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Low temperature synthesis of MgTa2O6 powders
Materials Letters 59 (2005) 3926 – 3928 www.elsevier.com/locate/matlet
Low temperature synthesis of MgTa2O6 powders S.C. Navale a, V. Samuel b, V. Ravi c,* a
c
Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India b Catalysis Division, National Chemical Laboratory, Pune 411008, India Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India Received 25 February 2005; accepted 14 July 2005 Available online 8 August 2005
Abstract A combination of digestion and further low temperature calcination to crystallize the product is employed to prepare MgTa2O6 (MT) ceramics. Freshly prepared niobium hydroxide gel is mixed with magnesium hydroxide thoroughly and allowed to react at 100 -C under refluxing and stirring conditions for 6 – 12 h. The X-ray amorphous product so formed is heated at 550 -C to form crystalline MgTa2O6. This is the lowest temperature so far reported for the formation of MgTa2O6. For comparison, MT powders were also prepared by the traditional solid state method. Transmission electron microscope (TEM) investigations revealed that the average particle size is 40 nm for the low temperature calcined powders. D 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Oxides; X-ray diffraction; Electron microscopy; Magnesium tantalate
1. Introduction Communication at microwave frequencies has lead to the proliferation of commercial wireless technologies such as cellular phones and global positioning systems. The requirement of ceramic dielectric resonators used at microwave frequencies are high dielectric constant, a high Q value (reciprocal of dielectric loss) and a low temperature coefficient of resonant frequency. Magnesium tantalate and its solid solution with magnesium niobate found applications as microwave dielectric materials due to their low dielectric loss and high dielectric constant. [1– 6]. MgTa2O6 has trirutile structure (space group P42/mnm)and ˚ and c = 9.20 A ˚ [2]. its unit cell parameters are a = 4.719 A The refractive index of MT for ordinary and extraordinary rays are 2.07 and 2.18, respectively, under white light while birefringence is 0.11. Accordingly MT single crystals can serve as polarizing devices [1]. There have been reports for MT as a photocatalyst for water purification [4] and in
* Corresponding author. Fax: +91 20 25893044. E-mail address: [email protected] (V. Ravi). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.035
luminescent applications [5]. Traditionally MT is prepared by ceramic method and that leads to inhomogeneity in composition and coarse particles. The properties of ceramics are greatly affected by the characteristics of the powder, such as particle size, morphology, purity and chemical composition. Using chemical methods, e.g. co-precipitation, sol –gel, hydrothermal and colloid emulsion technique have been confirmed to efficiently control the morphology and chemical composition of prepared powder. Among the reports of these wet chemical techniques sol –gel using alkoxides, hydrothermal and colloid emulsions are time consuming and involve highly unstable alkoxides and difficult to maintain reaction conditions. Here we communicate a simple procedure to prepare MgTa2O6 powders at low temperatures. The method of gel to crystalline conversion is reported in the literature for the preparation of multicomponent oxides such as perovskites and spinels [7]. We have used this technique to prepare a series of oxides such as TiO2 [8], SnO2 [9], ZnO[10] and Mn3O4 [11]. But this method is not yet reported for the preparation of MgTa2O6. It is found that considerable duration (¨15 h) of digestion at 100 -C does not yield crystalline MgTa2O6. A relatively higher temperature calcination is necessary to
(220) (114)
(202) (104)
(200) (201)
(112)
(011)
Intensity (a.u.)
3927
a period of 5 s per step in the step scan mode. Silicon was used as an internal standard. Least squares method was employed to determine the lattice parameters. The TEM picture was recorded with JEOL model 1200 EX instrument at the accelerating voltage of 100 kV. The fine powders were dispersed in amyl acetate on a carbon coated TEM copper grid.
(213)
Cu K α
(103)
(110)
S.C. Navale et al. / Materials Letters 59 (2005) 3926 – 3928
(b) 3. Results and discussion
(a) 20
30
40
50
60
2θ, degrees Fig. 1. XRD of MT precursor powder heated (a) after digestion at 100 -C and (b) at 550 -C.
form the crystalline product. This process can avoid complex steps such as refluxing of alkoxides, resulting in less time consumption compared to other techniques. This method is not reported for the preparation of MT powders in the literature.
