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1.
Lead titanate, barium titanate, and lead barium titanate powders (>99.9% pure), the particle size of which varied from 0.03 to 0.15 μm depending on the calcination temperature and the composition, was prepared from barium lead titanyl oxalate, which was previously prepared by reacting high-purity ammonium titanyl oxalate with barium and lead acetate. The critical crystallite size of BaTiO3 powder from the cubic to the tetragonal phase is around 1 μm. Pb0.3Ba0.7TiO3 powder with an average size of 0.057 μm showed the tetragonal phase.  相似文献   

2.
Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO2 (70 nm) with BaCO3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO3 in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO3 already leads to formation of the single-phase BaTiO3 after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d 50=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO3 and TiO2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.  相似文献   

3.
A process for synthesis of ultrafine YBa2Cu3O7–x powder by oxalate coprecipitation from nearly saturated solutions of the metal acetates and a 2-propanol solution of oxalic acid was developed. The coprecipitation was complete within 5 min in an ice bath at 0–2°C. The final stoichiometry was Y:Ba:Cu = 1:1.994:2.991, while the particle size and surface area in the homogeneous coprecipitated powder were 0.1–0.2 pm and 24.9 m2.g−1, respectively. Because of the uniformity and particle size of the coprecipitated material, reactive YBCO powder with a surface area of 1.7 m2.g−1 can be obtained at 780°C in about 12 h.  相似文献   

4.
A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC2O4·0.5H2O and TiO(OH)2·1.5H2O in a molar ratio of 1:4. Crystallized BaTi4O9 was obtained by the thermal decomposition of a precipitate precursor at 1300°C for 2 h in air. The dimensions of the powder calcined at 1000°C are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300°C are of the order of 10 to 30 μm. Dielectric properties of disk-shaped sintered specimens in the microwave frequency region were measured using the TE011 mode. Excellent microwave characteristics for BaTi4O9—ɛ= 38 ± 0.5, Q = 3800–4000 at 6–7 GHz and τ f = 11 ± 0.7 ppm/°C—were found.  相似文献   

5.
Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), with 3 wt% excess PbO content has been successfully prepared via a microemulsion processing technique. By stepwise hydrolysis using aqueous ammonia as the precipitant, hydroxide precursor was obtained from nitrate solutions dispersed in the nanosized aqueous domains of a microemulsion consisting of cyclohexane, mixed poly(oxyethylene)5 nonyl phenol ether (NP5) and poly(oxyethylene)9 nonyl phenol ether (NP9), and an aqueous phase. Upon calcination of the microemulsion-derived precursor at 780°C, an ultrafine perovskite PMN powder with less than 5% pyrochlore phase was obtained. The percentage of perovskite phase increases with increasing calcination temperature, reaching ∼98% at 900°C. The resulting PMN powder exhibits a near-spherical particle morphology although particle agglomerates of ∼0.3 µm in average diameter occur when they are calcined at 850°C. When sintered at 1150°C, the microemulsion-derived PMN showed a maximum relative density of ∼95.6% theoretical density, which gives a maximum dielectric constant of ∼11 000 at 1 kHz and a Curie temperature of -6°C.  相似文献   

6.
When preparing homogeneous, fine barium titanate powders, the major difficulty is to avoid the spontaneous self-condensation between the Ti-OH groups. In the usual way of preparing fine barium titanate powders, chelating agents (citrate, oxalate) or simply unidentate ligands (alkoxy or carboxyl groups) are used to complex titanium atoms. Another way is to mix barium and titanium precursors in a strongly basic medium. The condensation between the Ti(OH)2-6Ba2+ species directly gives the perovskite compound. Using an alkoxide-hydroxide route, a homogeneous Ba-Ti solution was prepared that completely advanced by condensation between the Ti(OH)2-6Ba2+ species and led to a controlled-stoichiometry powder. Concerning pure barium titanate, dried powders exhibited the cubic perovskite structure, and a direct sintering at 1150°C, without calcination, led to highly dense BaTiO3 bodies with fine-grained uniform microstructure (1 μm) that exhibited a high permittivity value at room temperature ( K = 5400). The alkoxide-hydroxide method was also used to prepare dense alkaline-earth perovskite ceramics with complex compositions.  相似文献   

