《材料制备与加工 Materials Synthesis and Processing》课程教学资源（书籍文献）Advanced Ceramics Processing

12 Advanced Ceramics Processing if 17 atom% LIQUID (L) more is added to zirconia.The cubic fluorite 3000 2500 CUBIC (C) solid solution is crystalline phase that can T (K) have variable composition.Solid solutions are 2000 very common in crystalline materials TETRAGONAL It is also evident from the phase diagram that ( the tetragonal phase for pure zirconia is unsta T+C ture phase by quench cool 1000 martensitic character of the tet onal-to This trans M+C accompani na 500-(M) 273 strain of 0080 so ifa co npact of pure zirco 0 5 10 20 nia is sintered above 1170C(a minimum Zr02 mol%YO15 iny de ng th cooling process.The expansion of the ceramic during cooling (caused by the phase transformation)gives of(tension)stresses in the ceramic and consequently the compact fractures to loose particles However.it is possible to obtain vttria-doped zirconia ceramics with zirconia in its tetragonal form at room temperature. These types of ceramics are used for several structural applications because of T y are often denoted as Y Y ttria-dop d ctragonal Zirco e is retain state at room Stabilisation of the tetragonal phase at room temperature is possible when certain microstru ctural conditions are fulfille n(qualitatively)be explai by regarding the tree ener y chang or the 100 cdriven but also kinetically.At temperatures below formation is more kinetically determined than thermodynamically.This kinetic term (an ene es with decreasing grain size,m all grair e when a hich no t formation occurs at room temperature (=d)is 30 nm.This critical grain size increases with yttria content.de values of 350 and 500 um are mentioned for zirconia ceramics doped with 4 and 6 mol%YOs respectively. M.G.Scott.J.Mate .ScL.10(1975)1521 (o-onainine

12 Advanced Ceramics Processing If one considers this phase diagram it can be concluded that the cubic fluorite structure is stable at room temperature if 17 atom % Y or more is added to zirconia. The cubic fluorite phase is stable in a broad composition range (at least up to 35 mol% YO1.5). That means that the crystalline material is a solid solution. A solid solution is crystalline phase that can have variable composition. Solid solutions are very common in crystalline materials. It is also evident from the phase diagram that the tetragonal phase for pure zirconia is unsta￾ble at room temperature. Retaining the high temperature phase by quenching (rapid cool￾ing) is not possible due to the diffusionless, martensitic, character of the tetragonal-to￾monoclinic phase transformation. This trans￾formation is accompanied by a dilatational strain (volume expansion) of 0.067 and a shear strain of 0.080. So if a compact of pure zirco￾nia is sintered above 1170 °C (a minimum temperature, required to obtain any densifica￾tion), the crystal structure transforms for the tetragonal to the monoclinic phase during the cooling process. The expansion of the ceramic during cooling (caused by the phase transformation) gives of (tension) stresses in the ceramic and consequently the compact fractures to loose particles. However, it is possible to obtain yttria-doped zirconia ceramics with zirconia in its tetragonal form at room temperature. These types of ceramics are used for several structural applications because of its high strength and toughness. They are often denoted as Y-TZP: Yttria-doped Tetragonal Zirco￾nia Polycrystals. In these ceramics the tetragonal phase is retained in a metastable state at room temperature. Stabilisation of the tetragonal phase at room temperature is possible when certain microstructural conditions are fulfilled. This can (qualitatively) be explained by regarding the free energy change of the tetragonal to monoclinic (t  m) phase transformation (∆Gtm). Transformation will not oc￾cur when ∆Gtm > 0. According to M. Yoshima et al.6 the t  m transformation is not only ther￾modynamically driven but also kinetically. At temperatures below ~ 1200 °C t  m phase trans￾formation is more kinetically determined than thermodynamically. This kinetic term (an energy barrier) increases with decreasing grain size, meaning that a metastable tetragonal zirconia phase can be obtained at room temperature when a sufficiently large molar surface area and subsequently a small grain is present. For undoped zirconia the critical grain size below which no t  m trans￾formation occurs at room temperature (= dc) is 30 nm. This critical grain size increases with yttria content; dc values of 350 and 500 µm are mentioned for zirconia ceramics doped with 4 and 6 mol% YO1.5 respectively. 5 M.G. Scott, J. Mater. Sci., 10 (1975) 1527 6 M. Yoshima, M. Kakihana and M. Yoshimura, “Metastable-stable phase diagrams in the zirconia-containing systems utilized in solid-oxide fuel cell application” Solid State Ionics, 86-88 [2] (1996) 1131-1149 Figure 1-7: Phase diagram of the zirconia-rich part of the zirconia-yttria system; after5

