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1.
Forsterite MgSiO4 and enstatite MgSiO3 were synthesized by two different aqueous processes. TEOS was directly hydrolyzed in aqueous solutions of magnesium nitrate, giving solutions of magnesium nitrate and silicic acid. For the first process these solutions were spray-dried and the powders heat treated to decompose the nitrate; and for the second one they were precipitated in a solution of ethylenediamine as a base, the resulting precipitate was filtered, washed and dried. Spray-dried or precipitated, no specific thermal event was detected by thermal analysis for the crystallization of forsterite (500–1000°C) while a strong and sharp exothermic peak traduced the crystallization of enstatite at 800°C. Very minor secondary phases could be detected by X-ray diffraction up to 1200°C for the spray-dried powders, while the precipitated powders presented a higher chemical homogeneity, but much care had to be taken for a quantitative precipitation. As some minor secondary phases like SiO2 or some polymorphs of MgSiO3 could be not detected by XRD up to 1300°C, higher thermal treatments were necessary to control the purity or the desired phase.  相似文献   

2.
Forsterite (Mg2SiO4) powders were prepared by mixing SiCl4 with aqueous solutions of either Mg(CH3COO)2·4H2O or Mg(NO3)2·6H2O and heating the powdered gel. The powders were characterised using thermal analysis (DTA and TGA), X-ray diffraction (XRD), nitrogen adsorption surface area analysis (BET) and transmission electron microscopy (TEM). On heating, MgO and enstatite (MgSiO3) were observed in addition to forsterite. On heating to 1200°C, forsterite was the dominant phase in the powders produced from Mg(NO3)2·6H2O, and MgO was the dominant phase in the powders produced from Mg(CH3COO)2·4H2O. The primary particle sizes of these powders were between 100 and 500 nm, which remained the same on heat treatment. However, higher temperatures gave rise to an increase in the size and densities of the agglomerates of primary particles.  相似文献   

3.
Pb(Zr0.50Ti0.50)O3 solid solution was prepared using lead acetate and transition metal n-propoxides in n-propanol or n-butoxides in n-butanol. The complex solutions were hydrolysed with an excess of H2O. The resultant powders were calcined up to 700°C for 30 minutes in a flowing oxygen atmosphere. Scanning electron microscope analysis showed different morphologies of the resultant powders. The n-propoxide derived powder consisted of gel fragments, while the n-butoxide derived one had agglomerated submicrometre particles. EDS analysis of the powders revealed no chemical heterogeneities in the examined samples upon calcining up to 600°C, notwithstanding the type of precursor used. Both samples, calcined at 700°C, exhibited a slight deficiency of lead in the pyrochlore type phase as compared to the perovskite phase.  相似文献   

4.
The effect of surface segregation in Sb- and In-doped SnO2 fine-grained powders has been analyzed in comparison with single-crystalline samples. The kinetics and thermodynamics of the Sb and In segregation processes were studied as a function of annealing temperature by X-ray photoelectron spectroscopy (XPS) after annealing in an oxygen-containing atmosphere. Significant differences between diffusion and segregation were revealed for doped powders and single crystals, obviously because of simultaneous diffusion and particle-growth processes proceeding during annealing of powders. For doped single crystals the thermodynamic equilibrium is approached after 24 h annealing above 850 °C and at 1000 °C for Sb and In, respectively. Higher effective activation energies of diffusion are observed for doped powders and the thermodynamic equilibrium is not achieved under technologically relevant annealing conditions. On the basis of dopant profile measurements anomalies in the electrical resistivity at 300 °C of Sb-doped SnO2 powders annealed at 700 and 900 °C were attributed to an Sb-depleted zone formed beneath the segregated surface during the kinetic regime. To achieve optimum resistivity behavior for commercial application, inhomogeneous doping of powders must be avoided by appropriate preparation steps.  相似文献   

5.
Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO x by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C4H9O)3. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N2 adsorption. The ETA curves show the 220Rn-release rate, E, of the samples, previously labelled with 228Th and 224Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.  相似文献   

