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Synthesis and sintering of nanocrystalline erbium oxide
NanoStructured Materials, Vol. 6, pp. 333-336, 1995 Copyright © 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0965-9773/95 $9.50 + .00
Pergamon 0965-9773(95)00065-8
SYNTHESIS AND SINTERING OF NANOCRYSTALLINE ERBIUM OXIDE
M.Lequitte, D.Autissier. CEA-DAM, B.P. 12, 91680 Bruy~res-le-Chfitel, France, tel: 33.1.69.26.59.86, fax: 33.1 ~69.26.61.28.
Abxtract: A chemical method for erbium oxide synthesis from nitrate precursor is desctqbed here. The powder is observed using Ttr~nsmission Electronic Microscopy and X-Ray diffraction. The grains are monodisperse and about 20 nm sized. The analysis still shows that the powder is agglomerated and g~ain coalescence occurs at the cristallization temperature. This powder is then coM isostatic pressed (CIP). A first pressing at 50 MPa is followed by a second one at 200 MPa. The green piece is then shltered under air at different temperatures between 1400 and 1600°(3. Ateasured densities are compared to those of commercial powders. The densities are higher and gra#1 sizes lower than those of commercial powders sintered at equivalent temperature. An important grain growth is also observed between 1450 and 1475°C. A kinetic study, based on dilatometry experiments, will be presented in this paper. INTRODUCTION Sintering mechanisms of ceramic powders depend on grain size. In this way shrinkage rates are very fast when diameter is small. Some authors try to determine the sintering mechanisms in the ease of metallic fine powders ( l ) . Some differences appear for low temperature mechanisms and for activation energy values in reference to the micromctric powders (2). Knowledge of these mechanisms is essential for understanding of bulky ceramics properties, for example thermic, mechanical or dielectric properties. An erbium oxyde (Er203) nanometric powder is chemicaly synthesized and its sintering behaviour is studied by dilatometry.
EXPERIMENTAL Erbium nitrate (Er(NO3)3) provides from Johnson Matthey. The phases are analyzed by X Ray Diffraction using the K a l Cobalt wavelength. Crystallisation and mass variations are observed by T.GA.. Agglomerates dimensions are measured on a Laser Diffusion granulometer. Grain sizes are estimated by XRD (Scherrer Calculation) and by Transmission Electronic Microscopy (TEM). Specific areas are measured by B.E.T. method. The powders are latex encapsulated and isostatically pressed. The latex container is replaced after each cycle. Powder s)~thesis The erbium oxyde synthesis was studied from different precursors (3). 0.4 moles erbium nitrate in 200 ml ethanol precipitate in a mixture of 100 ml concentrated ammonia and 200 mi ethanol. In the same way as sol-gel mechanism the pH of the mixture leads to a better electrophoretic stability and to a less agglomerated powder, via a stable sol. The precipitate is separated by centrifugation, washed twice in alcohol and dried at 50°C. The agglomerates size depends on furnace temperature. This
33,3
334
M LEQUrl-rEAND D AUTISSIER
provides from capillar strengths which cause an iinportmlt shrinkage and an important densitication during drying. The apparent density increases from 0.48 at 50°C (agglomerate size of 0.3 gm) to 1.1 at 80°C. This intermediate is then fired at 650°C. Crystallisation is monitored by T.G.A.-D.T.A. method. The cubic form is confirmed by X . R D ( 4 , 5 ) . Transmission Electronic Microscopy (TEM) shows 20 nm sized, strongly agglomerate grains. Somewhere, sintering necks seem to be formed (figure 1).