I Introduction 13 matrix acting on the zirconia particles.This matri constraint a/o depends on the Young's modulus(E)of the matrx Alarger E-modulus of the alumina is much higher than that of zirconia,so ti possible to stabilise the tetragonal structure to room 0.5um nsPoatcdnanhnin Alumina (ZTAhere1 5 weight o以1 ndoned tetrago nal zirconia is homogeneously dispersed in an alumina Aging of ceramics based on tetragonal zirconia Tetragonal zirconia ceramics are prone to aging in the of water 2410327Kx386k1:8m pres Th ans that a sl ter vanor at ter S0nd200C） Figure1-:SEM picture of zirconia-toughened ing starts in isolated grains on the surface by a stress corrosion type mechanism.St d that an radicals into the he of the t to m transformation Tensile s generated by this phase transformation can induce micr cks at the grain boundaries making i easier for water to diffuse inside the bulk grain stress state of the surface o Dry Siding wernCrun

1 Introduction 13 If zirconia particles are present as a second phase in a ceramic matrix of e.g. Al2O3 the kinetic term for phase transformation can increase more by a constraint of the matrix acting on the zirconia particles. This matrix constraint a/o depends on the Young’s modulus (E) of the matrix. A larger E-modulus of the matrix results in a more stable tetragonal structure. The E modulus of alumina is much higher than that of zirconia, so it is possible to stabilise the tetragonal structure to room temperature for undoped zirconia with grains of about 0.5 µm if this zirconia is incorporated in an alumina matrix. A well-known material is Zirconia Toughened Alumina (ZTA) where 15 weight % undoped tetrago￾nal zirconia is homogeneously dispersed in an alumina matrix. An example of such a microstructure is given in Figure 1-8. Aging of ceramics based on tetragonal zirconia Tetragonal zirconia ceramics are prone to aging in the presence of water. This means that a slow t to m phase transformation occurs in the presence of water or wa￾ter vapor at temperatures between 150 and 200°C. Ag￾ing starts in isolated grains on the surface by a stress corrosion type mechanism. Studies showed that an increase in internal stress, caused by diffusion of water radicals into the ZrO2 lattice, triggers the initiation of the t to m transformation. Tensile stresses generated by this phase transformation can induce microcracks at the grain boundaries, making it easier for water to diffuse inside the bulk of the material. In order to reduce aging effect, the densi￾ty, grain size, homogeneity of the phase distribution and the residual stress state of the surface of the material need to be controlled. 7 B. Kerkwijk, L. Winnubst, E.J. Mulder and H. Verweij, “Processing of Homogeneous Zirconia-Toughened Alumina Ceramics with High Dry-Sliding Wear Resistance” J. Am. Ceram. Soc., 82 [8] (1999) 2087-2093. Figure 1-8: SEM picture of zirconia-toughened alumina (ZTA)7: 15 wt% ZrO2 (white grains; grain size: 200 nm) - 85 wt% Al2O3 (dark grains; grain size: 500 nm)

15 Characteristics of powders and compacts In this chapter several microstructural characteristics of powders and compacts will be discussed. ructure dete mine the final properties of the ceramic but and size distributio density,flow an stics can be divided as follows .Properties dependent on the physical nature of the powder, particle size and particle size distribution particle form(form factor) surtace area tructure ties (e.g.flow) Properties dependent on the chemical nature of the powder. o chemical composition 0 puritv/impurity phase composition e energ and che mical nature. ties de dent on meration e properties of a green compact and a sintered product strongly depend on the characteristics of s.In Part of the b ed in given." The purity and n of owder partic cessing should rally spoken. Narrow particle size distribution. single-phase agglomeration or aggregation. contcntd ions during further processing 2.1 Definitions of ceramic powders In order to determine the p to make com have the dimensions of the solid particles to be taken into account but also their shape and mutual ie and epu ve tor s must b is course we use the definitions that are accepted for ASTM terminology [2] The particle is a common working unit used to describe particulate matter.It is that state of subdi- vision of matter whose shape depends on the process by which it was formed and on the intramo-

