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Dispersion of precursors for improving the homogeneity and sinterability of CaLa2S4 powders
64
Materials Chemistry and Physics, 35 (1993) 64-70
Dispersion of precursors for improving the homogeneity sinterability of CaLa,S, powders
and
Li Hsing Wang, Ming Shyng Tsai and Min Hsiung Hon Department of Materials Engineering, National Cheng Kung University, Tainan (Taiwan, ROC) (Received
June 22, 1992; accepted
December
28, 1992)
Abstract Cryochemical processing of carbonates dispersed in water and organic solutions is used to prepare CaLa,S,. Coprecipitated carbonates dispersed in cyclohexanol and t-butyl alcohol after spray-freeze/freeze-drying and sulfidization have an average particle size of 0.6 pm after grinding, which improves the sinterability significantly. The sintered pellet shows translucency after hot isostatic pressing and annealing in a Has atmosphere at 1400 “C.
Introduction It is well known that the sintering behavior of ceramic materials is greatly influenced by the characteristics of the starting powders, i.e., particle size distribution, degree of agglomeration, and chemical homogeneity [l, 21. However, the CaLa& powder obtained by a mixed oxide method [3] or coprecipitation method [4] has a broad particle size distribution and aggregation owing to high-temperature sulfidization over a long time. The CaLa& ceramic has lately become a promising optical window material for the 8-14 km IR range [5-71, but the agglomerated state and broad particle size distribution have resulted in poor packing and incomplete densification. One way to prepare a homogeneous powder is by using a precipitation method to obtain a mixed compound, which is then freeze-dried in order to decrease shrinkage and the formation of a hard agglomerate during sintering [8, 91. Another way is to use a sprayfreeze method to retard segregation of the powders
PI* To obtain a less agglomerated particle when drying, most coprecipitation techniques use ethanol instead of water [ll-141. However, the melting point (- 115 “C) of ethanol is too low to be suitable for freeze-drying suspensions of coprecipitates. On the other hand, tbutyl alcohol [15] and cyclohexanol [16] have melting points of 25.3 and 10 “C, respectively, with suitable vapor pressure and have easily controllable drying rates. The combination of spray-freezing and freeze-drying for the fabrication of oxide powders has a profound influence on their homogeneity and sinterability, be-
02540584/93/%6.00
cause an extremely small droplet of ice is formed at the instant of fast freezing [17-191. In this study the spray-freeze/freeze-drying of the coprecipitates has been investigated and its influence on the sulfide powder characteristics and sintered pellet properties has been evaluated in comparison with the effect of oven drying techniques. Experimental Commercial reagents La(NO,),+6H,O (99.0% pure) and Ca(NO,), (99.5% pure) were chosen for the carbonate formation and mixed in a ratio of La/Ca=2.20, while Pb(NO& (5 wt.%) was chosen as dopant. The carbonates were coprecipitated by adding the stock solution to the excess ammonium carbonate and ammonium hydroxide solution of ethanol for rapid coprecipitation. The stock solution was prepared by adding 0.5 mol of La(NO,), .6H,O and Ca(NO,), to a liter of distilled water. The ammonium carbonate and ammonium hydroxide solution was stirred in a mixer with the rotor at 1000 r-pm, and the stock solution was quickly dispersed in this torrent of solution. The pH values of the solution before and after precipitation were about 10.5 and 8.5, respectively. The precipitate was ultrasonically dispersed in the ethanol solution for 20 min. After filtration, the filtered cake was dispersed in tbutyl alcohol and cyclohexanol for the spray-freeze operation. The organic dispersed solution was sprayed through a nozzle into a large dewar of liquid nitrogen. This icy matrix was freeze-dried in VitrisB filter sealed bottles on a Vitris freeze-dryer for 3-6 days. The filtered cakes obtained were designated according to the syn-
0 1993 - Elsevier
Sequoia. All rights reserved
65
TABLE
1. Sample designation
and description
Sample designation
Description
CWDO CEDO CDFC CDFT
Carbonate Carbonate Carbonate Carbonate
of precursor
filtered
cake drying procedures
and procedure
precursor precursor precursor precursor
washed with water and dried in oven at 80 “C for 8 h washed with ethanol and dried in oven at 80 “C for 8 h dried by freeze-drying in cyclohexanol ice dried by freeze-drying in t-butyl alcohol ice
thesis reagents used and the organic or water removal method (Table 1). The processing flow chart is shown in Fig. 1. The dried coprecipitated carbonate precursors were calcined in pure oxygen and decomposed at 700 “C for 3 h, then sulfidized at 750 “C for 48 h in a H,S atmosphere. The sulfide powder was ground in a CS2 medium for 48 h. The resultant light green powder was cold isostatically pressed (CIP) at 200 MPa in a rubber mold to form a disk-shaped pellet 15 mm in diameter and 2.5 mm thick. CaLa,S, powder was packed in a graphite crucible and sintered in pure H,S at 1200-1400 “C for 8 h. The pellets were further hot isostatically pressed by heating the specimens to 1400 ‘C under 200 MPa of argon for 1 h. After hot isostatic
pressing (HIP), the specimens were annealed in a pure H,S atmosphere at 1400 “C for 2 h to restore the sulfur stoichiometry and IR transmission. The dried carbonates, the sulfidized powders and the pellets were examined by X-ray diffractometry (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The sulfur, nitrogen and carbon contents were analyzed using Tacussel coulometric titration and a Heraeus CHN-O-Rapid Elemental Analyzer. The particle size distribution was analyzed using the sedimentation method, and the surface area was measured by the BET adsorption method. The Archimedes method was used to estimate the density of the sintered pellet. Results and
I
coprecipitation in water t
coprecipitation in ethanol
ectra
I
filtration + washing in water
1 freezing drying
tl
1
CEDO
+ cakining in O2 atmosphere I sulfidizing in H$ atmosphere I pressing isostatically
sin&ring in H,S atmosphere i hot isostatic pressing in argon atmosphere J annealing in H,S atmosphere Fig.
