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Fast solid state synthesis of metal vanadates and chalcogenides using microwave irradiation
hint&daRescuchBulletia, Vol. 30,No. 9, pp. 1173~1177,159S chpyighto1995Elmvia5cialmLm RilItdintbUSA.AUightSrsrsnsd 002544Ow5$9.50+ .oO
0025-5408(95)ooo99-2
FAST SOLID STATE SYNTHESIS OF METAL VANADATES AND CHALCOGENIDES USING MICROWAVE IRRADIATION B. Vaidhyanathan,
Munia
Ganguli
and K.J.
Rae
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore012, India. (Received August 3 1, 1994; Communicatedby C.N.R Rae)
Metal vanadates and chalcogenides have been prepared in rapid time scales using a domestic microwave oven. The products exhibit high phase purity and homogeneity. The thermal and spectroscopic properties of the microwave prepared metal vanadates match well with those prepared by conventional methods. ABSTECACT:
MATERIALS
INDEX:
metal vanadates, metal chalcogenides
Introduction
Metal vanadates and chalcogenides belong to a rich category of inorganic solids with technologically important properties. Bismuth and lead vanadates are good ferroelectrics and metal chalcogenides are used in thermoelectric power generators, solar cells, scintillation counters etc. The conventional solid state methods of preparing these materials are quite difficult and rather involved. Recently, it has been realized that microwave radiation can be used to accelerate some of the kinetically slow solid state reactions[l-31. Certain ,metal oxides (like VZOS, WOS etc.) with high dielectric loss values can couple with microwaves efficiently and can get heated up to 1000 K or more. There are few reports wherein rapid heating of metal powders under microwave exposure has been used in the synthesis of metal chalcogenides[4,5]. The most important advantages in microwavca assisted synthesis appear to be the very short time scales required for the preparation and the selectivity in energy transfer from microwave field. In this communication, we report, 1173
B.VAIDHYANA'THANetol.
1174
Vol. 30,No. 9
a fast, clean and simple method of preparing some important metal vanadates and chalcogenides using a kitchen microwave oven.
Experimental The metal vanadates Bi4VzOll grade materials.
Well-mixed
and PbVzOe
stoichionetric
have been prepared starting with reagent mounts
of (VsOs + 2BisOs) and (VzOs +
PbO), with a batch weight of 5 grax~ were taken in clean silica crucibles and placed on a firebrick inside a domestic microwave oven (Batliboy
Eddy) operating at 2.45 GHz with
a tunable power level up to 980 W. The charges were exposed to microwaves for about 10-15 minutes and allowed to cool slowly inside the oven and the cooled products were then ground.
Bi4V20i1 and PbVzOe were then annealed separately in muffle furnace in
air at 973 K and 673 K respectively for two hours. Each of the oxides used in the above preparations were exposed to microwaves individually in order to check for their microwave absorbing ability. The metal chalcogenides were prepared as follows. Metal powders (Pb,Zn and Ag) and chalcogens (S, Se and Te) were taken in sealed quartz ampoules evacuated to 10e5 torr, and were exposed to microwaves. Typically 2-3 gram batches were taken and the microwave cxposure times varied from 5 to 20 minutes. The contents in the tube were rocked for proper redistribution two times in between by interrupting the microwave exposure. Generally in all the cases, coloured flashes ‘were seen emerging from the tube for the first few minutes of microwave exposure. The tubes did not explode or break up. The exact preparation times involved for the various compounds are given in the table 1. X-ray diffractograms were recorded for both annealed and unannealed samples of metal vanadates and for the as-prepared metal chalcogenides on a JEOL JDX-8P X-ray Diffractometer. The annealed vanadate samples were further characterized by Differential Scanning Calorimetry
(Perkin
Elmer DSC 2) and Infrared Spectroscopy (Perkin Elmer 580).
Results and Discussion The X-ray diffractograms of both annealed and unannealed metal vanadates are given in Fig.1.
Formation of metal vanadates in the unarmealed samples is evident from the
figure since the positions of the XRD peaks match well with the reported values. However there is a difference in the intensity distribution of the peaks in the unannealed samples as compared to the reported values. In the annealed samples, it is found that the peak positions as well as the intensities match well with those reported in the literature. The calculated lattice parameters of the annealed metal vanadates are given in table 1 along with the literature values given in parentheses. There is very good agreement.