15 2 Characteristics of powders and compacts In this chapter several microstructural characteristics of powders and compacts will be discussed. The properties of a ceramic product largely depend on the raw materials (powders) from which they are formed. It is therefore essential to know and to control powder properties. Not only the chemical composition, purity and crystal structure determine the final properties of the ceramic but also numerous other powder properties such as size and size distribution, packing density, flow and compaction characteristics. The most important microstructural properties and their characteristics can be divided as follows: • Properties dependent on the physical nature of the powder, ◊ particle size and particle size distribution ◊ particle form (form factor) ◊ surface area ◊ porosity ◊ pore size and pore size distribution ◊ packing density and packing structure ◊ dynamic properties (e.g. flow) • Properties dependent on the chemical nature of the powder, ◊ chemical composition ◊ purity/impurity ◊ phase composition ◊ surface energy ◊ surface reactivity ◊ surface composition • Properties dependent on both physical and chemical nature, ◊ degree of aggregation/agglomeration. The properties of a green compact and a sintered product strongly depend on the characteristics of the starting material. So, characterisation of powders is an important step in almost every ceramic process. In Part III of the book of J.S. Reed [1] important characteristics of ceramic powders are given. The purity and morphology of powders and the degree of agglomeration of powder particles show a strong correlation with the method of preparation. The general ideal morphology of powders for modern ceramic processing should, generally spoken, meet the following requirements in order to be able to prepare homogeneous compacts: • Narrow particle size distribution, • single-phase, • low degree of agglomeration or aggregation, • low content of unwanted impurities, • no destructive phase-transformations during further processing 2.1 Definitions of ceramic powders In order to determine the properties of (ceramic) powders it is necessary to make some agreements concerning nomenclature that will be used for the presentation and evaluation of powders. Not only have the dimensions of the solid particles to be taken into account but also their shape and mutual attractive and repulsive forces must be regarded. In this course we use the definitions that are accepted for ASTM terminology [2]. The particle is a common working unit used to describe particulate matter. It is that state of subdi￾vision of matter whose shape depends on the process by which it was formed and on the intramo-

16 Advanced Ceramics Processing lecular adhesive forces.Such a definition describes all particulate entities such as a single crystal,a onase system,a morpnous mater al. measuring proce s can b An uum is the smalles and chemical properties of that substance.These properties are also homogeneous on that scale. lomerate examp amorphous particles.This is the smallest group of molecules without a strictly ordered s,polymers) or cma ulsion.Thc ecule types c same ons can be applicd sEeko6Meohipmt Aggregate An aggregate is an assembly of solid particles held together by strong inter-or intramolecular or ne m can be ne pro ct of s state reaction and(sin icle bonds aggr or solid bridging does not only occur between two solid particles,but a solid particle can also eac with the carrying flui or gas,c.g ation)or w h a binder.Crystalli Aggregates are stable to normal handling and ordinary dispersion techniqucs such as high-speed mixing and eme grinding d ment of the microstructure during sintering (densification.grain growth)and can influence the final properties(e.g density and grain size).Non-uniform aggregates can give rise to a nor compact.In order to c a ceramic materal v n a w inthe formation of inhomoge ous products containing cracks and faults coy preven Agglomerate In agg les(crys charge which can he g enerated during handling and processing operations like sieving or drying The smaller the particle dimensions,the large the specific surface ch Be esides these ele In the case of sinter active fine-grained ceramic powders,agglomerates are always present.Control of agglomeration is therefore important.This means that the agglomerates must be uniform in size regular in structure and weak enough so that they are fractured during compaction,resulting in a