Sinte~ng The powder is isostatically pressed successively at 50 MPa and 200 MPa. The green densities are respectively 2.86 and 3.86 g/cm3. The samples are then sintered under air during 3 hours between 1400 to 1550°C. Figure 2 shows that density rises very quickly between 1450 and 1475°C to reach 8.39 at 1550°C. (97 % of the theorical density). The SEM and TEM micrographies show that an important grain growth occurs during the density rise (figure 3a and 3b). Till 1450°C grain diameter is about 400 mn whereas it is about 20 ~ n at 1475°C. For comparison, a conunercial powder densities at 76.6 % of the theoretical density at 1600°C. The grain growth has not still occured : in the case of micrometric powders, equivalent sintering mechmfisms occur at higher temperature (6).
Dilatonlet~ Samples (30*6*5) of 50 MPa uniaxially pressed nano-Er203 are introduced in a warm dilatometer fumace with an alumina reference. The linear shrinkages variations A are instantly measured. The results are represented on figure 4. DISCUSSION
Shrinkage occurs till 500°C mad begin4to be significant above 800°C. A step is reached rapidly and shrinkage rates tend to dinmlisk This step seems due to particulates rearrangement. They tend to lower the stress level generated during neck formation (1). It is necessary to study the low temperature sintering mechanisms because the porosity decreases quickly and the sphere stack models are not available anymore. In agreement with the sintering relation A" rm = kt ( 7 ) where r is the grain radius and n et m constants, we have observed the logarithmic variations of A versus time. Each curve is composed of two segments. The slopes of these segments are represented in table 1. We have shown ( 8 ) that water desorption occurs during sintering. The first part of the curve can be explained through water desorption and a rapid rearrangement which leads to an important shrinkage. After this transition time, sintering mechanisms can be estimated when they are slow enough to controll the reaction. From 500 to 600°C the slopes correspond to a diffusion mechanism. Incertitude of the measures don't permit to distinguish between volume or grain boundaries diffusion mechanisms. It is also difficult to take conclusions because the initial powder was ever treated at 650°C. Presence of sintering necks in the hfitial powder can be explained through these diffusion mechanisms which occur in the same time as crystallisation. Between 700-800°C, the slope is about 1 corresponding to a visquous flow. TABLE 1 Variation of the slopes of the curves in (A) = f(in (t)) for different temperatures (°C) Temp. (°C)
500
550
600
700
800
1000
1100
1200
1300
1400 comnl
l/n ~.a *a 1.69 ,a *a 2.81 3.63 2.68 '2.84 3.75 0 to - 5 mm 1/n 01'51 0.52 0.35 1.24 1.31 0.31 0.22 0.14 0.17 0.31 - 5 to 30 min Calculations are not possible because of the alumina dilatation disturbing. In each case the slope is > 1
SYNTHESIS ANDSINTERINGOF NANOCRYSTALUNEERBIUMOXIDE
335
Above IO00°C the second slope decreases with temperature. We can suppose that different mechanisms occur simultaneously and rapidly in the ftrst time. The second slope correspond to particulates rearrangement. The 1400°C curve (figure 5) shows 3 stages with slopes : 1.06; 0.36; 0.08. These stages look like a liquid phase sintering mechanism (7). The liquid phase appears in the first minute and the corresponding slope is 1. After this stage ef rearrangement, a dissolution-precipitation one appears and the slope ofln (A) = f(ln (t)) is equal to 0.3-0.5. The third stage corresponds to grain growth. This is in agreement with TEM and SEM observations described above. We can't explain in that case a liquid phase mechanism with one component and little impurities amount. For further comparisons we have observed the shrinkage of a commercial powder with 3-6 Ima grain size. The values of the two curve slopes (Table 1) and the time variation of A (figure 4) let us assume that the sintering mechanisms occur 500°C before the equivalent micrograins sintering mechanisans. The Herring sinfilitude law is applied after equilibrium time: (tl/t2)=(rl/r2) n where t is the necessary time to reach some sintering advancement for a grain size radius r. We found n = - 1, which corresponds to a plastic flow mechanism. CONCLUSION Tile dilatometric study made on erbium oxide has shown that file smtering rates are higher for fme grain powder than for a micrometric one through Herrmg's law. Above 800°C the rate is too fast to determine the mechanism. At 1400"C, three stages and a rapid grain growth for higher temperatures let us think to a liquid phase sintering mechanism which doesn't match with a one component system. ACKNOWLEDGMENTS We are grateful to .l.Alzin, L.Autissier, G.Amoux, J.P.Bdgu~, and E.Brtmeton for many helpful discussions.
