Pore size evolution during sintering of ceramic oxides

Ceramics International 16 (1990) 177-189

Pore Size Evolution during Sintering of Ceramic Oxides J. A. V a r e l a Instituto de Quimica, UNESP, CP 355, 14800 Araraquara, S~o Pauio State, Brazil

O. J. W h i t t e m o r e University of Washington, FB-10, Seattle, Washington 98195, USA

& E. Longo Departamento Quimica, UFSCar, CP 676, 13560 S. Carlos, S~o Paulo State, Brazil (Received 25 July 1989; accepted 8 October 1989)

Abstract: This paper reviews the influence of particle size distribution,

agglomerates, rearrangement, sintering atmospheres and impurities on the pore evolution of some commonly studied oxides. These factors largely affect sintering mechanisms due to modifications of diffusion coefficients or evaporation condensation. Very broad particle size distribution leads to grain growth and agglomerates densify first. Rearrangement of particles due to neck asymmetry mainly in the early stage ofsintering is responsible for a high rate of densification in the first minutes of sintering by collapse of large pores. Sintering atmospheres play an important role in both densification and pore evolution. The chemical interaction of water molecules with several oxides like MgO, ZnO and SnO z largely affects surface diffusion. As a consequence, there is an increase in the rates of pore growth and densification for MgO and ZnO and in the rate of pore growth for SnO 2. Carbon dioxide does not affect the rate ofsintering of MgO but greatly affects both rates of pore growth and densification of ZnO. Oxygen concentration in the atmosphere can especially affect semiconductor oxides but significantly affects the rate of pore growth of SnO 2. Impurities like chlorine ions increase the rate of pore growth in MgO due to evaporation of HC1 and Mg(OH)CI, increasing the rate of densification and particle cuboidization. CuO promotes densification in SnO2 and is more effective in dry air. The rate ofdensification decrease and pore widening are promoted in argon. An inert atmosphere favors SnO 2 evaporation due to reduction of CuO.

1 INTRODUCTION

is usually the object of sintering, and thus is the most common measurement in sintering studies. Since sintering is a thermal process that always results in reduction of surface area, many structural changes compatible with surface area reduction may occur, i.e. grain growth, pore shrinkage or growth,

T h e p r o c e s s i n g o f single-phase ceramics usually starts with a p o r o u s c o m p a c t o f a n a g g r e g a t i o n o f particles. D u r i n g sintering, besides density, the shape, size a n d d i s t r i b u t i o n o f p o r e s change. D e n s i f i c a t i o n 177

Ceramics International 0272-8842/90/$03"50 O 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

J. A. Varela, O. J. Whittemore, E. Longo

178

particle modification and variation in density. Quantification of sintering would then be best achieved not only by density determinations but also by pore size distribution and surface area estimates. The science of sintering has identified mechanisms of sintering as viscous flow, lattice diffusion, grain boundary diffusion, surface diffusion and evaporation-condensation. From models and theory, the first three should result in particle center approach or densification, while the latter should not result in densification. Rearrangment should also be considered in the list even though that results in particle movement instead of atomistic mass transport. 1'2 Early theoretical work attempted to identify the mechanisms of material transport during sintering. In the initial stage of sintering of an ideal ceramic compact with uniform particle size the mean pore size tends to decrease only when densifying mechanisms are operative. However, when surface diffusion or evaporation-condensation are dominant there is pore shape modification, due to rounding of pores. 3 In addition to temperature and time, several variables can affect pore kinetics: particle size distribution and shape, sintering atmosphere, impurities and anisotropy of surface tensions. With real materials the above variables together with rearrangement and the overlap of mechanisms make identification difficult. This paper will review experimental work on sintering in which pore size distribution, surface area and densities have been measured to describe the process. Theoretical work on interaction of atmosphere molecules with oxide surfaces was attempted to clarify the reactions of these molecules with the surface. An attempt was made to identify probable mechanisms responsible for pore size evolution during sintering. In addition, it is discussed how atmosphere and impurities affect pore evolution during sintering. The techniques used have been mercury intrusion porosimetry, nitrogen adsorption for surface area and mercury picnometry for appa.rent densities. For the theoretical studies, the semi-empirical quantum mechanics method CNDO/2 was used. 4 2 F A C T O R S A F F E C T I N G PORE EVOLUTION 2. 1 Particle size distribution and aggregates