6.
A novel and cost-effective sol-gel process for preparation of MgAl2O4 spinel nanometer powders has been developed in this study. A solution of magnesium and aluminum nitrates in stoichiometric proportion was successfully embraced in the biology polysaccharide gel network, formed by the synergistic interaction between xanthan gum (XG) and locust bean gum (LBG) utilizing their broad-spectrum stability of salt tolerance and character of transformation from sol to gel on the condition of proper temperature and relative proportion of polymeric components. Dry gel could be obtained by vacuum dehydration of aqueous gel at low temperatures. Monolithic MgAl2O4 spinel nanometer powders were produced by calcining the dry gel above 800°C, with average crystallite size of 20 nanometers.  相似文献   

7.
Cubic bismuth zinc niobate pyrochlore (base composition (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7) powders were successfully prepared by a chemical method. The formation mechanism of the pyrochlore phase was investigated by TG-DSC, FT-IR, Raman, and X-ray diffraction (XRD). The optical bandgap for the powders treated at temperatures ranging from 500 to 700 °C is 3.0-3.1 eV, indicating low crystallization temperature for the pyrochlore phase. No detectable intermediary phases as BiNbO4 or a pseudo-orthorhombic pyrochlore were observed at any time and the cubic-BZN phase was already formed after thermal treatment at temperatures as low as 500 °C. The phase formation study reveals that a well-crystallized single-phased nanopowder is obtained after calcination at 700 °C, indicating that the chemical synthesis conferred a higher chemical homogeneity and reactivity on the powder, modifying the crystallization mechanism.  相似文献   

8.
Application of Alumina Aerogels as Catalysts   总被引:2,自引:0,他引:2  
Al2O3 gels prepared by hydrolysis of Al-alkoxide were applied as catalysts for NO reduction by hydrocarbon. Xerogels were obtained by drying at 90°C, for 24 h, in air. Aerogels were prepared by supercritical drying of the wet gels in ethanol, using an autoclave. The catalyst activity of the gels for NO reduction with C3H6 as determined at 200–600°C in a fixed bed flow reactor. NO conversion to N2 was about 60% at 550°C with both catalysts, and at 600°C it was better with the aerogel than with the xerogel. The specific surface area of the xerogel, which was larger than that of the aerogel as-dried, decreased to 1/3 by annealing at >500°C. However, the surface area of the aerogel hardly changed by annealing at temperatures up to 700°C. The bimodal pore size distribution of the aerogel hardly changed by annealing, too. The microstructure of the aerogels is stable at high temperatures, and they are better catalysts at high temperatures.  相似文献   

9.
K2NbO3F powders were directly synthesized by an alternative solid-state method at low temperature. Stoichiometric ammonium niobium oxalate, K2C2O4 and KF were mixed with small amounts of water and then dried at room temperature. X-ray diffraction results show that layered perovskite K2NbO3F powders can be obtained by calcining the mixture in temperature range from 550 to 700 °C for 3 h. The elemental composition, powder morphology and particle size of calcination products were analyzed by scanning electron microscope-energy dispersive spectroscopy (SEM/EDS). The SEM images suggest that the particles of the powders obtained at 550 °C are irregular platelets with a diameter of 0.5-1 μm and a thickness of 100-200 nm. The platelets are 3-5 μm in diameter and 1-2 μm in thickness when the calcination temperature reaches 700 °C. K2NbO3F decomposes to K5(NbO3)4F and KF when the temperature reaches 800 °C.  相似文献   

10.
Cesium tungsten oxides (CsxWO3) were synthesized by solvothermal reactions using ethanol and 57.1 vol% ethanol aqueous solution at 200 °C for 12 h, and the effects of post annealing in ammonia atmosphere on the microstructure and electrical-optical properties were investigated. Agglomerated particles consisting of disk-like nanoparticles and nanorods of CsxWO3 were formed in the pure ethanol and ethanol aqueous solutions, respectively. The samples retained the original morphology and crystallinity after annealing in ammonia atmosphere up to 500 °C, while a small amount of nitrogen ion were incorporated in the lattice. The as-prepared CsxWO3 sample showed excellent near infrared (NIR) light shielding ability as well as high transparency in the visible light region. The electrical resistivity of the pressed pellets of the powders prepared in pure ethanol and 57.1 vol% ethanol aqueous solution greatly decreased after ammonia annealing at 500 °C, i.e., from 734 to 31.5 and 231 to 3.58 Ω cm, respectively.  相似文献   