")'~ t~l] 1
8,30 . 8.20 ~, 8.10
~'~-'-"~
80<) LgOt 1400
1450
1500
Slnterlng tempetalure
figure 1:TEM observation of the Er203 powder
figure 2: Variation density d = f(T*C)
1550
336
M LEOUrr'TE AND D AUTISSlER
100 n m
figure 3: TEM and SEM observationof the sintered n-Er203 at 1450 and 14750C
~oo
Lit)
10o0
2 2.,5 3 3.5 4 4.5 5 5,5 6 6.5 7 7.5
O.
i
"
•
i
i
-Q05
°1 x o
807c
•
12Z3~
•
13C0~
415 4
O07
-1 t
-Q15 -02 -025
D 0
o
%°°t~aa~a~m
,
,~'C=nJ
-Q3
-as I
figure 4: Variation of the linear shrinkage versus time for the n-Er203
figure5: Variation ofln (A) = In (T) at 1400°C for the n-Er203
REFERENCES 1
2 3 4 5 6 7 8
Bigot,L; Dominguez,O.; PM'94, Nanoscale Materials, p 1785, (1994). Siegel, R.W.; Nanostructured Mater., 4 (1), 121, (1994). Varnier, O.; These, Montpellier, France, (1992). Li, Z.; Hahn, H.; Siegel, R.W.; Mat.lett.; 6 (10), 342, (1988). Gasnier, M.; Mat.Lett.; 9 (4), 161, (1990). Averback, R.S.; HOfler, H.J.; Logas, J.C.; Nanostructured Mat.; 1, 173, (1992). Bernache-Assolant D.; Chimie Physique dufrittage, Ed. Hermes, Paris, p 192, (1993). Lequitte, M.; Autissier, D.; In press, (1994).
Pergamon 0965-9773(95)00065-8
SYNTHESIS AND SINTERING OF NANOCRYSTALLINE ERBIUM OXIDE
M.Lequitte, D.Autissier. CEA-DAM, B.P. 12, 91680 Bruy~res-le-Chfitel, France, tel: 33.1.69.26.59.86, fax: 33.1 ~69.26.61.28.
Abxtract: A chemical method for erbium oxide synthesis from nitrate precursor is desctqbed here. The powder is observed using Ttr~nsmission Electronic Microscopy and X-Ray diffraction. The grains are monodisperse and about 20 nm sized. The analysis still shows that the powder is agglomerated and g~ain coalescence occurs at the cristallization temperature. This powder is then coM isostatic pressed (CIP). A first pressing at 50 MPa is followed by a second one at 200 MPa. The green piece is then shltered under air at different temperatures between 1400 and 1600°(3. Ateasured densities are compared to those of commercial powders. The densities are higher and gra#1 sizes lower than those of commercial powders sintered at equivalent temperature. An important grain growth is also observed between 1450 and 1475°C. A kinetic study, based on dilatometry experiments, will be presented in this paper. INTRODUCTION Sintering mechanisms of ceramic powders depend on grain size. In this way shrinkage rates are very fast when diameter is small. Some authors try to determine the sintering mechanisms in the ease of metallic fine powders ( l ) . Some differences appear for low temperature mechanisms and for activation energy values in reference to the micromctric powders (2). Knowledge of these mechanisms is essential for understanding of bulky ceramics properties, for example thermic, mechanical or dielectric properties. An erbium oxyde (Er203) nanometric powder is chemicaly synthesized and its sintering behaviour is studied by dilatometry.