A wide distribution of particle sizes in a compact can result in pore growth. For example, products such as refractories and grinding wheels are composed of

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large dense particles with a bonding phase of small particles. During sintering, the bulk volume (and also porosity) remains essentially constant. The small bonding particles coalesce into large particles resulting in noticeable pore growth even at high temperatures. Watson et alfl showed pore growth in firing a wall tile body. They also correlated a finer pore structure in underfired brick with susceptibility to frost. Aggregates in a fine particle compact will densify first during sintering. Spots in dense alumina tubes used in sodium vapor lamps resulted from aggregates and their removal by milling was achieved. The sintering of compacts of unmilled alumina (from alum) at low temperatures resulted in the pore size distributions in Fig. 1.6 The large pores of 0.2 #m diameter are between aggregates, whereas the small pores of about 0.02#m are between particles in aggregates. Mercury porosimetry thus detects the presence of aggregates. Although the general tendency is to use single size colloidal particles for optimum sintered density and properties, Han et al. have shown that a mixture of three parts of 0.78~m alumina with one part of 0-21 ~tm alumina would sinter to 100% density at 1450°C, 100°C lower than with the 0"78 #m material alonefl Some pore growth was noted during sintering to 1200°C, followed by pore shrinkage at higher temperatures. Using two particle size distributions of alumina, one narrow and the other broad, Yeh and Sacks 8 showed that both powders densify to 99% of the theoretical density with similar grain size distribution. In this case both powders were unagglomerated and in the broad particle size distribution the smaller particles were fitting the voids between large particles. The green initial density in this case was very high (73% of theoretical density).

Pore size evolution during sintering of ceramic oxides

179

The specific pore frequency for both powders was similar, but the highest frequency for narrow particle size distribution corresponded to pores 3/5 in size of that corresponding of broad particle size distribution.

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Pores can be thermodynamically eliminated during sintering if the coordination number is smaller than a critical value R e . 9 The decrease of pore coordination number can be achieved by grain growth and/or rearrangement. Grain growth is a result of differential sintering of particles agglomerates. This leads to pore growth inter-agglomerates and pore shrinkage intra-agglomerates. As a consequence there is a decrease in pore coordination inside agglomerates. However, differential shrinkage of agglomerates leads to rearrangement, increasing pore size and coordination number. This nondensifying rearrangement is competing with densification. 1o Only in the final stage of sintering will the pore coordination number decrease due to differential sintering. Exner ~ has shown that rearrangement in two-dimensional arrays of spheres leads to pore growth. Neck asymmetry was suggested as the cause of this rearrangement. Weiser and De Jonghe ~2 showed that differential densification due to distribution of pore coordination number is the dominant cause of rearrangement in two-dimensional arrays of spherical copper particles. They suggested that in three-dimensional arrays the same mechanism should be dominant. Lange ~° showed that the sintering of agglomerated alumina particles leads to non-densifying rearrangement due to differential shrinkage. As a consequence, there is an increase in pore coordination and widening of pore size distribution. Similar results were observed during the sintering of MgO particles obtained by c o m b u s t i o n of metallic magnesium. 13 Densifying rearrangement in a compact during sintering due to neck asymmetry is an important mechanism, especially in the early stages. Exner 1~ found in compacts of spherical uniform copper particles that the relative linear shrinkage versus relative neck radius is higher than expected in the two-sphere model. Eloff and Lene114 showed similar results and Shumaker and Fulrath ~5 demonstrated rearrangement with a hot-stage scanning electron microscope on the surface of copper and nickel compacts. Varela and coworkers ~6'17 indicated rearrangement as an important factor in the sintering of MgO

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in dry argon where only pore shrinkage occurred. To relate densification with midpore size, the following equation was derived: A V / V o = 2[(d/do) a - 1]

(1)

where V0 is the initial sample volume, A V is the volumetric shrinkage, ). the pore fraction, d the midpore diameter and d o the initial midpore diameter. In Fig. 2 this equation is plotted as a straight line for MgO samples sintered in dry argon. The data points show more reduction of pore diameter initially related to shrinkage. With rearrangement this should occur because of the preferential decrease in large pores found by mercury porosimetry. This collapse of large pores was attributed to breaking of freshly formed necks due to development of stress gradients, a Considering total surface area variation during sintering as mainly due to neck growth and densifying mechanisms, the following equation was derived by Varela 17 for an ideal compact: A S / S o = [ 1 6 p o / P t ( A L / L o + 1) 3 - 2 ] ( A L / L o )