11.
The pressure-less sintering behavior of PbTiO3 powders synthesized by mechanical alloying TiO2 and PbO was investigated using dilatometry and Rietveld refinements of X-ray diffraction patterns. As-synthesized, the powders are nanocrystalline with a mean particle size of 20 nm. Pressure-less sintering in the range 500-1050°C gives single phase ceramics with densities of 85-90% and crystallite sizes in the range 80-400 nm. Cracking due to the paraelectric-ferroelectric phase transition was not observed in samples sintered below 700°C due to the small crystallite size whereas macroscopic cracks formed in samples sintered above 700°C. Rietveld analysis indicates the formation of Pb vacancies in samples sintered and held for 24 h at intermediate temperatures (600-1000°C) which gives some insight into the mechanism of Pb loss and second phase formation in this system.  相似文献   

12.
Nanosized-Ta2O5 powder photocatalyst was successfully synthesized by using sol-gel method via TaCl5 butanol solution as a precursor. Ta2O5 species can be formed under 500 °C via the decomposition of the precursor. The crystalline phase of Ta2O5 powder photocatalyst can be obtained after being calcined above 600 °C for 4 h. The crystal size and particle size of Ta2O5 powder photocatalyst was about 50 nm. A good photocatalytic performance for the degradation of gaseous formaldehyde was obtained for the nanosized-Ta2O5 powder. The Ta2O5 powder formed at 700 °C for 4 h and at 650 °C for 12 h showed the best performance. The calcination temperature and time play an important role in the crystallization and photocatalytical performance of nanosized-Ta2O5 powder.  相似文献   

13.
Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)3, and niobium pentaethoxide, [Nb(OEt)5]2, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb9O25. At lower temperatures, metastable solid solutions with TT-Nb2O5 structure (600 °C) and M-Nb2O5 structure (700 °C) are formed from the amorphous xerogel. The new VNb9O25 phase is structurally related to M-Nb2O5. The solid solution with M-Nb2O5 structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb9O25. The full transformation of this metastable phase to the well known tetragonal VNb9O25 requires a annealing temperature of about 1000 °C.  相似文献   

14.
Preparation and Characterization of Forsterite (Mg2SiO4) Xerogels   总被引:1,自引:0,他引:1  
Mg2SiO4 gels were prepared from alkoxide precursors, and the formation of the forsterite crystal phase was studied after heat treatments up to 1200°C. Prehydrolyzed TEOS in solution with 2-methoxyethanol was mixed with Mg(OEt)2, and the solution was hydrolyzed using excess water. The resultant gels were dried at 100°C to form xerogels which were subsequently powdered. These powders were characterized using thermal analysis (DTA and TGA), surface area analysis (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM).DTA and XRD indicated that forsterite crystallized at 770°C, and by 1000°C the powders were predominantly crystalline. BET gave powder surface areas between 400 and 550 m2 g–1. TEM revealed angular particles with sizes between 0.2 and 2 m. The low temperature of crystallization of forsterite indicates a high degree of intimate mixing between the precursor alkoxides, although XRD indicated some degree of inhomogeneity.  相似文献   

15.
The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li2O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn2O3 and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H2O2 decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg2MnO4, Mg6MnO8, MgMnO4 besides unreacted portions of MgO, Mn2O3 and Mn3O4). Li2O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn2O3 (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li2O added. This finding might suggest that the doping process enhanced the dissolution of Mn2O3 in MgO forming solid solution. This treatment led also to the formation of Li2MnO3. Furthermore, the doping with 3 and 6 mol% Li2O conducted at 800°C resulted in the conversion of Mn2O3 into Mn3O4, a process that took place at 1000°C in absence of Li2O. The produced Li2MnO3 phase remained stable by heating at up to 1000°C. Furthermore, Li2O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn2O3 in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k 20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li2O-doping of the samples having the formula 0.1 Mn2O3/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k 20°C due to Li2O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

16.
A novel microwave dielectric powder with composition of Ca2Zn4Ti16O38 was synthesized through a citrate sol-gel process. The development of crystalline phases with heat-treating temperature for the gel derived powders was evaluated by using thermo-gravimetric analysis and X-ray powder diffraction analysis techniques. The pure phase of Ca2Zn4Ti16O38 with crichtonite crystal structure was obtained at relatively low temperature of 1000 °C. The synthesized powder has high reactivity and the dense ceramics with single crystalline phase were obtained at low sintering temperature of 1100 °C. Impedance spectroscopy and microwave dielectric measurements on sintered samples showed the present compound to be a modest dielectric insulator with excellent dielectric properties of εr∼47-49, Qf value ∼27,800-31,600 GHz and τf∼+45 to +50 ppm/°C. It shows comparable microwave dielectric properties to other moderate-permittivity microwave dielectrics, but much lower sintering temperature of 1100 °C.  相似文献   