1. Processing
methods.
fiow sheet
for the
four
alternative
drying
Tl?e homogeneity of the precipitates was significantly influenced by the dispersion me&urn of the precursors. It was found that lanthanum carbonate was more hydrophilic than calcium carbonate. The normal carbonate, La,(CO,), .8H,O, was formed in aqueous solution, and hydrated to the basic carbonate form of La(OH)CO,sxH,O; however, this condition was influenced by the amount of water in the solution. Moreover, the presence of a trace amount of water in the ethanol solution also induced the formation of another crystalline form, La,O,(CO,).(l-2)H,O. The three lanthanum carbonates had different thermal behavior and physical properties 120-221. To eliminate segregation, pure ethanol was chosen as the coprecipitation medium, but ethanol did not completely wash the adsorbed nitrates from the coprecipitates (Fig. 2(a)). The IR peaks at 1052 and 826 cm-’ (NO, symmetric bend) showed that a residue of nitrates still existed in the coprecipitates 1231.This was also confirmed by eiemental analysis of N (6 wt.%) in the coprecipitates. After freeze-diying, nitrates still existed in the precipitates, as shown in Fig. 2(b). The presence of lanthanum hydroxide, which formed during the calcination of the dried coprecipitates in 0, at 700 “C, was identified by the O-H stretching at 3644 cm-‘, while the presence of calcium hydroxide was indicated by a peak at 3612 cm-l (Fig. 2(c)). The peak at 1360 cm-’ in the spectrum
66
I 3cKl
2wo
I2ca
1600
wavenumber
iloo
(cm”’)
I
40 28
20
60
Fig. 3. XRD patterns of (a) CWDO, @) CEDO, (c) CDFC, and (d) CDFI
i
0
0
CaLa-+
Fig. 4. XRD patterns of CaLa& powder obtained by sulfidizing (a) CWDO, (b) CEDO, (c) CDFC, and (d) CDT;? powders at 7.50 “C for 48 h.
400
Fig. 2. IR spectra of CDFC powder (a) before drying, (b) after freeze-drying, (c) after caicining in oxygen at 700 “C for 3 h; and fd) IR spectrum of CaLa& powder obtained by sulfidizing the calcined CDFC powder at 750 “C for 48 h.
t
0 0 I
powders.
of the CaLa& powder, obtained by sulfidizing the calcined powder at 750 “C for 48 h (Fig. 2(d)), represents the oxysulfide in p-La$,, because p-La& is known to have the actual com~sition La,,S,,_,O,. X-ray difiaction studies The XRD patterns showed that the coprecipitated carbonates were amorphous or poorly crystallized in various alcohols (CEDO, CDFC, CDFT), as shown in Fig. 3@)-(d), while the precursor powder (0490) obtained in the aqueous slurry or washed by water was identified as La,(CO,), *8H,O and crystalline, La,O,(CO,). (l-2)HZ0 (Fig. 3(a)). The impound obtained by sulfidizing the CWDO precursor contained some second phases (Fig. 4(a)), i.e., CaLa&, /3&a&
and CaS, owing to segregation of the lanthanum carbonates through the crystallization process in water. CaLa& and @La& were formed by su~fidiz~ng the coprecipitates dried by CDFC and CDFI’ techniques at 750 “C! for 48 h, as shown in Fig. 4(c) and (d). The &La& was then transfo~ed into ~-La& at 1300 “C during sintering [24]. The +A$, is a defect Th,P, cubic isostructure of CaLaZS, and exists as a solid solution [24,25]. The CaS in the compounds is harmful to the optical properties owing to its crystal structure, which is different from that of CaLa& [4]. The CaLa,S, powders obtaining @La& phase obtained using the CEDO, CDFT and CDFC methods were ready for further sulfidization and dens~cation. Particle size distributionand morphology The coprecipitated carbonates from CWDO had many shapes: they were wormlike, acicufar and spherical {Fig. 5(a)). These different particles had more than two phases. Two of the phases were identified as La2(CO& .8H,O and La,O,(CO,) * (l-2)HZ0 by XRD and TFM micrographs [27, 281. The CaLa,S, powder obtained by su~dizing the CWDO precursor at 750 “C for 48 h retained its carbonates morphology (Fig. 5(b)). The rate of crystallization of lanthanum carbonate was found to be speeded up by the addition of water, while calcium carbonate seemed to be indifferent to water in the presence of the former. Lanthanum carbonate’s affinity for water masked the tendency of calcium carbonate to hydrate. Particles with different morphologies have different compositions, as identified by EDX [27, 281. The coprecipitated carbonate from CEDO was gelatinous in appearance (Fig. 6(a)) and amorphous in character (Fig. 3(b)). The sulfide powder obtained by sulfidizing the CEDO precursor at 750 “C for 48 h (Fig. 6(b)) showed that the sulfides also retained their original
67
Fig. 5. Micrographs of (a) TEM of CWDO precursor and (b) SEM of sulfide powders obtained by sulfidizing at 750 “C for 24 h.