(2)

where A S / S o is the relative variation of surface area, Pt the theoretical density, Po the initial density and A L / L o the relative linear shrinkage. In Fig. 3 this equation is plotted as a solid line. The much higher initial shrinkage relative to loss of surface indicates rearrangement. Similar results were found on sintering of ZnO in dry air or oxygen, as shown in Fig. 4. ~8 In this figure the variation of surface area is related to apparent density by the following equation: A S / S o = [ 1 6 p / p t - 2 ] [ ( p / p t ) - 1/3(pO/Pt)l/3 -- 1] (3)

The experimental data points are above the theoretical curve, indicating a decrease of surface area larger than that due to densification. This fact can be explained by two mechanisms. For low

J. A. Varela, O. J. Whittemore, E. Longo

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temperatures and short sintering times, surface diffusion leads to rounding of pores that decrease the surface area with no densification. For higher temperatures, the most probable mechanism is particle coalescence with increasing grain growth. However, after surface loss due to surface diffusion, the data points tend to the theoretical curve, due mainly to rearrangement and densification. The above experiments indicating rearrangement reinforce the several arguments by Exner 11 that this mechanism is very important in the sintering of real systems.

2.3 Sintering atmosphere The atmosphere in which sintering is conducted can greatly affect pore kinetics. Much work has been I

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done on sintering of MgO in various pressures of water vapor. 7'19- 21 Compacts sintered in dry argon showed only pore shrinkage as plotted in Fig. 2. With only 2"3KPa of water vapor in argon, approximately the amount found in 'room air' which is often used by many investigators, pore growth was found in similar compacts up to l l00°C in isothermal studies. When sintered in 22.7 KPa water vapor in argon, considerable pore growth occurred in similar MgO compacts. The relation between volumetric shrinkage and pore diameter (as in Fig. 2) is plotted in Fig. 5 for the two pressures of water vapor. The straight line in the figures is from eqn (1). In contrast to pore shrinkage in inert atmosphere (Fig. 2), pore size in 2-3 KPa of water vapor can grow or shrink depending on the time and temperature. For low temperatures (900 to l l00°C) after pore shrinkage for short sintering times, pores grow during long sintering times, indicating the existence of two concurrent mechanisms. They are particle rearrangement for short sintering times, and surface diffusion that is accelerated with the interaction of the water molecule with the MgO surface for long sintering times. The increase of surface diffusion strengthens the necks between particles, avoiding new collapses of large pores and increases the rate of pore and grain growth. At 1200°C, after pore shrinkage during the first minutes, pore size remains constant with time. An equilibrium appears to be reached between the mechanisms responsible for

Pore size evolution during sintering of ceramic oxides

181

pore growth (surface diffusion) and that responsible for pore shrinkage (structural rearrangement and grain boundary diffusion). Figure 5 is a confirmation that rearrangement is temperature dependent and that surface diffusion increases with water vapor partial pressure. In these water vapor sintering experiments, shrinkage is more rapid than with dry atmospheres and this accelerated shrinkage has been noted by others. It is suggested that grain boundary diffusion with surface redistribution is the d o m i n a n t mechanism for densification. The interaction of water molecules with the MgO surface increases the surface diffusion. If surface redistribution is the controlling step, the densification is accelerated with increasing water vapor partial pressure. From an applied viewpoint, if rapid shrinkage of MgO is desired, water vapor pressure will assist, but if minimal grain growth is desired, dry atmospheres should be used. The shrinkage rate and grain growth are reported to be dependent on the water vapor pressure to the power ranging from 0-3 to 1.5, depending on the water vapor pressure. For low concentrations of water vapor (below 2.3 kPa) the rate of densification is proportional to water vapor pressure to the power ranging from 0-3 to 0.5. 2°'21 The rate ofdensification is proportional to water vapor partial pressure (vapor pressure ranging from 2"3 to 7.9kPa) to a power of 3/2 (vapor pressure above 7.9 kPa). Several mechanisms have been proposed to account the effect of water vapor on the sintering of MgO. Sata and Sasamoto 22 proposed two reactions which depend on the environment: MgO(s) + 1/2H20 MgO(s) + H 2 0