2. Experimental For preparing MgTa2O6, tantalum(V) oxide, magnesium nitrate and sodium hydroxide were used as starting materials, and all were of AR grade (Loba chemie). A stoichiometric amount of Mg(NO3)2I6H2O was dissolved in distilled water and added to TaF5 solution. TaF5 is prepared by dissolving required quantity of Ta2O5 in minimum amount of HF after heating at hot water bath for 20 h. Then, sodium hydroxide solution was added with constant stirring to the above solution mixture until pH > 12 to ensure complete precipitation. After filtration the mixed hydroxides precipitate was washed several times and transferred to flask fitted with a water condenser. The precipitate was continuously stirred for 6 h and temperature was maintained around 70 – 100 -C by heating in a Rotomantle. Then, the powder formed was filtered and oven dried. The dried powders are calcined at different temperatures ranging from 400 to 700 -C for 6 h. For comparison, MT samples are also prepared by ceramic method. The corresponding oxides or carbonates are taken in stoichiometric ratio and mixed, ground several times and heated at 900 -C for 12 h. Various techniques such as XRD (Rigaku miniflex Diffractometer) and TEM were employed to characterize these powders. The powder X-ray pattern were recorded for all the samples sintered at various temperatures by using Philips PW-1710 model X-ray diffractometer using Cu-K”. For lattice parameter and interplanar distance (d) calculation, the samples were scanned in the 2h range of 10– 80- for
Fig. 1 shows the XRD pattern of MgTa2O6 powder formed after digestion and after calcination at 550 -C. The product at 100 -C is X-ray amorphous and further annealing at 550 -C yields the crystalline product. This is the lowest temperature for formation of MT phase so far reported in the literature. The reported temperature of formation of MT phase by the solid state method is 900 -C [1 – 3]. There are no reports of wet chemical preparative routes for this compound. The crystal structure of MT is tetragonal (trirutile type) and all the d-line pattern matches with reported values [2]. It is to be noted that when precipitate is not washed completely, the reaction may proceed very slowly. The precipitate should not be aged also as this also impedes the reaction. The calculated lattice parameters ˚ and c = 9.213 A ˚ . Metal by least square fit are a = 4.761 A hydroxide gels are in general polymeric chains forming an entangled network in which solvent is entrapped. It is the osmotic pressure, which is the sum of rubber elasticity, polymer – polymer affinity and hydrogen ion pressure that contributes to stability of the gel. If any one of the factors is altered, the gel collapses irreversibly. Thus very minute nuclei of the product phase are forming at 100 -C. Conventional solid state method also forms MT phase at 900 -C [3] with comparatively larger particle size of ¨1 Am. The particle size and morphology of the calcined powders were examined by transmission electron microscopy. Particle morphology of calcined powder (550 -C for 6 h) prepared by the coprecipitation process was irregular in shape, with an average primary particle size around 40 nm (Fig. 2). The average particle size calculated from Scherrer’s formula (t = K k / Bcosh B) where t
Fig. 2. TEM of MT powder calcined at 550 -C.
3928
S.C. Navale et al. / Materials Letters 59 (2005) 3926 – 3928
is the average size of the particles, assuming particles are spherical, K = 0.9, k is the wavelength of X-ray radiation, B is the full width at half maximum of the diffracted peak and h B is the angle of diffraction is 50 nm. The average particle size of MT powders prepared by conventional ceramic method was 1 Am (not shown).
4. Conclusions A simple two step process is adopted for the preparation of ultrafine powders of MgTa2O6. The temperature at which MT phase forms is the lowest so far reported in the literature.
Acknowledgment One of the authors (V.R) acknowledges DST, Govt. of India (grant no.SP/S1/ H-19/2000) for financial assistance.
References [1] [2] [3] [4] [5] [6] [7] [8]
D.C. Sun, S. Senz, D. Hesse, J. Eur. Ceram. Soc. 24 (2004) 2453. C.R. Ferrai, A.C. Hernandes, J. Eur. Ceram. Soc. 22 (2002) 2101. M. Thirumal, A.K. Ganjuli, Mater. Res. Bull. 36 (2001) 2421. H. Kato, A. Kudo, Chem. Phys. Lett. 295 (1998) 487. G. Blasse, J. Solid State Chem. 72 (1998) 72. H.J. Lee, I.T. Kim, K.S. Hong, Jpn. J. Appl. Phys. 36 (1997) L1318. T.R.N. Kutty, P. Padmini, Mater. Res. Bull. 27 (8) (1992) 945. Sanjay R. Dhage, V. Choube, Violet Samuel, V. Ravi, Mater. Lett. 58 (2004) 2310. [9] S.R. Dhage, S.P. Gaikwad, V. Samuel, V. Ravi, Bull. Mater. Sci. 27 (2004) 221. [10] S.R. Dhage, R. Pasricha, V. Ravi, Mater. Lett. 59 (2005) 779. [11] M. Anilkumar, V. Ravi, Mater. Res. Bull. 40 (2005) 605.