16 Advanced Ceramics Processing lecular adhesive forces. Such a definition describes all particulate entities such as a single crystal, a multiphase system, an amorphous material, aggregates, agglomerates, etc. These working units do not change in size and other physical properties during the measuring procedure. Particles can be divided in ultimate particles, aggregates and agglomerates. Ultimate particle An ultimate particle of a substance is the smallest state of subdivision which retains all the physical and chemical properties of that substance. These properties are also homogeneous on that scale. Some examples of ultimate particles are: • Crystallites. A crystallite is a crystallographic ordered assembly of unit cells, • amorphous particles. This is the smallest group of molecules without a strictly ordered arrangement (glasses, polymers), • units of liquids (in gas or emulsion). These units can reach the dimension of a single mol￾ecule, • gas units in a liquid or solid state; e.g., pores. On all these types of ultimate particles the same mathematical compilations can be applied (size, size distribution, etc.). In this course the ultimate particle is in most cases the crystallite. If only this type of ultimate particle is regarded, an ultimate particle is often called a primary particle or crystallite. Aggregate An aggregate is an assembly of solid particles held together by strong inter- or intramolecular or atomic (adhesive) forces. These forces have a chemical character. In crystalline materials aggregates can be the product of solid state reactions and calcination (sin￾tering) treatments. During these temperature treatments sufficient diffusion of matter occurs into the neck regions between individual particles which create strong interparticle bonds. Aggregation or solid bridging does not only occur between two solid particles, but a solid particle can also react with the carrying fluid (liquid or gas; e.g. oxidation) or with a binder. Crystallisation of dissolved material at the point of particle contact (Oswalt ripening) must also be mentioned in this field. Fi￾nally partial melting and subsequent cooling can be a source for aggregation. Aggregates are stable to normal handling and ordinary dispersion techniques such as high-speed mixing and ultrasonic treatments. Extreme grinding decreases its dimensions. Aggregates are strong enough to retain their identity during green forming and can therefore affect the develop￾ment of the microstructure during sintering (densification, grain growth) and can influence the final properties (e.g. density and grain size). Non-uniform aggregates can give rise to a non-uniform compact. In order to obtain a ceramic material with a well-defined microstructure and well-defined properties it is important that aggregates are small and especially uniform in size, thereby prevent￾ing the formation of inhomogeneous products containing cracks and faults. Agglomerate In agglomerates solid ultimate particles (crystalline/amorphous) or aggregates are held together by relatively weak adhesive forces. In many cases these forces are due to an electrostatic surface charge, which can be generated during handling and processing operations like sieving or drying. The smaller the particle dimensions, the larger the specific surface charge density, hence the more severe agglomeration occurs. Besides these electrostatic forces, liquid bridges and Van der Waals forces also cause adhesive forces in agglomerates (see section 3.1). In the case of sinter active fine-grained ceramic powders, agglomerates are always present. Control of agglomeration is therefore important. This means that the agglomerates must be uniform in size, regular in structure and weak enough so that they are fractured during compaction, resulting in a Figure 2-1: Schematic representation of crystallites, ag￾gregates, and agglomerates

150nm and In thi for obtaining a well-defincd ceramic material schematic drawing containing these three particle working units is given in Figure 2-1. In literature the powder is not often cial sub separately but used as a part of,e.g.wet-chemical owder preparation or the other defir for particles are used For example Grain.This can be used for all types of parti- size.agglomerate size ompact). hard agglomerate ·o granule is often used when agglomeration oc- 2-3 aph ofa approximately 10 nm means which after drving and calcination results in irregu lar agglomerates (see Figure 2-3).Figure 2-4 s (agglomerates)n (o Some final remarks In literature sometimes different,or confusing,definitions of agglomerates and agglomerates are given.Sometimes it is just defined the other way around,like: Aggregate:loose ated assemblage of particles

2 Characteristics of powders and compacts 17 uniform stack of the individual crystallites or aggregates in the green compact. Agglomerates can be reduced in size by milling, ultrasonification or dispersion treatments, like fluidisation. Agglomerates can transform into aggregates dur￾ing temperature treatments like solid-state reaction and calcination. In this way irregular aggregate structures can arise which are even less favourable for obtaining a well-defined ceramic material. A schematic drawing containing these three particle working units is given in Figure 2-1. In literature the powder is not often a special sub￾ject for analysis. In many cases it is not treated separately but used as a part of, e.g. wet-chemical powder preparation or the study of sintering be￾haviour. In these cases sometimes other definitions for particles are used. For example: • Grain. This can be used for all types of parti￾cles (e.g. crystallite size, agglomerate size, crystallite size in a sintered compact), • an agglomerate is often denoted as a soft ag￾glomerate, while in that case an aggregate is a hard agglomerate, • flocculate or coagulate: an agglomerate in a liquid medium, • granule: granules are agglomerates according to the definitions mentioned above. The word granule is often used when agglomeration oc￾curs in a controlled way. This granulation pro￾cess can be very important for formation of the green compacts and will be discussed in chapter 6. In Figure 2-2 - Figure 2-4 several morphologies of a (zirconia) ceramic powder are shown. The TEM picture in Figure 2-2 shows an ultimate particle size or crystallite size of approximately 10 nm. This powder was prepared by means of a wet￾chemical method (gel-precipitation technique), which after drying and calcination results in irregu￾lar agglomerates (see Figure 2-3). Figure 2-4 shows a scanning electron microscope picture of spherical granules (agglomerates) made by spray drying. Some final remarks In literature sometimes different, or confusing, definitions of agglomerates and agglomerates are given. Sometimes it is just defined the other way around, like: • Aggregate: loose, unconsolidated assemblage of particles • Agglomerate: rigid, consolidated assemblage of particles Figure 2-2: Transmission micrograph of a zirconia pow￾der Figure 2-3 Scanning electron micrograph of a zirconia powder after dying and calcination Figure 2-4: Scanning electron micrograph of spherical granules made by spray drying (TOSOH Inc.)