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sintering with low partial pressure of water as follows: Mg 2 ÷ + O 2 - + H 2 0

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where Vo is oxygen vacancy. Quantum mechanical studies utilizing several models were conducted on the interaction of water with MgO. 23'24 These indicated that for low water vapor pressure the highest probability is to form a protonated magnesium vacancy plus MgOH" or to form a double vacancy of Mg and O plus Mg(OH)2, according to the equations: (MgO)3Mg + H 2 0 --+ (MgO)3 + Mg(OH)" + Vr~gn

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This agrees with results of Hamano et al. 21 and of Eastman and Cutler z° for low water vapor pressure. At intermediate water vapor pressures where densification rate is proportional to water vapor pressures, the quantum mechanical model predicts the formation of a diprotonated vacancy and Mg(OH)2, according to equation: (MgO)3Mg + 2H20 ~

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This agrees with results of Eastman and Cutler 2° and Varela and Whittemore. 19 For high water vapor pressures the model predicts the formation of a diprotonated vacancy and monohydrated magnesium hydroxide, according to the following equation: (MgO)aMg+3H20

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(MgO) 3 + Mg(OH)2. H20-4- V ~ (11) This model can explain results obtained by Eastman and Cutler 2° and by Varela and Whittemore 19 for high water vapor pressures. Calcium oxide compacts, in contrast with the MgO results, were found by Liu to show little change in pore size when sintered in air with water vapor pressures from 5"8 to 84.6 KPa. 25 Densification was found to be greatly accelerated by the presence of water vapor. The sintering constant was calculated to be a function of water vapor pressure with an exponential dependence of 0.75. The effect of water vapor on pore size during sintering of ZnO compacts at several water vapor pressures can be seen in Fig. 6. This figure shows that pore growth occurs beyond the initial stage of sintering, indicating that water vapor accelerates

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mid-pore diameter a, (d/do )3-1 Fig. 6. Relative volumetric shrinkage versus relative midpore diameter cubed of ZnO compacts sintercd in dry air from ] to 240min at various temperatures. The solid line represents homogeneous shrinkage. pore growth. The exact mechanism of water vapor interaction with ZnO surface is not known but it is believed that after the chemical interaction of water molecules with the ZnO surface, the water would dissociate forming H + and O H - , similar to the mechanism of water interaction with MgO. The result is the formation of surface defects by releasing Zn(OH)" or Zn(OH)2 species. The increase of surface diffusion would accelerate the rate of grain and pore growth. The sintering of ZnO is also affected by other atmospheres, as shown in Fig. 7. Sintering in dry oxygen is similar to sintering in dry air. In both atmospheres the rate of sintering is higher than in other less oxidizing or neutral atmospheres. However, water vapor accelerates both pore growth and shrinkage. The rate of densification of ZnO compacts is retarded in CO2 atmosphere. The surface area decrease, however, indicates that surface diffusion is responsible for pore rounding in the early stage of sintering. 3 A possible explanation for the influence of CO2 on sintering of ZnO would be the I

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In this case the CO2 would adsorb on the ZnO surface followed by the reaction forming ZnCOa. This carbonate can be doubly or singly bonded. This chemical adsorption would not affect significantly the mass transport on the surface but would affect the mobility of ions from the interior to the surface, inhibiting densification. The addition of water vapor to a CO 2 atmosphere (Pu2o = 49 KPa) accelerates the rate of sintering and pore growth, as can be seen in Fig. 7. The water molecule would compete with CO 2 molecules for chemical adsorption on the ZnO surface. Tin oxide samples do not densify during sintering in dry argon or in dry oxygen. 26'27 Figure 8 shows the pore size evolution for compacts sintered in dry argon for several temperatures. Rather than densification, pore growth is observed as well as grain growth (Fig. 9). Since the total pore volume remains constant with sintering, the decrease of surface area due to pore growth would follow an equation of the type: S = kVp/d

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where k is a geometric factor, that should be 4 for cylindrical pores and 6 for spherical pores, and lip is the total pore volume per gram of material. Figure 10 is a plot of surface area versus the reciprocal of midpore diameter. The straight line crossing the origin shows that the structure of compacts remains uniform. The geometric factor obtained in this condition (k = 4"8) is intermediate between cylindrical and spherical shapes. Mechanisms for grain and pore growth can be considered as either surface

