle surfaces and condensation at the neck between two particles, particularly for particle diameters of several micrometers or less, such as occur in ceramic fabrication. Although this evaporation condensation process produces changes in pore shape and joins particles together, the centre to centre distance between particles remains constant so that shrinkage and densification do not occur. The driving force for mass transport by solid state processes for ceramic powders with low vapour pressure is the difference in free energy between the neck region and surface of particles. As for the evaporation condensation pathway, transport from surface to neck by surface and lattice diffusion does not cause densification. This is produced only by diffusion from the grain boundary between particles and from the bulk lattice. Covalent ceramics such as Si3N4 are more difficult to sinter to high density than ionic solids, for example Al2O3, because of lower atomic mobilities, although difficulties can be overcome by using very fine powders ca. 0.1μm diameter, high temperature and high pressure.
Impurities such as oxygen and chlorine in Si3N4 often migrate during sintering to grain boundaries where they reduce the interfacial surface energy and impair densification, creep behaviour, oxidation resistance and high temperature strength.
Selected from "Chemical synthesis of advanced ceramic materials", David Segal, Cambridge University Press, Cambridge, 1989
Words and Expressions
sintering 烧结
porosity 气孔率
grain size 颗粒尺寸
microstructure 微结构
alumina 氧化铝
bauxite 铝土矿
impurity 杂质
gibbsite α三水铝石
bayerite β三水铝石
neutralisation 中和
supersaturation 超饱和
crystallisation 晶化
deposit 沉积
inhomogeneity 不均匀性
deleterious 有害的
agglomerate 结块
grinding 磨碎
oxalate 草酸盐
intercalate 插入
annealing 退火
multicomponent 多元的
calcine 煅烧
grain boundary 颗粒界面
additive 添加剂
pellet 片
binder 粘结剂
polyvinyl 聚乙烯
spraydrying 喷雾干燥
green body 生坯
yttrium 钇
rearrangement 重排
matrix 基体，矩阵
shrinkage 收缩
impair 削弱
creep 蠕变