Low temperature synthesis of MgTa2O6 powders S.C. Navale a, V. Samuel b, V. Ravi c,* a
c
Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India b Catalysis Division, National Chemical Laboratory, Pune 411008, India Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India Received 25 February 2005; accepted 14 July 2005 Available online 8 August 2005
Abstract A combination of digestion and further low temperature calcination to crystallize the product is employed to prepare MgTa2O6 (MT) ceramics. Freshly prepared niobium hydroxide gel is mixed with magnesium hydroxide thoroughly and allowed to react at 100 -C under refluxing and stirring conditions for 6 – 12 h. The X-ray amorphous product so formed is heated at 550 -C to form crystalline MgTa2O6. This is the lowest temperature so far reported for the formation of MgTa2O6. For comparison, MT powders were also prepared by the traditional solid state method. Transmission electron microscope (TEM) investigations revealed that the average particle size is 40 nm for the low temperature calcined powders. D 2005 Elsevier B.V. All rights reserved. Keywords: Ceramics; Oxides; X-ray diffraction; Electron microscopy; Magnesium tantalate
1. Introduction Communication at microwave frequencies has lead to the proliferation of commercial wireless technologies such as cellular phones and global positioning systems. The requirement of ceramic dielectric resonators used at microwave frequencies are high dielectric constant, a high Q value (reciprocal of dielectric loss) and a low temperature coefficient of resonant frequency. Magnesium tantalate and its solid solution with magnesium niobate found applications as microwave dielectric materials due to their low dielectric loss and high dielectric constant. [1– 6]. MgTa2O6 has trirutile structure (space group P42/mnm)and ˚ and c = 9.20 A ˚ [2]. its unit cell parameters are a = 4.719 A The refractive index of MT for ordinary and extraordinary rays are 2.07 and 2.18, respectively, under white light while birefringence is 0.11. Accordingly MT single crystals can serve as polarizing devices [1]. There have been reports for MT as a photocatalyst for water purification [4] and in
* Corresponding author. Fax: +91 20 25893044. E-mail address: [email protected] (V. Ravi). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.035
luminescent applications [5]. Traditionally MT is prepared by ceramic method and that leads to inhomogeneity in composition and coarse particles. The properties of ceramics are greatly affected by the characteristics of the powder, such as particle size, morphology, purity and chemical composition. Using chemical methods, e.g. co-precipitation, sol –gel, hydrothermal and colloid emulsion technique have been confirmed to efficiently control the morphology and chemical composition of prepared powder. Among the reports of these wet chemical techniques sol –gel using alkoxides, hydrothermal and colloid emulsions are time consuming and involve highly unstable alkoxides and difficult to maintain reaction conditions. Here we communicate a simple procedure to prepare MgTa2O6 powders at low temperatures. The method of gel to crystalline conversion is reported in the literature for the preparation of multicomponent oxides such as perovskites and spinels [7]. We have used this technique to prepare a series of oxides such as TiO2 [8], SnO2 [9], ZnO[10] and Mn3O4 [11]. But this method is not yet reported for the preparation of MgTa2O6. It is found that considerable duration (¨15 h) of digestion at 100 -C does not yield crystalline MgTa2O6. A relatively higher temperature calcination is necessary to
(220) (114)
(202) (104)
(200) (201)
(112)
(011)
Intensity (a.u.)
3927
a period of 5 s per step in the step scan mode. Silicon was used as an internal standard. Least squares method was employed to determine the lattice parameters. The TEM picture was recorded with JEOL model 1200 EX instrument at the accelerating voltage of 100 kV. The fine powders were dispersed in amyl acetate on a carbon coated TEM copper grid.
(213)
Cu K α
(103)
(110)
S.C. Navale et al. / Materials Letters 59 (2005) 3926 – 3928
(b) 3. Results and discussion
(a) 20
30
40
50
60
2θ, degrees Fig. 1. XRD of MT precursor powder heated (a) after digestion at 100 -C and (b) at 550 -C.
form the crystalline product. This process can avoid complex steps such as refluxing of alkoxides, resulting in less time consumption compared to other techniques. This method is not reported for the preparation of MT powders in the literature.
