for the preparation of a much lower than the tem-
PART III
micron dimensions (see salt decomposition particle, as is shown in
ln
MATERIALS CHARACTERIZATION
growth during decompo-
The properties of a fired product are very dependent on the characteristics of the starting material and their subsequent modification during processing. ln order to select and control these materiais, it is necessary to have a knowledge of their more important characteristics. Some of this information is supplied in the material specifications provided by the raw-material supplier. However, the material system will have to be characterized at different processing stages, and the processing engineer must be familiar with the techniques used and the information obtained. The general characteristics of a material system and common specifications of severa I commercial ceramic materiais are presented in Chapter 5. Chemical and microstructure analyses are discussed in Chapter 6. Chapters 7 and 8 describe principies and techniques for characterizing the size, shape, density, surface area, and porosity of ceramic particle systems.
,
'~
"I
67
CHAPTER 5
CHARACTERISTICS ANO SPECIFICATIONS OF CERAMIC MATERIALS
The characteristics of a material are those parameters that specify the chemical and physical aspects of its composition and structure. "Composition" denotes the proportions of chemically and physically different constituents. "Structure" refers to the spatial distribution, orientation, and association of these constit uents. The properties of a material are its responses to changes in the physical or chemical environment. Every particle system will have particular properties for example, a particular thermal conductivity, elastic modulus, and dielectric constant. Flow and deformation properties are commonly referred to as !:heo logical properties. Responses to the chemical environment such as adsorption or dissolution are chemical properties. Porous particle systems may have special properties such as capillarity, permeability, and electroosmotic flow. Dispersed systems have special properties such as settling rate, electrophoretic mobility, and optical scattering. A system is said to be with respect to a particular property if the property varies with direction in the material. This chapter will consider the general characteristics of particle systems and specifications provided for ceramic raw materiais. The student should keep in mind the distinct difference between the properties and characteristics of a material system.
5.1
PARTICLES, POWOERS, COLLOIOS, ANO AGGLOMERATES
The size range of particles used in ceramics processing covers a wide range of sizes, as is indicated in Fig. 5.1. A particle is a discrete, solid unit of material and may be single or multiphase in composition. Groups of particles that are 69
I
70
CHARACTERISTICS AND SPECIFICATIONS OF CERAMIC MATERIALS
TABLE 5.1 Characterisdc 10 18
Colloidal
Single ParticIe·'
Powder Sand
:2•
-
Cemenl
10 1 '
li 10 13
d
Q.
~ 10 10
eaclerla
c
::l
•
:; E
Palnl Plgmenl
10 7
o
II.
'. 1. PrimaI)' chemical complj 2. Impurity composition, dí and partitioning 3. Phase compositíon 4. Point and line defects, 5. Structure of phases, boII 6. Porosity and pore s~ 7. Size 8. Shape 9. Density 10. surface area
Tobaeeo Smoke
10 4 Vlru.e.
10 1
TABLE 5.2 Speci~ Particle Olameter (tlm)
Fig. 5.1 Increase in fonnula units per AI 2 0 3 particle with particle size and particle size range of granular materiaIs, powders, and col\oidal materiaIs and some common substances. ~~akly
bonde<.tJogether may behave as a fragile, larger pseudoparticle called an agglomerate (see Fig. 3.5). If ~trongly bonded together, the larger particle is not easily dispersed and is referred to as an aggregate or a hard agglomerate. Bonds in hard agglomerates are generally primary chemícal bonds formed by a chemical reaction or sintering. ln soft agglomerates, the relatively weak bonds may be of electrostatic, magnetic, Van der Waals, or capillary adhesion type. The magnitude of the inertial force of a particle relative to surface forces has a major effect on particle behavior. A particle system is said to be granular if the gravitational force is predominant (the material is free-flowing), a powder if the surface force is of the sarne order as the gravitational force (naturally agglomerates), and colloidal if the particles are so fine that the inertial force of a particle is insignificant and the surface forces dominate the behavior. The surface forces are dependent on the environment of the particles. But for prac tical purposes, particles larger than 44 pm (opening in a 325-mesh sieve) can generally be considered to be granular and particles smaller than I pm as colloidal. Colloids dispersed in a low-viscosity Iiquid typically exhibit Brown ian motion at 20°C. The behavior of powders and colloids can be markedly altered by adsorbed surfactants that modify the surface forces.
5.2
RAW-MATERIAL SPECIFICATIONS
The general characteristics of one particle and a system of particIes are listed in Table 5.1. Complete characterization is an impossible task, and for each
Characteristici
c I)' stal phase
"".1
Purity ( % ) 1 Impurity analysis of ce~ grade (ppm) !