7.
An important molecular precursor to barium titanate, namely, barium titanyl oxalate [BaTiO(C2O4)2.4H2O], has been synthesized by an alternative route. An alcoholic solution containing 1 mol of butyl titanate monomer [(C4H9O)4Ti] is reacted with alcoholic solution containing 2 mol of oxalic acid (H2C2O4:2H2O) to form an intermediate soluble oxalotitanic acid [H2TiO(C2O4)2.nH2O]. The oxalotitanic acid in alcoholic medium is subjected to cation exchange reaction with aqueous solution containing equimolar barium acetate to form an insoluble barium titanyl oxalate (BTO) in yields of 80–85% at room temperature. The pyrolysis of BTO in air at T .750°C/5 h produced barium titanate (BT) powders.  相似文献   

8.
The peroxo-oxalate complexation method is a method that can be used for the preparation of doped barium titanate. In this paper we focus on BaTi0.91Zr0.09O3, which can be used for discharge capacitors in lamp starters. The preparation method described here is based on the complexation and subsequent precipitation in basic environment of Ba, Ti, and Zr ions with hydrogen peroxide and oxalate. The influence of several process parameters, like precipitation temperature and pH, on powder properties is described. A single-phase perovskite crystal structure is obtained after calcination starting from a chloride precursor solution using a precipitation temperature of 40°C and a pH of 9. Because the peroxo-oxalate process starts with inexpensive chlorides and is performed in air, the peroxo-oxalate process is suitable for the commercial production of doped barium titanate.  相似文献   

9.
Two methods used to synthesize high-purity ferroelectric titanate powders in controlled, narrow size distributions, with average particle diameters <1000 A, were: (1) isothermal pyrolysis of barium titanyl oxalate or mixed calcium-barium titanyl oxalates as low as 550° and 825°C, respectively, average particle sizes depending strongly on the pyrolysis temperature; and (2) hydrolysis of titanate esters in barium hydroxide. Using solvent media of controlled polarity, high-purity stoichiometric BaTiO3 was obtained with average sizes as small as 100 A. Factors affecting stoichiometry and particle size are discussed in terms of assumed reaction mechanisms.  相似文献   

10.
Ultrafine 5.5 mol% CeO2—2 mol% YO1.5ZrO2 powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m2/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.  相似文献   

11.
A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH4AlY0.6(CO3)1.9(OH)2·0.9H2O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.  相似文献   

12.
Most of the chemistry-based preparation routes for bismuth titanate (BIT) involve calcination at elevated temperatures in order to realize precursor-to-ceramic conversion. In a completely different approach using an amorphous BIT hydroxide precursor, nanocrystalline particles of layered perovskite BIT are synthesized by mechanical activation, skipping the detrimental crystallite coarsening and particle aggregation encountered at high temperatures. Mechanical activation leads to nucleation and steady growth of BIT crystallites in the amorphous precursor matrix, while Bi2O3 is involved as an intermediate transitional phase. The activation-derived BIT particles demonstrate a rounded morphology of ∼50 nm in size. This is in contrast to the BIT derived from calcination of the coprecipitated precursor at 600°C that is dominated by coarsened platelike particles. The former is sintered to a density of >95% theoretical at 875°C for 2 h, leading to a dielectric constant of ∼1260 when measured at 1 MHz and the Curie temperature of 646°C.  相似文献   

13.
MgO (10.4 mol%)-doped ZrO2 powder, which was prepared by coprecipitation and calcination at 750°C, was leached and milled, respectively. The powders were characterized by Auger electron scanning microprobe and XRD technologies. Only the 2.3 mol% MgO was soluble in ZrO2; it stabilized ZrO2 as the metastable tetragonal phase (crystallite size ∼45 nm) at room temperature. The rest of the MgO (8.1 mol%) existed on the surface of the ZrO2 grains in the prepared powder.  相似文献   

14.
An experimental study has been conducted to evaluate the formation of nano α-Al2O3 under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al2O3 powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al2O3, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al2O3 was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.  相似文献   

15.
We report here the fabrication of transparent Sc2O3 ceramics via vacuum sintering. The starting Sc2O3 powders are pyrolyzed from a basic sulfate precursor (Sc(OH)2.6(SO4)0.2·H2O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc2O3 powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc2O3 powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc2O3 ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc2O3 single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.  相似文献   