EXPERIMENTAL Erbium nitrate (Er(NO3)3) provides from Johnson Matthey. The phases are analyzed by X Ray Diffraction using the K a l Cobalt wavelength. Crystallisation and mass variations are observed by T.GA.. Agglomerates dimensions are measured on a Laser Diffusion granulometer. Grain sizes are estimated by XRD (Scherrer Calculation) and by Transmission Electronic Microscopy (TEM). Specific areas are measured by B.E.T. method. The powders are latex encapsulated and isostatically pressed. The latex container is replaced after each cycle. Powder s)~thesis The erbium oxyde synthesis was studied from different precursors (3). 0.4 moles erbium nitrate in 200 ml ethanol precipitate in a mixture of 100 ml concentrated ammonia and 200 mi ethanol. In the same way as sol-gel mechanism the pH of the mixture leads to a better electrophoretic stability and to a less agglomerated powder, via a stable sol. The precipitate is separated by centrifugation, washed twice in alcohol and dried at 50°C. The agglomerates size depends on furnace temperature. This
33,3
334
M LEQUrl-rEAND D AUTISSIER
provides from capillar strengths which cause an iinportmlt shrinkage and an important densitication during drying. The apparent density increases from 0.48 at 50°C (agglomerate size of 0.3 gm) to 1.1 at 80°C. This intermediate is then fired at 650°C. Crystallisation is monitored by T.G.A.-D.T.A. method. The cubic form is confirmed by X . R D ( 4 , 5 ) . Transmission Electronic Microscopy (TEM) shows 20 nm sized, strongly agglomerate grains. Somewhere, sintering necks seem to be formed (figure 1).
Sinte~ng The powder is isostatically pressed successively at 50 MPa and 200 MPa. The green densities are respectively 2.86 and 3.86 g/cm3. The samples are then sintered under air during 3 hours between 1400 to 1550°C. Figure 2 shows that density rises very quickly between 1450 and 1475°C to reach 8.39 at 1550°C. (97 % of the theorical density). The SEM and TEM micrographies show that an important grain growth occurs during the density rise (figure 3a and 3b). Till 1450°C grain diameter is about 400 mn whereas it is about 20 ~ n at 1475°C. For comparison, a conunercial powder densities at 76.6 % of the theoretical density at 1600°C. The grain growth has not still occured : in the case of micrometric powders, equivalent sintering mechmfisms occur at higher temperature (6).
Dilatonlet~ Samples (30*6*5) of 50 MPa uniaxially pressed nano-Er203 are introduced in a warm dilatometer fumace with an alumina reference. The linear shrinkages variations A are instantly measured. The results are represented on figure 4. DISCUSSION
Shrinkage occurs till 500°C mad begin4to be significant above 800°C. A step is reached rapidly and shrinkage rates tend to dinmlisk This step seems due to particulates rearrangement. They tend to lower the stress level generated during neck formation (1). It is necessary to study the low temperature sintering mechanisms because the porosity decreases quickly and the sphere stack models are not available anymore. In agreement with the sintering relation A" rm = kt ( 7 ) where r is the grain radius and n et m constants, we have observed the logarithmic variations of A versus time. Each curve is composed of two segments. The slopes of these segments are represented in table 1. We have shown ( 8 ) that water desorption occurs during sintering. The first part of the curve can be explained through water desorption and a rapid rearrangement which leads to an important shrinkage. After this transition time, sintering mechanisms can be estimated when they are slow enough to controll the reaction. From 500 to 600°C the slopes correspond to a diffusion mechanism. Incertitude of the measures don't permit to distinguish between volume or grain boundaries diffusion mechanisms. It is also difficult to take conclusions because the initial powder was ever treated at 650°C. Presence of sintering necks in the hfitial powder can be explained through these diffusion mechanisms which occur in the same time as crystallisation. Between 700-800°C, the slope is about 1 corresponding to a visquous flow. TABLE 1 Variation of the slopes of the curves in (A) = f(in (t)) for different temperatures (°C) Temp. (°C)