Na! Pb Si
Cr Fe Ga Ca Mg Zn Ti
Mn
V
Cu Si02 Fe203
CaO
Na20
Ga203
Others Ultimate partic\e size (ILm] Specific surface area (mll. Agglomerate size (ILm) mi Cl)'stal density (Mg/m3) Apparent bulk density (M. "Products of 8aikowski Interl bproduct of Aluminum Comp
RAW-MATERIAL SPECIFICATIONS
71
TABLE 5.1 Characteristics of a Particle System Sing1e Particle
Particle System
1. Primary chemical composition 2. lmpurity composition, distribution, and partitioning 3. Phase composition 4. Point and line defects, domains, etc. 5. Structure of phases, boundaries 6. Porosity and pore structure 7. Size 8. Shape 9. Density 10. Specific surface area
1. Distribution of chemical composition 2. Distribution of impurities 3. Distribution of phase composition 4. Distribution of crystal defects 5. Porosity and pore structure 6. Particle structure distribution 7. Particle size distribution 8. Particle shape distribution 9. Particle density distribution 10. Bulk density 11. Specific surface area
TABLE 5.2 Speciftcation of SpeciaI High-Purity Aluminas Characteristic particle size and particle materiais and some common
pseudoparticle called , the larger particle or a hard agglomerate.
bonds formed by the relatively weak bonds capillary adhesion type. relative to surface forces is said to be granular
is free-flowing), a powder force (naturally fine that the inertial force the behavior. The particles. But for prac in a 325-mesh sieve) can smaller than 1 Ilm as typícally exhibit Browncolloids can be markedly forces.
Crystal phase Purity (%) Impurity analysis of ceramic grade (ppm) Na Pb Si Cr Fe Ga Ca Mg Zn Ti
Mn V
Cu Si02 Fe20J CaO Na20 Ga203 Others Ultimate particle size (Ilm) Specific surface area (m 2 /g) Agglomerate size (JLm) mean Crystal density (Mg/m 3 ) Apparent bulk density (Mg/m J )
CalcinedG
C alcinedG
Calcined b
>90% gamma 99.99
85% alpha 99.99
alpha 99.99
20
20
4
4
18
18
4
4
10 15 10 5
10
15
10
5
4
4
5 3 3 2
5 3 3 2
0.01 115 2 3.67 0.12
"Products of Baikowski Intemational Corp., Charlotte, NC. bProduct of Aluminum Company of America, Pittsburgh, PA.
0.15 10 0.6 3.98 0.51
<50 <20 <10 <10 <10 <10 <0.5 5-50 0.5 3.98
72
CHARACTERISTICS ANO SPECIFICATIONS OF CERAMIC MATERIALS
material and application we must consider what characterization is necessary and sufficient. Sorne of this information may be supplied by the raw-material vendor on a specification sheet for the material. Table 5.2 lists typical speci fications for special very high purity, very fine alumina materiais. The impurity composition is quite complete, and the crystalline phase is identified. The specific surface area and nominal information about agglomeration and crystal size are presented. Since these are relatively expensive materiais, the customer will certainly determine additional characteristics of each lot and perhaps pro cess and fabricate a small amount of the material in the laboratory or factory to verify that it is satisfactory. Typical specifications of three calcined Bayer process aluminas are listed in Table 5.3. The parameters specified are similar to those for the purer alumina, but we can readily see from the specifications that these aluminas are quite different in chemical purity and particle size (see Figs. 3.5 and 4.2). A plant TABLE 5.3 Speci6cations of Three Bayer Process Aluminas
Characteristic Chemical analysis (wt%) AI 20 3 Si02 Na20 Fe2O) CaO LOI (l100°C) Total water" ex Alumina phase (%) Ultimate crystal size (/Lm) Particle size distribution Sieve analysis (wt %) + 100 mesh +200 mesh +325 mesh -325 mesh Sedimentation analysisb (/Lm) 90%< 50%< 10%< Specific surface areac (m2 /g) Specific gravity Bulk density (Mg/m 3 )
Calcined Intennediate Soda
Reactive Low Soda
Tabular (-325 Mesh)
engineer examines the preshiprn lhe particular 10t of material. De applícation, a seI of characteristic standardized test procedures, in or rejecting a shipment. Specifications for three dilfere ceramics are listed in Table 5.4 chemical and particle size chame concentration of CO2 and S03 m tants during calcination. DilfereJ reflect the variations in the cheIJI of the raw materiais and microsl Specifications for the COIIllll4 basic chemical and particle size eral types and mineral impuritie index is a relative indication 01 sorption of methylene blue dye. sions, and the pH index of a
TABLE 5.4 Typical Specificatio 99.4 0.02 0.25 0.04 0.04 0.2 0.3 90+ <5.0
99.7 0.02 0.08 0.01 0.01
-100 >0.5
99 0.2 0.10 0.3 0.07
-100 >95
40 12 3 1.0 3.8 1.0
1.5 0.5 0.2 3-6 3.98
Source: Products of Alumínum Company of America, Pittsburgh, PA.