morphology. The coprecipitated carbonates from CDFC (Fig. 7(a)) and CDFT precursors had an appearance similar to that of CEDO, and the CaLa,S, powders obtained by sulfidizing the precursor were masses of aggregates, as shown in Fig. 7(b). The CaLa,S, powders with interconnected necks sulfidized from the CDFC precursor can easily be broken by grinding (Fig. 8). The different crystallinity and homogeneity of the coprecipitates and sulfides from CEDO, CDFC and CDFT can be explained by the different affinities of the three alcohols. According to the general principles of colloidal chemistry, polarization at the solid-liquid interface is required for the preparation of a stable suspension [29-311. When the precursor dispersions in three kinds of alcohol were compared, ethanol was seen to have the highest dielectric constant and the strongest affinity for the coprecipitates. When the drying rates of the alcohol with the precipitate were compared, cyclohexanol was seen to have the highest drying rate and lowest affinity for the precipitate (CDFC). The reason why CDFC precursor showed the lowest affinity for the carbonate and cyclohexanol is related to the steric hindrance of the large cyclohexanol molecule if it attaches to the precipitate [29-311. A liquid with a proper dielectric constant favors a stable dispersion of the powder [29-311. A liquid with a proper dielectric constant favors a stable dispersion of the powder [29-311.
Fig. 6. TEM micrographs of (a) CEDO precursor and (b) sulfide powder obtained by iulfidizing at 750 “C for 48 h.
(b)
‘l&m
Fig. 7. TEM micrographs of (a) CDFC precursor and (b) sulfide powder obtained by sulfidizing at 750 “C for 48 h.
From this study of the morphology of CEDO, CDFC and CDFT precursors, similar results showed that the CDFC powder was the proper precursor for suhidization and densification. The particle size distribution determined by Sedigraph experiments showed that the CaLa,S, powder obtained using CWDO precursors had a median agglomerate particle size of 3.5 pm, while
Elemental analyses The compounds containing carbonate
Fig. 8. TEM micrograph of ground sulfide powder obtained via CDFC route.
and a trace of nitrate were homogeneous in composition, and the residue could be diminished by calcining the precursor (CDFC, CDFT) before sulfidization. The carbon contents of the CaLa,!& powders were 3-5 wt.% for CDFC and CDFT precursors before calcination owing to the incomplete decomposition of organic materials, which were reduced (to 0.01-0.02 wt.%) by calcining in O2 at 700 “C. The nitrogen content was also reduced by calcining the precursors in an 0, atmosphere owing to the easy decomposition of nitrate in oxygen. However, the nitrate did not decompose much after calcination in the presence of a H,S atmosphere, because 5 wt.% N still existed. The sulfur content of the CaLa,!& powder obtained by sulfidization of the precursor at 750 “C for 48 h was about 24.50 wt.%, which increased to the theoretical value of 28.75 wt.% when the sulfidization temperature was increased to 950 “C for 48 h. Densifcation
EQUIVALENT
SPHERICAL
DIAMETER
(urn)
Fig. 9. Particle size distribution of sulfide from the (a) CWDO and (b) CEDO routes before grinding; from the (c) CWDO, (d) CEDO, and (e) CDFC routes after grinding. TABLE 2. BET surface analysis and median particle size from surface area’ CaLa&
CWDO CEDO CDFC CDFT
powder
Median particle size
Surface area (m2 g-7
(pm)
3.2 5.5 6.4 5.3
0.41 0.24 0.21 0.25
The density of the pellet sintered from the CWDO precursor was low, owing to the acicular, dendritic particles or hard agglomerate formed by coprecipitation and the severe hydrogen bonding of water with particles during drying (Fig. 5(a) and (b)). The relative densities of the pellets obtained by sintering the compacts from CDFC and CDFT precursors in a H,S atmosphere at 1400 “C for 8 h were 97.2% and 95.3%, respectively (Fig. 10(c) and (d)). The improvement can perhaps be explained in light of the relative degrees of agglomeration in the three powders, as shown in Fig. 9(e). All these results (Figs. 10 and 11) taken together indicate that both the freezing and the subsequent removal of water or organic material influenced the ultimate sinterability of the powders synthesized. After sintering, HIP treatment and annealing in a H$ atmosphere at 1400 “C, the theoretical density of CaLa& could be
&&It!
