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Chromia microspheres by the sol-gel technique
December 1995
Materials Letters 25 ( 1995) 261-263
ELSEVIER
Chromia microspheres by the sol-gel technique M. Chatterjee, B. Siladitya, D. Ganguli * Sol-Gel Laboratory,
Central Glass and Ceramic Research Institute, Calcutta 700 032. India
Received 19 July 1995; accepted 25 July 1995
Abstract Chromia microspheres were synthesized by the sol-emulsion-gel method. The solvent extraction technique was followed for the preparation of the ~01s.Cr,03 crystallized as single phase in the temperature range 400 to 1300°C. Complete crystallization occurred at 1300°C. Spherical morphology of the powder was confirmed by scanning electron microscopy. A sfze range of 330 pm was observed in the powder calcined at 1300°C.
1. Introduction
2. Experimental procedure
The efficiency of ceramic powders as feed for good quality plasma-sprayed coatings is determined, among others, by the very important practical parameter of specific size range for each composition [ 1,2]. Sphericity of the powder is considered to be an added advantage as it can lead to facile flowability. Both these characteristics are known to be easily achievable by the sol-gel processing route; such powders [3-71 therefore give rise to very efficient thermally sprayed coatings. Chromium( III) oxide powders find wide use in the preparation of plasma-sprayed wear-resistant coatings in e.g. textile, mechanical and chemical industries [ 81. However, little information is available in the published literature on preparative aspects of such powders [ 41. The present communication gives a brief account of sol-emulsion-gel synthesis of spherical Cr203 powders (a process which can be used in the manufacture of plasma sprayable Cr,03) and some of their properties.
The preparative process involves two steps: (i) preparation of a Cr(II1) sol via hydrolysis of the corresponding chloride solution with an organic amine solution followed by the solvent extraction of the hydrolytic acid (HCl) generated; and (ii) formation of gel microspheres by producing an emulsion of sol droplets and their subsequent neutralization with a base. The starting material was a C? + solution of molarity 3.49; the chloride concentration was 8.55 M (Cl-/ Cr” molar ratio= 2.45). The solution was obtained from an aqueous solution of CrO,, acidified with 30% w/v hydrochloric acid and treated with ethanol, used as a reducing agent. A part of the Cl- was extracted [ 41 by a triethylamine (TEA) solution in l,l,l-trichloroethane (TCE).Two layers of liquid were produced during this step, i.e. an aqueous phase with C? + and a heavier organic phase with the extracted Cl-. As the Cl- concentration of the original C? + solution decreased due to such extraction, the Cl- /CrS’ molar ratio in the aqueous phase also decreased. The hydrolysis reaction may be represented as [ 9,101:
* Corresponding author. 0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO167-577x(95)00179-4
pCr”
+qH,O
+
[Cr,(OH),J”“-Y’
+ +qH+.
where p and y represent moles of Cr’ + and H,O, respectively. Thus, the aqueous phase enriched in OH compared to Cl -, led to the formation of a sol [ 41. By changing the extraction parameters with different concentrations of Cr” in the starting solution and of TEA in the organic diluent, sols with a wide range of polymeric hydroxyl species were obtained. The Cl - /C? * molar ratio of the sols during extraction was restricted within the range of about 2.0 to I .30. Further extraction beyond this limit led to relatively quick formation of gels. The sols were used in preparing water-in-oil type emulsions; gel microspheres were obtained using earlier procedures [ 5.61. The microspheres were washed with organic solvents. The dried microspheres were heat-treated at 2001300°C with I h soak at different temperatures. A separate sample was held for 5 h at 1300°C ,for obtaining complete crystallization. Both the gel and calcined microspheres were characterized by differential thermal analysis (DTA) and thermogravimetry (TG) (Model TA 50 I, Shimadzu, Japan), X-ray diffractometry (XRD) (Philips PW: 1730 X-ray unit) using Nifiltered Cu Ko radiation, scanning electron microscopy (SEM) (S 250, Cambridge) and particle size analysis (Model 5OOOD, Micromeritics. USA).