"I 100°C ígnítíon loss after adsorption ai 44% relatíve humidity.
hOravity settling.
'Nilrogen adsorption.
>3.4
Character Chemical analysis (wt%) Si02 AI 20) Ti0 2 srO BaO
Na20
S03
CO 2
LOI
Size analysis (/Lmt
90%<
50%<
10%<
+325 Mesh (%) Bulk density (Mg/m 3 ) Electrical Property Analyses (bodl magnesium zirconate):
Dielectric constant (25°C)
Dissipation factor (% at 25°C)
.:l Dielectric constant (\0ü0C)
.:l Dielectric constant (- 10°C)
Fired density (Mg/mJ )
"Products of TAM Ceramics lnc.,
NÍI
RAW·MATERIAL SPECIFICATIONS
cha.racterization is necessary by the raw-material 5.2 lists typicaJ speci materiais. The impurity phase is identified. The agglomeration and crystal materiais, the customer each lot and perhaps pro in the laboratory or factory aluminas are listed in for the purer alumina, these aluminas are quite 3.5 and 4.2). A plant
Reactive
Low Soda
Tabular (-325 Mesh)
73
engineer examines the preshipment specifications supplied by the vendor for the particular lot of material. Depending on past experience and the particular application, a set of characteristics may be determined for a small sample using standardized test procedures, in the purchaser' s labo rato ry , before authorizing or rejecting a shipment. Specifications for three different barium titanate powders used for electronic ceramics are listed in Table 5.4. These materiaIs are prepared with different chemical and particle size characteristics, as indicated in the specifications. The concentration of CO 2 and S03 indicates the incomplete decomposition of reac tants during calcination. Differences in the electrical properties of a fired body reflect the variations in the chemical stoichiometry and physical characteristics of the raw materiaIs and microstructure developed during firing. Specifications for the commercial kaolins listed in Table 5.5 include the basic chemicaI and particle size characteristics. The composition of c1ay min eral types and mineral impurities such as free quartz are not listed. The MBI index is a relative indication of the specific surface area determined by ad sorption of methylene blue dye. Clay bodies are usually processed as suspen sions, and the pH index of a suspension may suggest the compatibility or TABLE 5.4 Typical Specifications of Calcined Barium Titanates
99.7 0.02 0.08 0.01 0.01
-100
99 0.2 0.10 0.3 0.07
-100
>0,5 >95
1.5 0.5 0.2 3-6 3.98 PA.
>3.4
Character
Capacito r
MLC
Piezoelectric
Chemical analysís (wt %) 0.10 0.12 0.15 8i02 0.10 0.14 AI 2 0 i 0.16 Ti02 34.64 33.95 33.38 0.90 0.78 8rO 0.91 63.59 64.28 BaO 64.18 0.10 0.17 0.15 Na20 0.15 0.14 0.18 803 0.15 0.09 0.43 CO2 0.17 0.32 LOI 0.57 8ize analysis (/lmt 4.5 5.5 5.4 90%< 1.6 2.3 2.0 50%< 0.8 1.0 0.8 10%< 0.02 0.02 +325 Mesh (%) 0.02 Bulk density (Mg/m 3 ) 1.8 2.4 2.0 Electrical Property Analyses (body contains 10% calciurn zirconate and 1% magnesium zirconate): Dielectric constant (25°C) 5250 4000 4400 1.18 0.83 Dissipation factor (% at 25°C) 0.67 -48.9 -52.9 -54.0 Ll Dielectric constant (l00°C) -33.7 -2.4 -4.9 Ll Dielectric constant (-10°C) 5.54 Fired density (Mg/rn J ) 5.30 5.60 "Produets of TAM Ceramics Ine., Niagara Falis, NY.