“Assuming a spherical particle.
powder derived from CEDO had a median agglomerate particle size of 1.5 pm with a similar distribution, as shown in Fig. 9(a) and (b). After grinding, the CaLa,S, powders made by CEDO, CDEC and CDFT methods had similar distributions, with median particle sizes of 0.6 pm, as shown in Fig. 9(d) and (e), but the CaLa,S, powder obtained using CWDO had the broadest particle size distribution (Fig. 9(c)). As expected, the BET surface areas of the sulfides from the four precursors (Table 2) were very similar for materials with the same crystallite size.
n
$o-
2’ ___.--------+(a)
4
__--0
ii
ml
d
1200 SINTERING
I
1300 TEMPERATURE
I
1400 (‘C)
Fig. 10. Effect of precursors from the routes of (a) CWDO, (b) CEDO, (c) CDFT, and (d) CDFC on densification of CaLa,S, pellet.
69
Fig. 11. SEM micrographs of the polished surface of CaLa& CWDO, (b) CEDO, (c) CDFT, and (d) CDFC routes. WAVELENGTH
(urn
)
Thickness
4ooa
XXNI
2aM
xoo
WAVENUMBER
lzal
800
400
( cm-‘)
Fig. 12. IR transmission spectra of hot isostatically pressed CaLa& pellet from (a) CDFC, (b) CEDO, and (c) CDFT precursors.
obtained. The pellet from the CDFC precursor had a thickness of 0.55 mm. Its IR transmittance was about 62% in the 13 Frn region (Fig. 12) while other pellets from CEDO and CDFT precursors showed poor transmittance.
pellet sintered
at 1400 “C for 8 h from sulfide powders
via the (a)
was retarded and sinterability was improved. t-Butyl alcohol (CDFT) and cyclohexanol (CDFC), replacing ethanol as the dispersing and drying media, also improved the sinterability and transmittance of the annealed pellets. The homogeneity of the coprecipitated carbonates was improved by mild mixing and retarded segregation, which was achieved by spray-freezing/freeze-drying in cyclohexanol. Investigation of the morphology of the carbonates from CDFC showed that they were interconnected. The sulfide powder obtained by sulfidizing the CDFC precursor at 750 “C for 48 h also showed interconnected particles. XRD study of the CaLa,S, powder obtained by sulfidizing the dried and calcined carbonates at 750 “C for 48 h showed that no CaS second phase existed. After sintering, HIP treatment and annealing in a H,S atmosphere at 1400 “C, the theoretical density of CaLa&, could be obtained, and the IR transmittance was about 62% in the 13 pm region for the CDFC precursor with a thickness of 0.55 mm.
Conclusions
Acknowledgements
When ethanol was used in place of water as the coprecipitate medium, segregation of the carbonates
The support from the Chung Shan Institute of Science and Technology and the National Science Council of
70
the Republic of China under contract No. CS-79-0210D006-28 is appreciated.
References 1 D. W. Johnson, Jr., P. K. Gallagher, D. J. Nitti and R. Schrey, Am. Ceram. Sot. BUN., 53 (1974) 163. 2 S. L. Dole, R. W. Scheidecker, L. E. Shiers, M. F. Berard and 0. Hunter, Jr., Mater. Sci. Eng., 32 (1978) 277. 3 D. L. Chess, C. A. Chess, J. A. Biggers and W. B. White, J. Am. Ceram. Sot., 66 (1983) 18. D. C. Harris, M. E. Hills, B. L. Gentilman, K. J. Saunders and T. Y. Wong, Adv. Ceram. Mater., 2 (1987) 74. J. A. Savage and K. J. Marsh, Proc. SPZE, 297 (1981) 35. S. Musikant, Proc. SPZE, 297 (1981) 2. R. L. Gentihnan, B. Dekosky, T. Y. Wong and R. W. Tustison, Proc. SPZE, 929 (1988) 58. D. E. Niesz and R. B. Bennett, in G. Y. Onoda, Jr., and L. L. Hench (eds.), Ceramic Processing before Firing, Wiley, New York, 1978, p. 61. 9 A. Roosen and H. Hausner, in P. Vincenzini (ed.), Ceramic Powders, Elsevier, Amsterdam, 1983, p. 773. 10 R. E. Jaeger, T. J. Miller and J. C. Williams, Am. Ceram. Sot. Bull., 53 (1974) 850. 11 P. Pampuch and K. Haberko, in P. Vincenzini (ed.), Ceramic Powders, Elsevier, Amsterdam, 1983, p. 623. 12 K. R. Reszka and J. Reszka, in P. Vincenzini (ed.), High Tech Ceramics, Elsevier, Amsterdam, 1988, p. 707. 13 M. S. Kaliszewski and A. H. Heuer, J. Am. Ceram. Sot., 73 (1990) 1504.