500
550
600
700
800
1000
1100
1200
1300
1400 comnl
l/n ~.a *a 1.69 ,a *a 2.81 3.63 2.68 '2.84 3.75 0 to - 5 mm 1/n 01'51 0.52 0.35 1.24 1.31 0.31 0.22 0.14 0.17 0.31 - 5 to 30 min Calculations are not possible because of the alumina dilatation disturbing. In each case the slope is > 1
SYNTHESIS ANDSINTERINGOF NANOCRYSTALUNEERBIUMOXIDE
335
Above IO00°C the second slope decreases with temperature. We can suppose that different mechanisms occur simultaneously and rapidly in the ftrst time. The second slope correspond to particulates rearrangement. The 1400°C curve (figure 5) shows 3 stages with slopes : 1.06; 0.36; 0.08. These stages look like a liquid phase sintering mechanism (7). The liquid phase appears in the first minute and the corresponding slope is 1. After this stage ef rearrangement, a dissolution-precipitation one appears and the slope ofln (A) = f(ln (t)) is equal to 0.3-0.5. The third stage corresponds to grain growth. This is in agreement with TEM and SEM observations described above. We can't explain in that case a liquid phase mechanism with one component and little impurities amount. For further comparisons we have observed the shrinkage of a commercial powder with 3-6 Ima grain size. The values of the two curve slopes (Table 1) and the time variation of A (figure 4) let us assume that the sintering mechanisms occur 500°C before the equivalent micrograins sintering mechanisans. The Herring sinfilitude law is applied after equilibrium time: (tl/t2)=(rl/r2) n where t is the necessary time to reach some sintering advancement for a grain size radius r. We found n = - 1, which corresponds to a plastic flow mechanism. CONCLUSION Tile dilatometric study made on erbium oxide has shown that file smtering rates are higher for fme grain powder than for a micrometric one through Herrmg's law. Above 800°C the rate is too fast to determine the mechanism. At 1400"C, three stages and a rapid grain growth for higher temperatures let us think to a liquid phase sintering mechanism which doesn't match with a one component system. ACKNOWLEDGMENTS We are grateful to .l.Alzin, L.Autissier, G.Amoux, J.P.Bdgu~, and E.Brtmeton for many helpful discussions.
")'~ t~l] 1
8,30 . 8.20 ~, 8.10
~'~-'-"~
80<) LgOt 1400
1450
1500
Slnterlng tempetalure
figure 1:TEM observation of the Er203 powder
figure 2: Variation density d = f(T*C)
1550
336
M LEOUrr'TE AND D AUTISSlER
100 n m
figure 3: TEM and SEM observationof the sintered n-Er203 at 1450 and 14750C
~oo
Lit)
10o0
2 2.,5 3 3.5 4 4.5 5 5,5 6 6.5 7 7.5
O.
i
"
•
i
i
-Q05
°1 x o
807c
•
12Z3~
•
13C0~
415 4
O07
-1 t
-Q15 -02 -025
D 0
o
%°°t~aa~a~m
,
,~'C=nJ
-Q3
-as I
figure 4: Variation of the linear shrinkage versus time for the n-Er203
figure5: Variation ofln (A) = In (T) at 1400°C for the n-Er203
REFERENCES 1
2 3 4 5 6 7 8
Bigot,L; Dominguez,O.; PM'94, Nanoscale Materials, p 1785, (1994). Siegel, R.W.; Nanostructured Mater., 4 (1), 121, (1994). Varnier, O.; These, Montpellier, France, (1992). Li, Z.; Hahn, H.; Siegel, R.W.; Mat.lett.; 6 (10), 342, (1988). Gasnier, M.; Mat.Lett.; 9 (4), 161, (1990). Averback, R.S.; HOfler, H.J.; Logas, J.C.; Nanostructured Mat.; 1, 173, (1992). Bernache-Assolant D.; Chimie Physique dufrittage, Ed. Hermes, Paris, p 192, (1993). Lequitte, M.; Autissier, D.; In press, (1994).