74
CHARACTERISTICS ANO SPECIFICATIONS OF CERAMIC MATERIALS
TABLE 5.5 Typical Specifications of Ceramic-Grade Kaolins Characteristic Chemical analysis (%) Si0 2 AIP3 Fel O) Ti0 2 CaO MgO
Kp
Na20 LOI Total Particle size analysis (cumulative mass percent finer) 20 (/-tm) 10
5 2 I
NC a
GA-p b
GA-C b
47.72 37.53 1.16 0.08
45.36 38.26 0.36 1.52 0.47 0.04 0.21 0.11 13.47 99.80
45.74 38.25 0.41 1.55
0.06
0.12 0.06 0.14 13.66 99.99
98.0 93.5 83.0 65.0 48.5 32.0 15.0 7.8 7.2 34-35 3.4
97.0 88.0 74.5 54.0
38.0
21.5 11.0 2.0 4.3 34-35 0.9
1.17 0.15 14.04 99.85
97.5 89.0 75.0 53.0 35.0
0.5 0.2 MBI (meq/IOO g)
pH PCE Dry MOR (MPa)
- c 5 33-34 1.2
"North Carolina kaolin, Harris Mining Co. Inc., Spruce Pine, NC.
bGeorgia kaolin, Cyprus Industrial Minerais, Inc., Sandersville, GA. (GA-P tor plastic forming,
GA-C for casting.) 'Contains halloysite with tubular particle shape.
change in pH if one clay is substituted for another. The pyrometric cone equiv alent (PCE) indicates the relative resistance of a material to vitrification and creep on heating. The MOR is the flexural strength of dried bars formed by extrusion. The pH, PCE, and MOR are not characteristics; rather, they are indices that indicate something about effects of soluble chemical impurities, impurity phases, and the particle size distribution on the chemical, thermal, and mechanical behavior, respectively.
SUMMARV
The characteristics of a material are the parameters necessary for its identifi cation or description. Specifications provided by suppliers of materiais provide some of these characteristics. More complete specifications or tighter specifi
cations of the lot-to-Iol reprocl material cost. Processing alten terials processors should deter uisite for control of the proces
SUGGESTED READING
1. F. H. Norton, Fine Ceramics,
2. W. M. Flock, "Characterizati before Firing, George Y. Ono New York, 1978, Chapter 4.
3. G. Y. Onoda]r. and L. L. f Ceramic Processing before Fil Wiley-Jnterscience. New Yorl 4. Y. S. Kim, "Effects of Powd and Technology, Vol. 9, Fm
1976, pp. 51-67.
PROBLEMS
5.1 What is the relatíonship porting the impurity in a
5.2 Calculate and compare t
in the two alpha alumil1ll
5.3 Calculate and compare til size 50% < for the two
5.4 Calculate the bulk densi and compare your resull 5.3.
5.5 Ca1culate the impurity I
barium titanate powders rities?
5.6 The chemical analyses (
grade barium titanate 3.1 SrO BaO Ti02
What is the reproducibi
PROBlEMS
MATERIALS
cations of the lot-to-Iot reproducibility of a material commonly increase the material cost. Processíng alters the characteristics of the particle system. Ma teriaIs processors should detennine the materiaIs characteristics that are req uisite for control of the processing and the properties of their products.
Kaolins
45.36 38.26 0.36 1.52 0.47 0.21 0.1I 13.47 99.80
45.74 38.25 0.41 1.55 0.06 0.12 0.06 0.14 13.66 99.99
98.0 93.5 83.0 65.0 48.5 32.0 15.0 7.8 7.2 34-35 3.4
97.0 88.0 74.5 54.0 38.0 21.5 11.0 2.0 4.3 34-35 0.9
0.04
75
(GA-P for plastic fonning,
pyrometric cone equiv to vitrification and dried bars fonned by ; rather, they are chemical impurities, the chemical, thennal,
.ece:ssalty for its identifi of materiaIs provide _ltiOllS or tighter specifi-
SUGGESTED READING l. F. H. Norton, Fine Ceramics, Krieger, Malabar, FL, 1978. 2. W. M. Flock, "Characterization and Process Interactions," in Ceramic Processing before Firing, George Y. Onoda Jr. and Lany L. Hench, Eds., Wiley-Interscience, New York, 1978, Chapter 4. 3. G. Y. Onoda Jr. and L. L. Hench, "Physícal Characterization Tennínology," in Ceramic Processing before Firing, George Y. Onoda Jr. and Lany L. Hench, Eds., Wiley-Interscience, New York, 1978, Chapter 5. 4. Y. S. Kim, "Effects of Powder Characteristics," in Treatise on MateriaIs Science and Technology, Vol. 9, Franklin F. Y. Wang, Ed., Academic Press, New York, 1976, pp. 51-67.
PROBLEMS
5.1 What is the relationship between parts per million and percent when re porting the impurity in a solid substance? 5.1 CaIculate and compare the impurity analysis of the five major impurities in the two alpha aluminas in Table 5.2. Use an elementary oxide basis.