3. Results and discussion DTA/TG of the obtained gel microspheres showed: (i) a shallow and broad endotherm at about 108°C indicating expulsion of water and organics, (ii) a sharp exothermic peak at about 425”C, presumably indicating onset of crystallization of the gel, (iii) no further heating effect up to 1200°C and (iv) weight loss of about 2 I .5% up to 45O“C, and very little ( l-l .5%) loss thereafter. The gel mass was found to remain X-ray amorphous at 2OO”C, but did show initial crystallization of CrzO, at 400°C. which compared well with the DTA results. In fact, up to 1300°C of crystallization, the only crystal phase generated was Cr203. Further, the increase in the degree of crystallization of the gel microspheres with increasing calcination temperature’ was evident from the increase in the area of XRD peaks under the same conditions of recording. The area under the peak of the
OY ml
1
404
600
800
low
tmo
wo
Tapaohre I ‘C I Fig. 1. Effect of calcination temperature on the crystallization of CrA
( 104) reflection (relative intensity = 100) of Cr,O, was computed in each case on the basis of the relation [ I I ] area = height X fwhm (full width at half maximum). The results, along with that for the 13OO”C/5 h sample, are plotted in Fig. I, which indicates that crystallization of Cr203 in the present gels was probably complete only at 1300°C. The pathway of crystallization of chromia gels obtained in this work is straightforward: an X-ray amorphous gel to Cr203 (eskolaite) crystals starting to form at around 400°C. No hydrate, oxyhydrate or transient Cr,O, polymorphs were obtained as intermediate crystalline products. Similar results have also been obtained by other workers [ 121. This is in sharp contrast with the results reported by various workers in obtaining aA&O, (corundum) from amorphous alumina gels [ 5,13 I, where crystallization of corundum takes place via boehmite and/or different transition phases of alumina. Powders prepared by varying Cl- /C? * molar ratio were examined under SEM. Under certain conditions (Cl - /Cr’ + molar ratio = 1.4) unbroken spheres could be obtained. The retainment of sphericity was observed even after heat-treatment at 1300°C. Fig. 2a shows the morphology of the calcined samples. The porous nature of these microspheres is obvious from the micrograph of Fig. 2b, obtained at a higher magnification. A size range of 3-30 pm with d,, (average size) at I6 pm was obtained in powders heat-treated at 1300°C. The
M. Chatterjee et al. /Materials
Letters 25 (1995) 261-263
263
(ii) Cr,O, crystallized as single phase throughout the temperature range 400 to 13OO”C,and the degree of crystallization increased with the increase in calcination temperature. Complete crystallization took place at 1300°C. (iii) Spherical morphology of both the gel and calcined microspheres was confirmed by SEM. The calcined powder ( 1300°C) showed a particle size range of 3-30 p,m with a & value of 16 pm. Acknowledgements The authors are thankful to Dr. B.K. Sarkar, Director of the Institute, for his kind permission to publish this paper. They also thank the colleagues of the X-ray Laboratory, Electroceramics and Refractory Divisions for their kind help in material characterization. The help rendered by Mrs. A. Laskar in obtaining the scanning electron micrographs is gratefully acknowledged.
References
Fig. 2. Scanning electron micrographs of Crz09 microspheres, cined at 13OO“C, at two different magnifications.
cal-
above size range may be considered as a typical example. By changing the process parameters, the present method can yield Cr,O, microspheres of any desired size range.
4. Conclusions (i) Cr,,O, microspheres were synthesized by the solgel method. Sols were prepared following the solvent extraction technique with different Cl-/C?’ molar ratios. Water-in-oil type emulsion was used in obtaining gel microspheres.
[ 1] Anon., Techno Japan 23 ( 1990) 8. [ 21 R. Shivakumar, in: Powder metallurgy - recent advances, eds. VS. Arunachalam and O.V. Roman (Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi, 1989) p.203. [3] F.G. Sheriff andL.J. Shyu, J. Am. Ceram. Sot. 74 (1991). [4] J.L. Woodhead, J. Mater. Educ. 6 ( 1984) 887. [5] J. Ray, M. Chattejee and D. Ganguli, J. Mater. Sci. Letters 12 (1993) 1755. [6] M. Chatterjee, J. Ray, A. Chattetjee, D. Ganguli, S.V. Joshi and M.P. Srivastava, J. Mater. Sci. 28 (1993) 2803. [7] S.V. Joshi, M.P. Srivasmva, M. Chattejee, J. Ray, A. Chattejee and D. Ganguli, Bull. Mater. Sci. 16 (1993) 19. [ 81 D. Chuanxian, H. Bingtang and L. Huiling, in: Proceedings of the 11th International Thermal Spraying Conference (Pergamon Press, New York, 1986) p. 533. [9] P.L. Brown and R.N. Sylva, J. Chem. Sot. Dalton Trans. (1985) 1967. [ 101 R.E. Mesmer andC.F. Baes, Jr., Inorg. Chem. 10 ( 1971) 2290. [ 111 N. Enomoto, M. Katsumoto and Z. Nakagawa, J. Ceram. Sot. JapanlO2(1994)1105. [ 121 M. Patel, Indian Ceram. 29 ( 1986) 113. [ 131 R.K. Dwivedi and G. Gowda, J. Mater. Sci. Letters 4 ( 1985) 331.