14 L. A. Xue, F. L. Riley and R. J. Brook, Br. Ceram. Trans. J., 85 (1986) 47. 15 Data from R. H. Perry and C. H. Chilton (eds.), Vapor Pressure of Organic Compounds, Chemical Engineering Handbook, McGraw-Hill, New York, 5th edn., 1973, pp. 3-50 and 3-52. 16 S. J. Visco and J. H. Kennedy, Solid State Zonics, 9 (1983) 885. 17 L. A. Beyer, C. E. Kalnas, D. Roy and I. B. Lloyd, Am Ceram. Sot. Bull., 66 (1987) 1647. 18 M. D. Rasmussen, M. Akinc and M. F. Berard, Ceram. Znt., 10 (1984) 99. 19 M. Akinc and D. Sordelet, Adv. Ceram. Mater., 2 (1987) 232. 20 M. L. Salutsky and L. L. Quill, .I. Am. Chem. Sot., 72 (1950) 3306. 21 R. L. N. Sastry, S. R. Yoganarasimhan, P. N. Mehrrotra and C. N. R. Rao, J. Znorg. Nucl. Chem., 28 (1966) 1165. 22 L. M. Seaverson, S.-Q. Luo, P.-L. Chien and J. F. McCeIIand, J. Am. Ceram. Sot., 69 (1986) 423. 23 J. A. Savage and K. L. Lewis, Proc. SPZE, 683 (1986) 79. 24 K. J. Saunders, T. Y. Wong, T. M. Hartnett, R. W. Tustion and R. L. Gentilman, Proc. SPZE, 683 (1986) 72. 25 H. Medelius and D. J. Rowcliffe, Mater. Sci. Eng. A, 109 (1989) 289. 26 T. J. Gardner and G. L. Messing, Am. Ceram. Sot. Bull., 12 (1984) 1498. 27 L. H. Wang, M. H. Hon and W. L. Huang, Ceram. Znt., 18 (1992) 27. 28 L. H. Wang and M. H. Hon, J Mater. Sci., 28 (1993) 2304. 29 S. Mizuta, M. Parish and H. K. Bowen, Ceram. Znt., 10 (1984) 43. 30 S. Mizuta, M. Parish and H. K. Bowen, Ceram. Znt., 10 (1984) 83. 31 B. C. Mutsuddy, J. Am. Cemm. Sot., 73 (1990) 2747.
Materials Chemistry and Physics, 35 (1993) 64-70
Dispersion of precursors for improving the homogeneity sinterability of CaLa,S, powders
and
Li Hsing Wang, Ming Shyng Tsai and Min Hsiung Hon Department of Materials Engineering, National Cheng Kung University, Tainan (Taiwan, ROC) (Received
June 22, 1992; accepted
December
28, 1992)
Abstract Cryochemical processing of carbonates dispersed in water and organic solutions is used to prepare CaLa,S,. Coprecipitated carbonates dispersed in cyclohexanol and t-butyl alcohol after spray-freeze/freeze-drying and sulfidization have an average particle size of 0.6 pm after grinding, which improves the sinterability significantly. The sintered pellet shows translucency after hot isostatic pressing and annealing in a Has atmosphere at 1400 “C.
Introduction It is well known that the sintering behavior of ceramic materials is greatly influenced by the characteristics of the starting powders, i.e., particle size distribution, degree of agglomeration, and chemical homogeneity [l, 21. However, the CaLa& powder obtained by a mixed oxide method [3] or coprecipitation method [4] has a broad particle size distribution and aggregation owing to high-temperature sulfidization over a long time. The CaLa& ceramic has lately become a promising optical window material for the 8-14 km IR range [5-71, but the agglomerated state and broad particle size distribution have resulted in poor packing and incomplete densification. One way to prepare a homogeneous powder is by using a precipitation method to obtain a mixed compound, which is then freeze-dried in order to decrease shrinkage and the formation of a hard agglomerate during sintering [8, 91. Another way is to use a sprayfreeze method to retard segregation of the powders
PI* To obtain a less agglomerated particle when drying, most coprecipitation techniques use ethanol instead of water [ll-141. However, the melting point (- 115 “C) of ethanol is too low to be suitable for freeze-drying suspensions of coprecipitates. On the other hand, tbutyl alcohol [15] and cyclohexanol [16] have melting points of 25.3 and 10 “C, respectively, with suitable vapor pressure and have easily controllable drying rates. The combination of spray-freezing and freeze-drying for the fabrication of oxide powders has a profound influence on their homogeneity and sinterability, be-
02540584/93/%6.00
cause an extremely small droplet of ice is formed at the instant of fast freezing [17-191. In this study the spray-freeze/freeze-drying of the coprecipitates has been investigated and its influence on the sulfide powder characteristics and sintered pellet properties has been evaluated in comparison with the effect of oven drying techniques. Experimental Commercial reagents La(NO,),+6H,O (99.0% pure) and Ca(NO,), (99.5% pure) were chosen for the carbonate formation and mixed in a ratio of La/Ca=2.20, while Pb(NO& (5 wt.