2. Experimental For preparing MgTa2O6, tantalum(V) oxide, magnesium nitrate and sodium hydroxide were used as starting materials, and all were of AR grade (Loba chemie). A stoichiometric amount of Mg(NO3)2I6H2O was dissolved in distilled water and added to TaF5 solution. TaF5 is prepared by dissolving required quantity of Ta2O5 in minimum amount of HF after heating at hot water bath for 20 h. Then, sodium hydroxide solution was added with constant stirring to the above solution mixture until pH > 12 to ensure complete precipitation. After filtration the mixed hydroxides precipitate was washed several times and transferred to flask fitted with a water condenser. The precipitate was continuously stirred for 6 h and temperature was maintained around 70 – 100 -C by heating in a Rotomantle. Then, the powder formed was filtered and oven dried. The dried powders are calcined at different temperatures ranging from 400 to 700 -C for 6 h. For comparison, MT samples are also prepared by ceramic method. The corresponding oxides or carbonates are taken in stoichiometric ratio and mixed, ground several times and heated at 900 -C for 12 h. Various techniques such as XRD (Rigaku miniflex Diffractometer) and TEM were employed to characterize these powders. The powder X-ray pattern were recorded for all the samples sintered at various temperatures by using Philips PW-1710 model X-ray diffractometer using Cu-K”. For lattice parameter and interplanar distance (d) calculation, the samples were scanned in the 2h range of 10– 80- for
Fig. 1 shows the XRD pattern of MgTa2O6 powder formed after digestion and after calcination at 550 -C. The product at 100 -C is X-ray amorphous and further annealing at 550 -C yields the crystalline product. This is the lowest temperature for formation of MT phase so far reported in the literature. The reported temperature of formation of MT phase by the solid state method is 900 -C [1 – 3]. There are no reports of wet chemical preparative routes for this compound. The crystal structure of MT is tetragonal (trirutile type) and all the d-line pattern matches with reported values [2]. It is to be noted that when precipitate is not washed completely, the reaction may proceed very slowly. The precipitate should not be aged also as this also impedes the reaction. The calculated lattice parameters ˚ and c = 9.213 A ˚ . Metal by least square fit are a = 4.761 A hydroxide gels are in general polymeric chains forming an entangled network in which solvent is entrapped. It is the osmotic pressure, which is the sum of rubber elasticity, polymer – polymer affinity and hydrogen ion pressure that contributes to stability of the gel. If any one of the factors is altered, the gel collapses irreversibly. Thus very minute nuclei of the product phase are forming at 100 -C. Conventional solid state method also forms MT phase at 900 -C [3] with comparatively larger particle size of ¨1 Am. The particle size and morphology of the calcined powders were examined by transmission electron microscopy. Particle morphology of calcined powder (550 -C for 6 h) prepared by the coprecipitation process was irregular in shape, with an average primary particle size around 40 nm (Fig. 2). The average particle size calculated from Scherrer’s formula (t = K k / Bcosh B) where t
Fig. 2. TEM of MT powder calcined at 550 -C.
3928
S.C. Navale et al. / Materials Letters 59 (2005) 3926 – 3928
is the average size of the particles, assuming particles are spherical, K = 0.9, k is the wavelength of X-ray radiation, B is the full width at half maximum of the diffracted peak and h B is the angle of diffraction is 50 nm. The average particle size of MT powders prepared by conventional ceramic method was 1 Am (not shown).
4. Conclusions A simple two step process is adopted for the preparation of ultrafine powders of MgTa2O6. The temperature at which MT phase forms is the lowest so far reported in the literature.
Acknowledgment One of the authors (V.R) acknowledges DST, Govt. of India (grant no.SP/S1/ H-19/2000) for financial assistance.
References [1] [2] [3] [4] [5] [6] [7] [8]
D.C. Sun, S. Senz, D. Hesse, J. Eur. Ceram. Soc. 24 (2004) 2453. C.R. Ferrai, A.C. Hernandes, J. Eur. Ceram. Soc. 22 (2002) 2101. M. Thirumal, A.K. Ganjuli, Mater. Res. Bull. 36 (2001) 2421. H. Kato, A. Kudo, Chem. Phys. Lett. 295 (1998) 487. G. Blasse, J. Solid State Chem. 72 (1998) 72. H.J. Lee, I.T. Kim, K.S. Hong, Jpn. J. Appl. Phys. 36 (1997) L1318. T.R.N. Kutty, P. Padmini, Mater. Res. Bull. 27 (8) (1992) 945. Sanjay R. Dhage, V. Choube, Violet Samuel, V. Ravi, Mater. Lett. 58 (2004) 2310. [9] S.R. Dhage, S.P. Gaikwad, V. Samuel, V. Ravi, Bull. Mater. Sci. 27 (2004) 221. [10] S.R. Dhage, R. Pasricha, V. Ravi, Mater. Lett. 59 (2005) 779. [11] M. Anilkumar, V. Ravi, Mater. Res. Bull. 40 (2005) 605.