5.3 Calculate and compare the sharpness index (size 90% < size 10% <)1 size 50% < for the two Bayer process aluminas in Table 5.3. 5.4 Calculate the bulk density as a percent for the two aluminas in Table 5.2 and compare your results to the value for the calcined alumina in Table 5.3.
5.5 Calculate the impurity leveI of CO2 and S03 in parts per million for the barium titanate powders in Table 5.4. What is the source of these impu rities? 5.6 The chemical analyses (wt %) of three production lots of caIcined capacitor grade barium titanate are as follows: SrO BaO Ti0 2
0.95 63.71 34.67
0.82 64.00 34.61
What is the reproducibilíty of the molar ratio (BaO
0.84 63.94 34.59
+ SrO)/Ti02?
76
CHAAACTEAISTICS ANO SPECIFICATlDNS DF CEAAMIC MATEAIALS
5.7 Which parameters in Table 5.5 are characteristics and which are pro per ties? Explain your reasoning.
Calculating in a similar mannel MLC~
5.8 Compare the content of colloidal sizes for the two Georgia kaolins in Table 5.5.
Piezoel
5.9 Explain why the tubular particles in NC kaolin may cause a misinterpre tation of dye adsorption index MBI (see Table 5.5).
It is observed, after sintering, the mole ratio > I; the grain s
What are the in powders, and colloids?
Example 5.4
EXAMPLES
What are diíferences ín agglomerate characteristics for the spe cial caIcined aluminas in Table 5.2.
Solution. The diíferences in p:
Solution. The mostly gamma phase alumina has the largest mean agglomerate size of 2 ,um and the smallest crystallite size (ultimate particle size) of 0.01 ,um; the size ratio is 2/0.01 or 200. For the other aluminas calcined at a higher
Parameter
Granul~
Size (/km)
FA vs. Fw
>44 FA «
Flowabilíty
GoodC
Agglomeration Vol. Ads./Vol. Par.
InsigniJ
Example 5. 1
temperature, the agglomerate size is smaller and the crystallite size is larger; therefore, the agglomerate size and the number of particles in a mean agglom erate is lower. The greater agglomeration of the gamma alumina is reftected in its lower apparent bulk density. Example 5.2
What is the nominal purity of the barium titanates in Table
5.4? Solution. The elementary oxides forming the perovskite structure are BaO,
SrO, and Ti02 • Summing the weights of these gives 99.1 % purity for the capacitor, 99.0% for the MLC, and 98.5% for the piezoelectric type of ma terial. The nominal purity is 99 % . Example 5.3 Compare the stoichiometry for the three types of barium titan ates in Table 5.4.
Solution. Ideally, stoichiometric barium títanate contains 1 mole of alkaline earth oxides (BaO + SrO) and 1 mole of Ti02 • The moles of the oxide com ponents are found by dividing each by its molecular weight. For the capacitor grade material,
moles BaO = 63.54 g/153.34 g/mol
=
0.415
moles SrO = 0.90 g/103.62 g/mol = 0.009 moles Ti02 = 34.64 g179.88 g/mol = 0.434 (BaO
+ SrO)/Ti02 mole ratio
(0.415
+ 0.009)/0.434
=
0.976
in the table below:
Minim~
Note: FA is Van der Waals attractivl volume of adsorbed processing additi
EXAMPLES
and which are proper-
Calculating in a similar manner, the mole ratio for the other types is MLC Type Piezoelectric
Georgia kaolins in Table may cause a misinterpre-
5.5).
77
1.005 1.02
It is observed, after sintering, that a much larger grain size is obtained when the mole ratio > 1; the grain size strongly influences the electrical properties. Example 5.4
What are the important differences between granular materiais, powders, and colloids?
characteristics for the spe-
So/ution. The differences in particle characteristics and behavior may be seen in the table below:
largest mean agglomerate particIe size) of O. O1 calcined at a higher crystallite size is larger; in a mean agglom alumina is reftected in
Parameter
Granular Material
Powder
Colloid
Size (ltm) FA vs. Fw Flowability Agglomeration Vol. Ads.lVol. Par.
>44 FA « Fw Good (free ftowing) Minimal Insignificant
44 - 1 FA = Fw Poor Spontaneous Significant
<1 F A » Fw Very poor Spontaneous Very significant
barium titanates in Table
structure are Baú, 99.1 % purity for the piezoelectric type of ma-
types of barium titan-
I mole of alkaline moles of the oxide com weight. For the capacitor
.434
= 0.976
Note: F A is Van der Waals attractive force. Fw is particle weight. Vol. Ads.lVol. Par. is lhe volume of adsorbed processing additive relative to the volume of the partícle.