%) was chosen as dopant. The carbonates were coprecipitated by adding the stock solution to the excess ammonium carbonate and ammonium hydroxide solution of ethanol for rapid coprecipitation. The stock solution was prepared by adding 0.5 mol of La(NO,), .6H,O and Ca(NO,), to a liter of distilled water. The ammonium carbonate and ammonium hydroxide solution was stirred in a mixer with the rotor at 1000 r-pm, and the stock solution was quickly dispersed in this torrent of solution. The pH values of the solution before and after precipitation were about 10.5 and 8.5, respectively. The precipitate was ultrasonically dispersed in the ethanol solution for 20 min. After filtration, the filtered cake was dispersed in tbutyl alcohol and cyclohexanol for the spray-freeze operation. The organic dispersed solution was sprayed through a nozzle into a large dewar of liquid nitrogen. This icy matrix was freeze-dried in VitrisB filter sealed bottles on a Vitris freeze-dryer for 3-6 days. The filtered cakes obtained were designated according to the syn-
0 1993 - Elsevier
Sequoia. All rights reserved
65
TABLE
1. Sample designation
and description
Sample designation
Description
CWDO CEDO CDFC CDFT
Carbonate Carbonate Carbonate Carbonate
of precursor
filtered
cake drying procedures
and procedure
precursor precursor precursor precursor
washed with water and dried in oven at 80 “C for 8 h washed with ethanol and dried in oven at 80 “C for 8 h dried by freeze-drying in cyclohexanol ice dried by freeze-drying in t-butyl alcohol ice
thesis reagents used and the organic or water removal method (Table 1). The processing flow chart is shown in Fig. 1. The dried coprecipitated carbonate precursors were calcined in pure oxygen and decomposed at 700 “C for 3 h, then sulfidized at 750 “C for 48 h in a H,S atmosphere. The sulfide powder was ground in a CS2 medium for 48 h. The resultant light green powder was cold isostatically pressed (CIP) at 200 MPa in a rubber mold to form a disk-shaped pellet 15 mm in diameter and 2.5 mm thick. CaLa,S, powder was packed in a graphite crucible and sintered in pure H,S at 1200-1400 “C for 8 h. The pellets were further hot isostatically pressed by heating the specimens to 1400 ‘C under 200 MPa of argon for 1 h. After hot isostatic
pressing (HIP), the specimens were annealed in a pure H,S atmosphere at 1400 “C for 2 h to restore the sulfur stoichiometry and IR transmission. The dried carbonates, the sulfidized powders and the pellets were examined by X-ray diffractometry (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The sulfur, nitrogen and carbon contents were analyzed using Tacussel coulometric titration and a Heraeus CHN-O-Rapid Elemental Analyzer. The particle size distribution was analyzed using the sedimentation method, and the surface area was measured by the BET adsorption method. The Archimedes method was used to estimate the density of the sintered pellet. Results and
I
coprecipitation in water t
coprecipitation in ethanol
ectra
I
filtration + washing in water
1 freezing drying
tl
1
CEDO
+ cakining in O2 atmosphere I sulfidizing in H$ atmosphere I pressing isostatically
sin&ring in H,S atmosphere i hot isostatic pressing in argon atmosphere J annealing in H,S atmosphere Fig.
1. Processing
methods.
fiow sheet
for the
four
alternative
drying
Tl?e homogeneity of the precipitates was significantly influenced by the dispersion me&urn of the precursors. It was found that lanthanum carbonate was more hydrophilic than calcium carbonate. The normal carbonate, La,(CO,), .8H,O, was formed in aqueous solution, and hydrated to the basic carbonate form of La(OH)CO,sxH,O; however, this condition was influenced by the amount of water in the solution. Moreover, the presence of a trace amount of water in the ethanol solution also induced the formation of another crystalline form, La,O,(CO,).(l-2)H,O. The three lanthanum carbonates had different thermal behavior and physical properties 120-221. To eliminate segregation, pure ethanol was chosen as the coprecipitation medium, but ethanol did not completely wash the adsorbed nitrates from the coprecipitates (Fig. 2(a)). The IR peaks at 1052 and 826 cm-’ (NO, symmetric bend) showed that a residue of nitrates still existed in the coprecipitates 1231.This was also confirmed by eiemental analysis of N (6 wt.%) in the coprecipitates. After freeze-diying, nitrates still existed in the precipitates, as shown in Fig. 2(b). The presence of lanthanum hydroxide, which formed during the calcination of the dried coprecipitates in 0, at 700 “C, was identified by the O-H stretching at 3644 cm-‘, while the presence of calcium hydroxide was indicated by a peak at 3612 cm-l (Fig. 2(c)). The peak at 1360 cm-’ in the spectrum
66
I 3cKl
2wo
I2ca
1600
wavenumber
iloo
(cm”’)
I
40 28
20
60
Fig. 3. XRD patterns of (a) CWDO, @) CEDO, (c) CDFC, and (d) CDFI
i
0
0
CaLa-+
Fig. 4. XRD patterns of CaLa& powder obtained by sulfidizing (a) CWDO, (b) CEDO, (c) CDFC, and (d) CDT;? powders at 7.50 “C for 48 h.