Pore size evolution during sintering of ceramic oxides

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(a)

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diffusion followed by grain boundary migration as proposed by Greskovich and Lay 2a or by evaporation-condensation. If the latter mechanism is dominant, one expects that atmosphere would have a large effect on sintering. Figure 11 shows the plot of relative midpores versus time at 1000 and 1200°C for both dry argon and dry oxygen. Comparatively, the oxygen atmosphere retards pore widening. The measurement of grain size shows that an oxygen atmosphere also retards grain growth. By considering the equilibrium reaction: SnO2(s) -- : SnO(s) + 1/202(g) SnO2(s ) ~ r SnO(g) + 1/202(g)

(14) (15)

reaction (14) is found more thermodynamically favorable. The partial pressure calculated at 1495 K is similar to the experimental value determined by Hoenig. 29 In a dry atmosphere of argon one expects a high rate of SnO2 evaporation and a higher rate of pore growth. Otherwise, for an atmosphere of dry oxygen, the evaporation of SnOa is buffered, leading to less grain and pore growth. Then an evaporation--condensation mechanism can explain pore i

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The effect of impurities on the sintering of oxides was theoretically discussed by Johnson. 3° Depending on the nature of the oxide and on the quantity and nature of the impurity, two types of structure can be formed: solid solution or second phase. Solid solution can improve the rate of sintering by increasing the chemical potential gradient and by increasing the concentration of defects in the crystals. The behavior of real systems is more complicated. If impurities are segregated at the grain boundaries, the width and the grain boundary diffusion are modified. 31 - 3 3 The segregation of impurities on the oxide surface can affect surface mobility. The surface composition determines the type of chemical interaction with the atmosphere as well as the electrical properties of the material. McCune and K u 34 determined by Auger spectroscopy and lowenergy ion scattering that the maximum segregation of Ca 2 + ions on MgO surface is 50%. In A120 3 the limit of Ca 2÷ on the surface is about 9%. Most of the powders used in sintering studies in the literature have a relatively significant level of impurities, although they are considered pure. Results in sintering of MgO is a typical example. Handwerker 35 studied the sintering of MgO powder of high purity (prepared i n a clean dry box). Sintering of this high purity MgO powder from 1350 to 1600°C produced samples of 65-85% theoretical density, which is very low compared to samples of lower purity sintered at the same temperature. 17 Grain growth in high-purity samples was insignificant. It was concluded that the sintering inhibition was caused by decreased diffusivities of the highpurity material. Chlorine ions have a great effect on rate of sintering and pore kinetics. Figure 12 shows the pore size distribution of compacts sintered in temperatures from 900 to 1300°C in dry argon. From the green sample to the one sintered at l l00°C a significant amount of pore growth is observed. 14 Comparing these results with pure MgO compacts sintered in the same conditions shows that chlorine ions retard densification and cause pore growth. Hamano et al. 36 also studied the effect of chlorine addition on the sintering of MgO. They also found

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the decrease of the densification rate but obtained 99% of the theoretical density due to uniformity of particles. A scanning electron microscopy study of MgO with chlorine additions 37 showed that for green compacts grains are spherical. The widening of pores seems to be related to grain shape. At 850°C the pore size starts to grow after 120min of sintering. At 900°C the grains start attaining cubic shapes instead of spherical. At 1000°C the relative number of cubic grains increases and at 1300°C (Fig. 13) the grains are perfectly cubic with smooth (100) faces. 37 At 900, 950 and 1000°C a time of 120, 60 and 10 min respectively is necessary for pores to start growing. At low temperatures an incubation time period is required before the grains start becoming cubes and as a consequence, pore growth. These

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Micrographs of fractured surface o f M g O - 1 % MgCl 2 compact sintered in argon for 1 h at 1200°C.