Materials Letters 25 ( 1995) 261-263
ELSEVIER
Chromia microspheres by the sol-gel technique M. Chatterjee, B. Siladitya, D. Ganguli * Sol-Gel Laboratory,
Central Glass and Ceramic Research Institute, Calcutta 700 032. India
Received 19 July 1995; accepted 25 July 1995
Abstract Chromia microspheres were synthesized by the sol-emulsion-gel method. The solvent extraction technique was followed for the preparation of the ~01s.Cr,03 crystallized as single phase in the temperature range 400 to 1300°C. Complete crystallization occurred at 1300°C. Spherical morphology of the powder was confirmed by scanning electron microscopy. A sfze range of 330 pm was observed in the powder calcined at 1300°C.
1. Introduction
2. Experimental procedure
The efficiency of ceramic powders as feed for good quality plasma-sprayed coatings is determined, among others, by the very important practical parameter of specific size range for each composition [ 1,2]. Sphericity of the powder is considered to be an added advantage as it can lead to facile flowability. Both these characteristics are known to be easily achievable by the sol-gel processing route; such powders [3-71 therefore give rise to very efficient thermally sprayed coatings. Chromium( III) oxide powders find wide use in the preparation of plasma-sprayed wear-resistant coatings in e.g. textile, mechanical and chemical industries [ 81. However, little information is available in the published literature on preparative aspects of such powders [ 41. The present communication gives a brief account of sol-emulsion-gel synthesis of spherical Cr203 powders (a process which can be used in the manufacture of plasma sprayable Cr,03) and some of their properties.
The preparative process involves two steps: (i) preparation of a Cr(II1) sol via hydrolysis of the corresponding chloride solution with an organic amine solution followed by the solvent extraction of the hydrolytic acid (HCl) generated; and (ii) formation of gel microspheres by producing an emulsion of sol droplets and their subsequent neutralization with a base. The starting material was a C? + solution of molarity 3.49; the chloride concentration was 8.55 M (Cl-/ Cr” molar ratio= 2.45). The solution was obtained from an aqueous solution of CrO,, acidified with 30% w/v hydrochloric acid and treated with ethanol, used as a reducing agent. A part of the Cl- was extracted [ 41 by a triethylamine (TEA) solution in l,l,l-trichloroethane (TCE).Two layers of liquid were produced during this step, i.e. an aqueous phase with C? + and a heavier organic phase with the extracted Cl-. As the Cl- concentration of the original C? + solution decreased due to such extraction, the Cl- /CrS’ molar ratio in the aqueous phase also decreased. The hydrolysis reaction may be represented as [ 9,101:
* Corresponding author. 0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO167-577x(95)00179-4
pCr”
+qH,O
+
[Cr,(OH),J”“-Y’
+ +qH+.
where p and y represent moles of Cr’ + and H,O, respectively. Thus, the aqueous phase enriched in OH compared to Cl -, led to the formation of a sol [ 41. By changing the extraction parameters with different concentrations of Cr” in the starting solution and of TEA in the organic diluent, sols with a wide range of polymeric hydroxyl species were obtained. The Cl - /C? * molar ratio of the sols during extraction was restricted within the range of about 2.0 to I .30. Further extraction beyond this limit led to relatively quick formation of gels. The sols were used in preparing water-in-oil type emulsions; gel microspheres were obtained using earlier procedures [ 5.61. The microspheres were washed with organic solvents. The dried microspheres were heat-treated at 2001300°C with I h soak at different temperatures. A separate sample was held for 5 h at 1300°C ,for obtaining complete crystallization. Both the gel and calcined microspheres were characterized by differential thermal analysis (DTA) and thermogravimetry (TG) (Model TA 50 I, Shimadzu, Japan), X-ray diffractometry (XRD) (Philips PW: 1730 X-ray unit) using Nifiltered Cu Ko radiation, scanning electron microscopy (SEM) (S 250, Cambridge) and particle size analysis (Model 5OOOD, Micromeritics. USA).