400
Fig. 2. IR spectra of CDFC powder (a) before drying, (b) after freeze-drying, (c) after caicining in oxygen at 700 “C for 3 h; and fd) IR spectrum of CaLa& powder obtained by sulfidizing the calcined CDFC powder at 750 “C for 48 h.
t
0 0 I
powders.
of the CaLa& powder, obtained by sulfidizing the calcined powder at 750 “C for 48 h (Fig. 2(d)), represents the oxysulfide in p-La$,, because p-La& is known to have the actual com~sition La,,S,,_,O,. X-ray difiaction studies The XRD patterns showed that the coprecipitated carbonates were amorphous or poorly crystallized in various alcohols (CEDO, CDFC, CDFT), as shown in Fig. 3@)-(d), while the precursor powder (0490) obtained in the aqueous slurry or washed by water was identified as La,(CO,), *8H,O and crystalline, La,O,(CO,). (l-2)HZ0 (Fig. 3(a)). The impound obtained by sulfidizing the CWDO precursor contained some second phases (Fig. 4(a)), i.e., CaLa&, /3&a&
and CaS, owing to segregation of the lanthanum carbonates through the crystallization process in water. CaLa& and @La& were formed by su~fidiz~ng the coprecipitates dried by CDFC and CDFI’ techniques at 750 “C! for 48 h, as shown in Fig. 4(c) and (d). The &La& was then transfo~ed into ~-La& at 1300 “C during sintering [24]. The +A$, is a defect Th,P, cubic isostructure of CaLaZS, and exists as a solid solution [24,25]. The CaS in the compounds is harmful to the optical properties owing to its crystal structure, which is different from that of CaLa& [4]. The CaLa,S, powders obtaining @La& phase obtained using the CEDO, CDFT and CDFC methods were ready for further sulfidization and dens~cation. Particle size distributionand morphology The coprecipitated carbonates from CWDO had many shapes: they were wormlike, acicufar and spherical {Fig. 5(a)). These different particles had more than two phases. Two of the phases were identified as La2(CO& .8H,O and La,O,(CO,) * (l-2)HZ0 by XRD and TFM micrographs [27, 281. The CaLa,S, powder obtained by su~dizing the CWDO precursor at 750 “C for 48 h retained its carbonates morphology (Fig. 5(b)). The rate of crystallization of lanthanum carbonate was found to be speeded up by the addition of water, while calcium carbonate seemed to be indifferent to water in the presence of the former. Lanthanum carbonate’s affinity for water masked the tendency of calcium carbonate to hydrate. Particles with different morphologies have different compositions, as identified by EDX [27, 281. The coprecipitated carbonate from CEDO was gelatinous in appearance (Fig. 6(a)) and amorphous in character (Fig. 3(b)). The sulfide powder obtained by sulfidizing the CEDO precursor at 750 “C for 48 h (Fig. 6(b)) showed that the sulfides also retained their original
67
Fig. 5. Micrographs of (a) TEM of CWDO precursor and (b) SEM of sulfide powders obtained by sulfidizing at 750 “C for 24 h.
morphology. The coprecipitated carbonates from CDFC (Fig. 7(a)) and CDFT precursors had an appearance similar to that of CEDO, and the CaLa,S, powders obtained by sulfidizing the precursor were masses of aggregates, as shown in Fig. 7(b). The CaLa,S, powders with interconnected necks sulfidized from the CDFC precursor can easily be broken by grinding (Fig. 8). The different crystallinity and homogeneity of the coprecipitates and sulfides from CEDO, CDFC and CDFT can be explained by the different affinities of the three alcohols. According to the general principles of colloidal chemistry, polarization at the solid-liquid interface is required for the preparation of a stable suspension [29-311. When the precursor dispersions in three kinds of alcohol were compared, ethanol was seen to have the highest dielectric constant and the strongest affinity for the coprecipitates. When the drying rates of the alcohol with the precipitate were compared, cyclohexanol was seen to have the highest drying rate and lowest affinity for the precipitate (CDFC). The reason why CDFC precursor showed the lowest affinity for the carbonate and cyclohexanol is related to the steric hindrance of the large cyclohexanol molecule if it attaches to the precipitate [29-311. A liquid with a proper dielectric constant favors a stable dispersion of the powder [29-311. A liquid with a proper dielectric constant favors a stable dispersion of the powder [29-311.
Fig. 6. TEM micrographs of (a) CEDO precursor and (b) sulfide powder obtained by iulfidizing at 750 “C for 48 h.
(b)
‘l&m
Fig. 7. TEM micrographs of (a) CDFC precursor and (b) sulfide powder obtained by sulfidizing at 750 “C for 48 h.