16.
Fine-grained powder of the mixed oxide (CeO2)0.9(Gd2O3)0.1, which is an ionic conductor for oxygen ions, was prepared by coprecipitation of the corresponding oxalates followed by calcination. The powder was used to prepare pellets sintered at a relatively low temperature of 1000°C compared with the usual sintering temperature of 1700° to 1800°C. The size of the powder grains was determined from BET surface area (SBET) measurements. The effect of precipitation conditions and calcination temperature on Sbet was examined. The largest surface area measured was 88 m2/g. Decomposition of the oxalate powder was followed using an optical dilatometer. The decomposition was indicated by a large shrinkage and it was completed below 300°C (for a heating rate of 3.3°C/min). The formation of the oxide was verified by X–ray diffraction analysis. It shows that the product of decomposition is the oxide and that decomposition can be carried to completion at 250°C if the heating lasts for 1 h. The pellets had a density of 83% of theoretical, small grains (0.5 μm), and a conductivity which, at 900°C, is two–thirds of the conductivity of dense samples obtained from the same raw material, but calcined and fired at much higher temperatures.  相似文献   

17.
Monodisperse and spherical barium titanate (BaTiO3) powders with diameters of 200–470 nm were directly prepared by a low-temperature hydrothermal method at 90°C. Spherical titania (TiO2) powders, ranging in size from 150 to 420 nm, were initially prepared by a controlled hydrolysis and condensation reaction, aged in a highly alkaline solution for 12 h, and then hydrothermally reacted with barium hydroxide to be converted to BaTiO3 without a morphological change. The aging step of the TiO2, where the surface of TiO2 was highly densified through elimination of the pores, was indispensable to retain the sizes and shapes of TiO2 in the resulting BaTiO3. This was due to the fact that the formation of BaTiO3 proceeded by an in situ reaction mechanism. The resulting BaTiO3 powders exhibited dense and nonporous structures even after calcination at 1000°C.  相似文献   

18.
Nanosized lead zirconate (PbZrO3) powder was synthesized from its oxalate precursor, namely lead zirconyl oxalate (LZO). LZO heated in a microwave heating system for 1 h yielded the PbZrO3 at 600°C. The same precursor (LZO), when heated in a resistance-heated furnace at 850°C for 3 h, does not give a pure product. Thermogravimetry, differential thermal analysis, and X-ray diffraction techniques were used to characterize the precursor and optimize the conditions for microwave processing. The particle size of PbZrO3 powder prepared at 600°C using microwave heating was measured using transmission electron microscopy (TEM). The TEM images show that the particles of PbZrO3 are spherical in shape and that the particle size varies between 20 and 22 nm.  相似文献   

19.
Fine Strontium Ferrite Powders from an Ethanol-Based Microemulsion   总被引:1,自引:0,他引:1  
A fine strontium ferrite powder with high coercivity was successfully prepared by forming hydroxide precursor particles in the continuous ethanol-based phase of a microemulsion consisting of iso-octane, NP9, and an ethanol solution containing Sr2+ and Fe3+ cations at a molar ratio of 1:12. The microemulsion-derived hydroxide precursor was calcined at various temperatures, ranging from 600° to 1100°C, to develop the hexagonal strontium ferrite phase. X-ray diffractometry and infrared characterizations revealed that the formation mechanisms of strontium ferrite in the microemulsion-derived precursor differed from those of the precursor derived by conventional coprecipitation. The microemulsion resulted in a strontium ferrite of finer particle size and better magnetic properties than those of the conventionally coprecipitated strontium ferrite. The microemulsion-derived strontium ferrite exhibited an intrinsic coercivity of 6195 Oe and a saturation magnetization of 58.28 emu/g when calcined at 900oC. The saturation magnetization increased further, to 69.75 emu/g, when the microemulsion-derived precursor was calcined at 1100oC.  相似文献   

20.
Reactive Ceria Nanopowders via Carbonate Precipitation   总被引:3,自引:0,他引:3  
Nanocrystalline CeO2 powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce3+ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO3·2.5H2O, decomposes to CeO2 at temperatures ≥300°C. The CeO2 powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.  相似文献   

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