185

results indicate that pore widening is dependent on temperature and time. At 950°C the pore growth process is still going even after 120 min of sintering while at 1200°C it is practically over after 15 min and then pores start to shrink. This shows that grain growth and cuboidization are competing with densification. For low temperatures grain growth and cuboidization are dominant, resulting in pore growth, while for higher temperatures the rate of grain growth decreases, densification is dominant and pores start to shrink (Fig. 12). This is in agreement with large surface area reduction for temperatures between 900 and 1000°C where cuboidization and grain growth take place. Since densification in this condition is very small, grain growth and cuboidization lead to pore growth. The most likely mechanisms causing these modifications in MgO samples are surface diffusion or evaporationcondensation. Vacuum sintering did not result in the same cuboidization so evaporation-condensation was thought to be the principal mechanism rather than surface diffusion. The surface quality of the cuboidal material is particularly interesting. Stepped isotherms are obtained by Kr adsorption, indicating a homogeneous surface which is more stable than that of MgO smoke, the only other oxide for which stepped isotherms have been found. Calcium oxide does not form an extensive solid solution with MgO. The main reason is that the size of Ca 2+ is 1.4 times larger than Mg 2+. In calciadoped magnesia there is a tendency of segregation of Ca 2+ ions at grain boundaries and surfaces. To study the effect of C a 2 + on the physical characteristics of the powder and sintering of MgO, small amounts of calcium hydroxide were mixed with magnesium hydroxide and calcined at 900°C. Table 1 gives the characteristics of the calcined powder with different concentrations of Ca 2 +. From Table 1 it can be inferred that calcium ions inhibit particle growth during calcination of MgO. However, the compaction of small particles is more difficult, resulting in lower green density. Figure 14 shows pore size distributions for MgO compacts with 65 p p m of calcium ions sintered at several temperatures in dry argon. The decrease of midpore size is verified. The same behavior is verified for other concentrations of calcium ions in MgO sintered in the same conditions. However, large pores decrease faster than small pores, indicating rearrangement, as can be inferred from Fig. 15. Resulting densities show that densification is independent of calcium concentrations in MgO.

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rLtO

relative decrease in surface area, AS/So

Fig. 15. Relative linear shrinkage versus relative decrease in surface area for MgO sintered in argon for 1 h from ll00 to 1300°C. O, 65ppm; A, 740ppm; ©, 1730ppmo

0.02 0

L. I ~ / !

0

5

J

J ~..~--~--11,00°C--j

10 15 20 25 3O absolute pressure ( M P o )

Fig. 16. Poresize distributions for SnO2 with 2 mol% of CuO sintered in argon (dashed lines)and in dry air (solidlines)for I h. atmosphere. In dry air pores grow at low temperatures and shrink at higher temperatures. The effect of CuO on the densification of S n O 2 c a n be explained by either liquid-phase formation or solid solution of CuO in the S n O 2 lattice. The eutectic points of the systems C u O - S n O 2 and C u 2 0 - S n O 2 a r e about 1060°C and liquid phase is formed when samples of S n O 2 containing CuO are sintered above this temperature. However, in Fig. 16 pore growth for temperatures below 1000°C and substantial shrinkage before liquid formation is seen, indicating solidstate diffusion as the main mechanism for shrinkage. After liquid formation above 1060°C the rate of densification of the system S n O 2 - C u O increases, as observed by Dolet et a/. 39 They observed that the densification rate of S n O 2 with additions of 1-2-8 wt% of CuO is independent of the amount of liquid formed at 1100°C. However, they observed a large a m o u n t of microstrains originating from dislocations. Such microstrains are released at 1200°C when most of dislocations vanish upon twinning of the microcrystallites. The solubility of S n O 2 in liquid rich in CuO is small a8 and it is believed that the main role of the liquid phase is to rearrange S n O 2 particles and to homogenize the composition in order to increase bulk diffusion in S n O 2. Therefore most of shrinkage above the liquid formation is more likely to occur by

Physical characteristics of M g O green samples w i t h t h r e e c o n c e n t r a t i o n s of Ca 2÷ ions after calcination at 900°C for 4 h

Concentration

0047

I

0.10

/t

Fig. 14. Pore size distributions of MgO with 65 ppm of CaO sintered in dry argon for 1 h.

Table 1.