From this study of the morphology of CEDO, CDFC and CDFT precursors, similar results showed that the CDFC powder was the proper precursor for suhidization and densification. The particle size distribution determined by Sedigraph experiments showed that the CaLa,S, powder obtained using CWDO precursors had a median agglomerate particle size of 3.5 pm, while
Elemental analyses The compounds containing carbonate
Fig. 8. TEM micrograph of ground sulfide powder obtained via CDFC route.
and a trace of nitrate were homogeneous in composition, and the residue could be diminished by calcining the precursor (CDFC, CDFT) before sulfidization. The carbon contents of the CaLa,!& powders were 3-5 wt.% for CDFC and CDFT precursors before calcination owing to the incomplete decomposition of organic materials, which were reduced (to 0.01-0.02 wt.%) by calcining in O2 at 700 “C. The nitrogen content was also reduced by calcining the precursors in an 0, atmosphere owing to the easy decomposition of nitrate in oxygen. However, the nitrate did not decompose much after calcination in the presence of a H,S atmosphere, because 5 wt.% N still existed. The sulfur content of the CaLa,!& powder obtained by sulfidization of the precursor at 750 “C for 48 h was about 24.50 wt.%, which increased to the theoretical value of 28.75 wt.% when the sulfidization temperature was increased to 950 “C for 48 h. Densifcation
EQUIVALENT
SPHERICAL
DIAMETER
(urn)
Fig. 9. Particle size distribution of sulfide from the (a) CWDO and (b) CEDO routes before grinding; from the (c) CWDO, (d) CEDO, and (e) CDFC routes after grinding. TABLE 2. BET surface analysis and median particle size from surface area’ CaLa&
CWDO CEDO CDFC CDFT
powder
Median particle size
Surface area (m2 g-7
(pm)
3.2 5.5 6.4 5.3
0.41 0.24 0.21 0.25
The density of the pellet sintered from the CWDO precursor was low, owing to the acicular, dendritic particles or hard agglomerate formed by coprecipitation and the severe hydrogen bonding of water with particles during drying (Fig. 5(a) and (b)). The relative densities of the pellets obtained by sintering the compacts from CDFC and CDFT precursors in a H,S atmosphere at 1400 “C for 8 h were 97.2% and 95.3%, respectively (Fig. 10(c) and (d)). The improvement can perhaps be explained in light of the relative degrees of agglomeration in the three powders, as shown in Fig. 9(e). All these results (Figs. 10 and 11) taken together indicate that both the freezing and the subsequent removal of water or organic material influenced the ultimate sinterability of the powders synthesized. After sintering, HIP treatment and annealing in a H$ atmosphere at 1400 “C, the theoretical density of CaLa& could be
&&It!
“Assuming a spherical particle.
powder derived from CEDO had a median agglomerate particle size of 1.5 pm with a similar distribution, as shown in Fig. 9(a) and (b). After grinding, the CaLa,S, powders made by CEDO, CDEC and CDFT methods had similar distributions, with median particle sizes of 0.6 pm, as shown in Fig. 9(d) and (e), but the CaLa,S, powder obtained using CWDO had the broadest particle size distribution (Fig. 9(c)). As expected, the BET surface areas of the sulfides from the four precursors (Table 2) were very similar for materials with the same crystallite size.
n
$o-
2’ ___.--------+(a)
4
__--0
ii
ml
d
1200 SINTERING
I
1300 TEMPERATURE
I
1400 (‘C)
Fig. 10. Effect of precursors from the routes of (a) CWDO, (b) CEDO, (c) CDFT, and (d) CDFC on densification of CaLa,S, pellet.
69
Fig. 11. SEM micrographs of the polished surface of CaLa& CWDO, (b) CEDO, (c) CDFT, and (d) CDFC routes. WAVELENGTH
(urn
)
Thickness
4ooa
XXNI
2aM
xoo
WAVENUMBER
lzal
800
400
( cm-‘)
Fig. 12. IR transmission spectra of hot isostatically pressed CaLa& pellet from (a) CDFC, (b) CEDO, and (c) CDFT precursors.
obtained. The pellet from the CDFC precursor had a thickness of 0.55 mm. Its IR transmittance was about 62% in the 13 Frn region (Fig. 12) while other pellets from CEDO and CDFT precursors showed poor transmittance.
pellet sintered
at 1400 “C for 8 h from sulfide powders
via the (a)
was retarded and sinterability was improved. t-Butyl alcohol (CDFT) and cyclohexanol (CDFC), replacing ethanol as the dispersing and drying media, also improved the sinterability and transmittance of the annealed pellets. The homogeneity of the coprecipitated carbonates was improved by mild mixing and retarded segregation, which was achieved by spray-freezing/freeze-drying in cyclohexanol. Investigation of the morphology of the carbonates from CDFC showed that they were interconnected. The sulfide powder obtained by sulfidizing the CDFC precursor at 750 “C for 48 h also showed interconnected particles. XRD study of the CaLa,S, powder obtained by sulfidizing the dried and calcined carbonates at 750 “C for 48 h showed that no CaS second phase existed. After sintering, HIP treatment and annealing in a H,S atmosphere at 1400 “C, the theoretical density of CaLa&, could be obtained, and the IR transmittance was about 62% in the 13 pm region for the CDFC precursor with a thickness of 0.55 mm.
Conclusions
Acknowledgements
When ethanol was used in place of water as the coprecipitate medium, segregation of the carbonates
The support from the Chung Shan Institute of Science and Technology and the National Science Council of
70
the Republic of China under contract No. CS-79-0210D006-28 is appreciated.
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