FAST SYNTHESIS
Vol. 30. No. 9
1175
‘lhble 1. Preparative conditions and structural parameters of the metal vanadates and chalcogenides. Compound B4VzOu
PbVs Oe
PbSe PbTe ZnS
Preparation time 15 minutes + (2 hrs. annealing at 973 K) 10 minutes + (2 hrs annealing at 673 K) 10 minutes 4 minutes 20 minutes
Crystal System Orthorhombic
Orthorhombic
Cubic Cubic Hexagonal Cubic Monoclinic
The annealed B4VsOii sample is found to exhibit an endothermic a->@ transition at, around 725 K during a DSC run as reported by earlier workers[6]. We may note that the unannealed sample exhibits only a weak transition in this region. The IR spectrum of annealed PbV206 has a sharp band at 960 cm-’ and broad bands at 870,830, 750,700,530, 470,420 cm-’ all of which agree very well with the earlier reports(7]. The consequences of annealing suggest that the initial unannealed product is perhaps trapped in a metestable state whose structure is close to that of the annealed product. The metastable state may be structurally disordered in which bismuth-oxygen or vanadium-oxygen polyhedra arc randomly oriented. Fig.2 shows the X-ray diffraction patterns of the as prepared binary chalcogenides. The diffractograms do not show any impurity peaks indicating good phase purity. Both the peak positions and the intensities match well with the reported data. The lattice parameters also agree well (See table 1). It is found from individual experiments that VsOs can couple with microwaves efficiently and gets heated up to 980 K whereas BisOs and PbO do not exhibit significant, microwave absorption. It is likely that V20s absorbs microwaves and transfers the energy to the reaction mixture resulting in heating up of the complete charge. In the case of PbV206, the reaction mixture gets melted. (In fact the melt can be quenched into a glass ]81).
1176
Vol. 30. No. 9
B. VAIDHYANATHAN et al.
In the case of metal chalcogenides the fine metal powders are likely to be responsible for the observed microwave absorption because eddy currents are known to be generated
I
10
20
30 10 50 28 (Degrees)
60
FIG. 1. X-ray diffraction patterns of the metal vanadates
10
20
30 LO 50 28 (Dcgrccs)
6
FIG. 2. XRD patterns of the as-prepared metal chalcogenides
in small metal particles which subsequently causes fast heating of the charge. The reaction enthalpies (for PbSe = -18 K cal /g mol; for ZnS = -45.3 K Cal/g mol; for AgzS = -7.01 K Cal/g mol) may also help the rates of the reaction to escalate rapidly. The rapid reaction between metal and chalcogen powders is associated with the coloured flashes seen within the sealed tube during initial stages of microwave exposure. Evidently the rapid reactions involve excitation of covalent bonds in chalcogens. The deexcitation partly results in fluorescent flashes observed in the reaction. Though the tube becomes red hot in most cases, indicating that quite high temperatures (around 1000 K) have been reached within such short times, in no case do the tubes explode. Generally it is found that the microwave exposure time for the preparation of metal chalcogenides varies as sulfides > selenides > tellurides.
Vol. 30. No. 9
FAST SYNTHESIS
1177
It may be appropriate to note here that the conventionel solid state method of preparation of metal vanadates involves stepwise heating (upto 1050 K) of the initial oxide mixtures for about 24 hours in air with intermediate grinding[9]. Similarly the conventional synthesis of metal chalcogenides is also quite diEcult and tedious. For example, the preparation of PbSe involves the direct reaction of the metal and chalcogen mixture at around 1300 K in an evacuated sealed tube for several hours in a rotating furnace[lO]. Controlled heating is necessary to avoid explosion of the tube. The present method, however, is very rapid and simple and yields highly homogeneous products.
Conclusions Metal vanadates and chalcogenides have been prepared using a novel microwave heating method in a fraction of time required for conventional solid state procedures. The products are found to have excellent phase purity and homogeneity.
Acknowledgements The authors thank Prof. C.N.R. l&o, F.R.S for encouragement.
References 1. DRBaghurst,
A.M. Chippindale and D.M.P. Mingos, Nature 332,311
(1988).
2. DR. Baghurst and D.M.P. Mingos, J. Chem. Sot., Chem. Commun. 829 (1988). 3. P.D. Ramesh, B. Vaidhyanathan, M. Ganguli and K.J. Rae, J. Mater. Rev. 9,3026 (1994). 4. CC. Landry and AR. Barron, Science 260, 1653 (1993). 5. A.G. Whittaker and D.M.P. Mingos, J. Chem.>Sot. Dalton ‘Ikans. 2751 (1992). 6. F. Abraham, M.F. Debreuille-Gresse, G. Mairesse and G. Nowogrocki, Sol. St. Ion. 28-30, 529 (1988). 7. A. Tsuzuki, K. Kani, K. Watari and Y. Torii, J. Mater. Sci. 27, 5091 (1992). 8. Unpublished results from Prof. K.J. Rae’s laboratory. 9. K.B.R. Varma, G.N. Subbanna, T.N. Guru Row and C.N.R. Rao, J. Mater. Res. 5(11), 2718 (1990). 10. F.W. Abel, Comprehensive Inorganic Chemistry, edited by A.F. lkouman-Dickenson (Pragmon press, Oxford), 2, 134 (1973).