I

I I

pressure ( M Po)

~_..I

pore diameter (jJ m) 0.14 (~07

Midpore diameter (nm)

Surface area (m=/g)

Green density

of Ca=+ (ppm)

(g/cm 3)

Particle size (nm)

65 740 1 730

51 "7 48"2 45"2

24"0 26"2 31 "0

1 "70 1 '64 1 "61

69'8 64'0 54"1

Pore size evolution during sintering of ceramic oxides

solid-state diffusion due to increase of oxygen vacancies in SnO2, given by: Sn02

CuO ,-- - - - * 2Cu~r + Vo + Oo

(16)

Sintering SnO2 with 2 m o l % of CuO in dry argon is quite different than in dry air. There is less densification and the midpore always grows with temperature. These differences as compared with sintering in dry air can be explained by the reduction of CuO to C u 2 0 and by the increase in the rate of evaporation of SnO2. In the former the oxygen vacancies would increase by: SnO2

C u 2 0 ÷ - --+ 2Cu~', + 3Vo + O o

(17)

These two mechanisms are competitive. While the creation of lattice defects increases the sintering rate, the evaporation-condensation mechanism increases the rates of grain and pore growth, limiting the densification. 3 SUMMARY

AND CONCLUSIONS

Pore size evolution during sintering of ceramic oxides is intimately related to the sintering atmosphere and impurities. During sintering of MgO in a neutral atmosphere like argon pores shrink in the first minutes of sintering. This is due to rearrangement and other densifying mechanisms. In contrast, during sintering of ZnO in dry argon at low temperatures, there is a slight widening of pores (with no grain growth) due to surface diffusion. Otherwise, sintering of SnO 2 in dry argon leads to fast pore widening and grain growth due to coalescence of small particles into large ones. Evaporati0n-condensation is the main mechanism and is governed by reaction (14). The effect of water vapor added to most atmospheres is to increase surface diffusion due to chemical interaction of water with oxide surfaces. In the case of sintering of MgO in argon with water vapor, after rearrangement of particles in the first minutes there is an increase in surface diffusion that leads to pore widening. This is due to coalescence of small particles into large ones leading to grain growth. Water vapor also accelerates the rate of pore widening and grain growth when SnO2 is sintered in argon. In this way the increase in surface diffusion due to interaction of water with S n O 2 surface is parallel to evaporation-condensation increasing the rate of pore widening. Water vapor accelerates the rate of pore growth when ZnO is sintered in carbon dioxide. In this case there is a competition between

187 water molecules and carbon dioxide interacting with the ZnO surface. While carbon dioxide molecules decrease the ion mobility to the surface, the water molecules increase surface diffusion, increasing the rate of pore widening. The effect of an oxygen atmosphere depends on the nature of the oxides and on the dominant mechanism. An oxygen atmosphere inhibits the evaporative decomposition of S n O 2 given by eqn (14). Then the rate of pore growth is smaller. Otherwise oxygen atmospheres increase the rates of ZnO densification and pore shrinkage. Carbon dioxide does not affect the rate of densification or pore change during sintering of MgO compacts. However, CO2 inhibits densification of ZnO due to interaction of CO 2 with the ZnO surface and formation of Z n C O 3. Although CO2 affects densification, surface diffusion is not significantly affected on ZnO. Thus CO 2 has little effect on pore evolution. Impurities have great effect on both rates of densification and pore evolution. Introduction of calcium ions in the MgO lattice is inhibited by the solubility limit of C a 2 + in MgO. Then these ions segregate at grain boundaries and surface. Although the presence of calcium ions in MgO affects the surface area obtained during calcination of Mg(OH)2 at 700°C, these ions have little effect on the rate of densification during sintering of MgO. Since the powder with higher concentration of C a 2 ÷ has higher surface area, the calcium ions inhibit surface transport, decreasing the rates of densification and grain growth. Chlorine ions on the MgO surface decrease the rate of densification and cause pore widening. During the sintering of MgO with MgC12 dopants there is formation of HC1. This reacts with the MgO surface forming Mg(OH)C1 that evaporates as small particles and which condense on large ones. Thus there is grain growth (and pore widening) with a cuboidization of MgO particles. The dominant mechanism during sintering of pure S n O 2 s e e m s to be evaporation-condensation. The introduction of 2 mol% of CuO helps to densify S n O 2 in an atmosphere of dry air due to formation of oxygen vacancies and particle rearrangement during the initial stage of sintering. By changing the sintering atmosphere to argon there is a decrease in the rate of densification and increase in the rate of pore growth due to two concurrent mechanisms: evaporation-condensation and increasing oxygen vacancies. In argon the rate of evaporation of S n O 2 is increased and there is a reduction of CuO into

188

Cu20, increasing oxygen vacancy concentration. Thus although the number of oxygen vacancies increases, grain growth (and pore widening) is concurrent with densification. ACKNOWLEDGEMENT

The authors acknowledge FAPESP, CNPq and FINEP for financial support of this project.

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