3. Results and discussion DTA/TG of the obtained gel microspheres showed: (i) a shallow and broad endotherm at about 108°C indicating expulsion of water and organics, (ii) a sharp exothermic peak at about 425”C, presumably indicating onset of crystallization of the gel, (iii) no further heating effect up to 1200°C and (iv) weight loss of about 2 I .5% up to 45O“C, and very little ( l-l .5%) loss thereafter. The gel mass was found to remain X-ray amorphous at 2OO”C, but did show initial crystallization of CrzO, at 400°C. which compared well with the DTA results. In fact, up to 1300°C of crystallization, the only crystal phase generated was Cr203. Further, the increase in the degree of crystallization of the gel microspheres with increasing calcination temperature’ was evident from the increase in the area of XRD peaks under the same conditions of recording. The area under the peak of the
OY ml
1
404
600
800
low
tmo
wo
Tapaohre I ‘C I Fig. 1. Effect of calcination temperature on the crystallization of CrA
( 104) reflection (relative intensity = 100) of Cr,O, was computed in each case on the basis of the relation [ I I ] area = height X fwhm (full width at half maximum). The results, along with that for the 13OO”C/5 h sample, are plotted in Fig. I, which indicates that crystallization of Cr203 in the present gels was probably complete only at 1300°C. The pathway of crystallization of chromia gels obtained in this work is straightforward: an X-ray amorphous gel to Cr203 (eskolaite) crystals starting to form at around 400°C. No hydrate, oxyhydrate or transient Cr,O, polymorphs were obtained as intermediate crystalline products. Similar results have also been obtained by other workers [ 121. This is in sharp contrast with the results reported by various workers in obtaining aA&O, (corundum) from amorphous alumina gels [ 5,13 I, where crystallization of corundum takes place via boehmite and/or different transition phases of alumina. Powders prepared by varying Cl- /C? * molar ratio were examined under SEM. Under certain conditions (Cl - /Cr’ + molar ratio = 1.4) unbroken spheres could be obtained. The retainment of sphericity was observed even after heat-treatment at 1300°C. Fig. 2a shows the morphology of the calcined samples. The porous nature of these microspheres is obvious from the micrograph of Fig. 2b, obtained at a higher magnification. A size range of 3-30 pm with d,, (average size) at I6 pm was obtained in powders heat-treated at 1300°C. The
M. Chatterjee et al. /Materials
Letters 25 (1995) 261-263
263
(ii) Cr,O, crystallized as single phase throughout the temperature range 400 to 13OO”C,and the degree of crystallization increased with the increase in calcination temperature. Complete crystallization took place at 1300°C. (iii) Spherical morphology of both the gel and calcined microspheres was confirmed by SEM. The calcined powder ( 1300°C) showed a particle size range of 3-30 p,m with a & value of 16 pm. Acknowledgements The authors are thankful to Dr. B.K. Sarkar, Director of the Institute, for his kind permission to publish this paper. They also thank the colleagues of the X-ray Laboratory, Electroceramics and Refractory Divisions for their kind help in material characterization. The help rendered by Mrs. A. Laskar in obtaining the scanning electron micrographs is gratefully acknowledged.
References
Fig. 2. Scanning electron micrographs of Crz09 microspheres, cined at 13OO“C, at two different magnifications.
cal-
above size range may be considered as a typical example. By changing the process parameters, the present method can yield Cr,O, microspheres of any desired size range.
4. Conclusions (i) Cr,,O, microspheres were synthesized by the solgel method. Sols were prepared following the solvent extraction technique with different Cl-/C?’ molar ratios. Water-in-oil type emulsion was used in obtaining gel microspheres.
[ 1] Anon., Techno Japan 23 ( 1990) 8. [ 21 R. Shivakumar, in: Powder metallurgy - recent advances, eds. VS. Arunachalam and O.V. Roman (Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi, 1989) p.203. [3] F.G. Sheriff andL.J. Shyu, J. Am. Ceram. Sot. 74 (1991). [4] J.L. Woodhead, J. Mater. Educ. 6 ( 1984) 887. [5] J. Ray, M. Chattejee and D. Ganguli, J. Mater. Sci. Letters 12 (1993) 1755. [6] M. Chatterjee, J. Ray, A. Chattetjee, D. Ganguli, S.V. Joshi and M.P. Srivastava, J. Mater. Sci. 28 (1993) 2803. [7] S.V. Joshi, M.P. Srivasmva, M. Chattejee, J. Ray, A. Chattejee and D. Ganguli, Bull. Mater. Sci. 16 (1993) 19. [ 81 D. Chuanxian, H. Bingtang and L. Huiling, in: Proceedings of the 11th International Thermal Spraying Conference (Pergamon Press, New York, 1986) p. 533. [9] P.L. Brown and R.N. Sylva, J. Chem. Sot. Dalton Trans. (1985) 1967. [ 101 R.E. Mesmer andC.F. Baes, Jr., Inorg. Chem. 10 ( 1971) 2290. [ 111 N. Enomoto, M. Katsumoto and Z. Nakagawa, J. Ceram. Sot. JapanlO2(1994)1105. [ 121 M. Patel, Indian Ceram. 29 ( 1986) 113. [ 131 R.K. Dwivedi and G. Gowda, J. Mater. Sci. Letters 4 ( 1985) 331.