0025-5408(95)ooo99-2
FAST SOLID STATE SYNTHESIS OF METAL VANADATES AND CHALCOGENIDES USING MICROWAVE IRRADIATION B. Vaidhyanathan,
Munia
Ganguli
and K.J.
Rae
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore012, India. (Received August 3 1, 1994; Communicatedby C.N.R Rae)
Metal vanadates and chalcogenides have been prepared in rapid time scales using a domestic microwave oven. The products exhibit high phase purity and homogeneity. The thermal and spectroscopic properties of the microwave prepared metal vanadates match well with those prepared by conventional methods. ABSTECACT:
MATERIALS
INDEX:
metal vanadates, metal chalcogenides
Introduction
Metal vanadates and chalcogenides belong to a rich category of inorganic solids with technologically important properties. Bismuth and lead vanadates are good ferroelectrics and metal chalcogenides are used in thermoelectric power generators, solar cells, scintillation counters etc. The conventional solid state methods of preparing these materials are quite difficult and rather involved. Recently, it has been realized that microwave radiation can be used to accelerate some of the kinetically slow solid state reactions[l-31. Certain ,metal oxides (like VZOS, WOS etc.) with high dielectric loss values can couple with microwaves efficiently and can get heated up to 1000 K or more. There are few reports wherein rapid heating of metal powders under microwave exposure has been used in the synthesis of metal chalcogenides[4,5]. The most important advantages in microwavca assisted synthesis appear to be the very short time scales required for the preparation and the selectivity in energy transfer from microwave field. In this communication, we report, 1173
B.VAIDHYANA'THANetol.
1174
Vol. 30,No. 9
a fast, clean and simple method of preparing some important metal vanadates and chalcogenides using a kitchen microwave oven.
Experimental The metal vanadates Bi4VzOll grade materials.
Well-mixed
and PbVzOe
stoichionetric
have been prepared starting with reagent mounts
of (VsOs + 2BisOs) and (VzOs +
PbO), with a batch weight of 5 grax~ were taken in clean silica crucibles and placed on a firebrick inside a domestic microwave oven (Batliboy
Eddy) operating at 2.45 GHz with
a tunable power level up to 980 W. The charges were exposed to microwaves for about 10-15 minutes and allowed to cool slowly inside the oven and the cooled products were then ground.
Bi4V20i1 and PbVzOe were then annealed separately in muffle furnace in
air at 973 K and 673 K respectively for two hours. Each of the oxides used in the above preparations were exposed to microwaves individually in order to check for their microwave absorbing ability. The metal chalcogenides were prepared as follows. Metal powders (Pb,Zn and Ag) and chalcogens (S, Se and Te) were taken in sealed quartz ampoules evacuated to 10e5 torr, and were exposed to microwaves. Typically 2-3 gram batches were taken and the microwave cxposure times varied from 5 to 20 minutes. The contents in the tube were rocked for proper redistribution two times in between by interrupting the microwave exposure. Generally in all the cases, coloured flashes ‘were seen emerging from the tube for the first few minutes of microwave exposure. The tubes did not explode or break up. The exact preparation times involved for the various compounds are given in the table 1. X-ray diffractograms were recorded for both annealed and unannealed samples of metal vanadates and for the as-prepared metal chalcogenides on a JEOL JDX-8P X-ray Diffractometer. The annealed vanadate samples were further characterized by Differential Scanning Calorimetry
(Perkin
Elmer DSC 2) and Infrared Spectroscopy (Perkin Elmer 580).
Results and Discussion The X-ray diffractograms of both annealed and unannealed metal vanadates are given in Fig.1.
Formation of metal vanadates in the unarmealed samples is evident from the
figure since the positions of the XRD peaks match well with the reported values. However there is a difference in the intensity distribution of the peaks in the unannealed samples as compared to the reported values. In the annealed samples, it is found that the peak positions as well as the intensities match well with those reported in the literature. The calculated lattice parameters of the annealed metal vanadates are given in table 1 along with the literature values given in parentheses. There is very good agreement.
FAST SYNTHESIS
Vol. 30. No. 9
1175
‘lhble 1. Preparative conditions and structural parameters of the metal vanadates and chalcogenides. Compound B4VzOu
PbVs Oe
PbSe PbTe ZnS
Preparation time 15 minutes + (2 hrs. annealing at 973 K) 10 minutes + (2 hrs annealing at 673 K) 10 minutes 4 minutes 20 minutes
Crystal System Orthorhombic
Orthorhombic
Cubic Cubic Hexagonal Cubic Monoclinic
The annealed B4VsOii sample is found to exhibit an endothermic a->@ transition at, around 725 K during a DSC run as reported by earlier workers[6]. We may note that the unannealed sample exhibits only a weak transition in this region. The IR spectrum of annealed PbV206 has a sharp band at 960 cm-’ and broad bands at 870,830, 750,700,530, 470,420 cm-’ all of which agree very well with the earlier reports(7]. The consequences of annealing suggest that the initial unannealed product is perhaps trapped in a metestable state whose structure is close to that of the annealed product. The metastable state may be structurally disordered in which bismuth-oxygen or vanadium-oxygen polyhedra arc randomly oriented. Fig.2 shows the X-ray diffraction patterns of the as prepared binary chalcogenides. The diffractograms do not show any impurity peaks indicating good phase purity. Both the peak positions and the intensities match well with the reported data. The lattice parameters also agree well (See table 1). It is found from individual experiments that VsOs can couple with microwaves efficiently and gets heated up to 980 K whereas BisOs and PbO do not exhibit significant, microwave absorption. It is likely that V20s absorbs microwaves and transfers the energy to the reaction mixture resulting in heating up of the complete charge. In the case of PbV206, the reaction mixture gets melted. (In fact the melt can be quenched into a glass ]81).
1176
Vol. 30. No. 9
B. VAIDHYANATHAN et al.
In the case of metal chalcogenides the fine metal powders are likely to be responsible for the observed microwave absorption because eddy currents are known to be generated
I
10
20
30 10 50 28 (Degrees)
60
FIG. 1. X-ray diffraction patterns of the metal vanadates
10
20
30 LO 50 28 (Dcgrccs)
6
FIG. 2. XRD patterns of the as-prepared metal chalcogenides
in small metal particles which subsequently causes fast heating of the charge. The reaction enthalpies (for PbSe = -18 K cal /g mol; for ZnS = -45.3 K Cal/g mol; for AgzS = -7.01 K Cal/g mol) may also help the rates of the reaction to escalate rapidly. The rapid reaction between metal and chalcogen powders is associated with the coloured flashes seen within the sealed tube during initial stages of microwave exposure. Evidently the rapid reactions involve excitation of covalent bonds in chalcogens. The deexcitation partly results in fluorescent flashes observed in the reaction. Though the tube becomes red hot in most cases, indicating that quite high temperatures (around 1000 K) have been reached within such short times, in no case do the tubes explode. Generally it is found that the microwave exposure time for the preparation of metal chalcogenides varies as sulfides > selenides > tellurides.
Vol. 30. No. 9
FAST SYNTHESIS
1177
It may be appropriate to note here that the conventionel solid state method of preparation of metal vanadates involves stepwise heating (upto 1050 K) of the initial oxide mixtures for about 24 hours in air with intermediate grinding[9]. Similarly the conventional synthesis of metal chalcogenides is also quite diEcult and tedious. For example, the preparation of PbSe involves the direct reaction of the metal and chalcogen mixture at around 1300 K in an evacuated sealed tube for several hours in a rotating furnace[lO]. Controlled heating is necessary to avoid explosion of the tube. The present method, however, is very rapid and simple and yields highly homogeneous products.
Conclusions Metal vanadates and chalcogenides have been prepared using a novel microwave heating method in a fraction of time required for conventional solid state procedures. The products are found to have excellent phase purity and homogeneity.
Acknowledgements The authors thank Prof. C.N.R. l&o, F.R.S for encouragement.
References 1. DRBaghurst,
A.M. Chippindale and D.M.P. Mingos, Nature 332,311
(1988).
2. DR. Baghurst and D.M.P. Mingos, J. Chem. Sot., Chem. Commun. 829 (1988). 3. P.D. Ramesh, B. Vaidhyanathan, M. Ganguli and K.J. Rae, J. Mater. Rev. 9,3026 (1994). 4. CC. Landry and AR. Barron, Science 260, 1653 (1993). 5. A.G. Whittaker and D.M.P. Mingos, J. Chem.>Sot. Dalton ‘Ikans. 2751 (1992). 6. F. Abraham, M.F. Debreuille-Gresse, G. Mairesse and G. Nowogrocki, Sol. St. Ion. 28-30, 529 (1988). 7. A. Tsuzuki, K. Kani, K. Watari and Y. Torii, J. Mater. Sci. 27, 5091 (1992). 8. Unpublished results from Prof. K.J. Rae’s laboratory. 9. K.B.R. Varma, G.N. Subbanna, T.N. Guru Row and C.N.R. Rao, J. Mater. Res. 5(11), 2718 (1990). 10. F.W. Abel, Comprehensive Inorganic Chemistry, edited by A.F. lkouman-Dickenson (Pragmon press, Oxford), 2, 134 (1973).