17.
ZnO/TiO2/SnO2 mixture was prepared by mixing its component solid oxides ZnO, TiO2 and SnO2 in the molar ratio of 4?1?1, followed by calcining the solid mixture at 200-1300 °C. The products and solid-state reaction process during the calcinations were characterized with powder X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and Brunauer-Emmett-Teller measurement of specific surface area. Neither solid-state reaction nor change of crystal phase composition took place among the ZnO, TiO2 and SnO2 powders on the calcinations up to 600 °C. However, formation of the inverse spinel Zn2TiO4 and Zn2SnO4 was detected at 700-900 and 1100-1200 °C, respectively. Further increase of the calcination temperature enabled the mixture to form a single-phase solid solution Zn2Ti0.5Sn0.5O4 with an inverse spinel structure in the space group of . The ZnO/TiO2/SnO2 mixture was photocatalytically active for the degradation of methyl orange in water; its photocatalytic mass activity was 16.4 times that of SnO2, 2.0 times that of TiO2, and 0.92 times that of ZnO after calcination at 500 °C for 2 h. But, the mass activity of the mixture decreased with increasing the calcination temperature at above 700 °C because of the formation of the photoinactive Zn2TiO4, Zn2SnO4 and Zn2Ti0.5Sn0.5O4. The sample became completely inert for the photocatalysis after prolonged calcination at 1300 °C (42 h), since all of the active component oxides were reacted to form the solid solution Zn2Ti0.5Sn0.5O4 with no photocatalytic activity.  相似文献   

18.
The first purely alkoxide-based sol-gel route to nano-phase powders and thin films of perovskite La0.75Sr0.25MnO3 is described. The phase and microstructure evolution on heat treatment of free gel films to form the target nano-phase oxide were investigated by TGA, IR spectroscopy, powder XRD, SEM and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, without remaining alkoxo groups or solvent. Heating at 5°C·min–1 decomposed the carbonate groups and yielded the pure perovskite La0.75Sr0.25MnO3 at 760°C. The cell dimensions were virtually unchanged from the first observation of perovskite at 680°C, to 1000°C, 4 h. The monoclinic cell of La0.75Sr0.25MnO3 obtained at 1000°C, 4 h, had the dimensions a = 5.475(1), b = 5.504(2), c = 7.771(1) Å, = 90.50(2), fitting the literature data quite well. Crack-free, homogenous, 150 nm thick La0.75Sr0.25MnO3 films were prepared by spin-coating Si/SiO2/TiO2/Pt and polycrystalline -Al2O3 substrates with a 0.6 M alkoxide solution, followed by heating at 5°C·min–1 to 800°C, 30 min.  相似文献   

19.
Lithium manganese oxide (LiMn2O4) powder with spinel structure has been synthesized by a sol-gel method using an aqueous solution of metal nitrates containing polyvinyl alcohol (PVA). The role of PVA and the calcination conditions for the formation of LiMn2O4 have been studied. Homogeneity and reactivity of the precursor powder are enhanced with an increase in the amount of PVA in the starting solution. When the amount of PVA is low, an impurity phase-Mn2O3 is formed at low temperature. On the other hand, when the vinyl alcohol monomer unit of PVA to metal ion ratio is 2 : 1 in the starting solution, only spinel phase is formed at 180°C and organic-free LiMn2O4 powder is obtained at as low as 400°C. Nanosized LiMn2O4 particles with a narrow size distribution have been successfully prepared by this technique. This method with proper amount of PVA results in much lower calcination temperature and shorter calcination time for producing the single spinel phase in comparison with the conventional solid state reaction and other solution techniques.  相似文献   

20.
Dip- or spin-coating and characterization of titania (TiO2) thin films from various aqueous solutions have been studied. The aqueous titanium solutions mainly used in this study were halogen- and chelate-free solutions with the concentrations up to 1.4 M derived from titanium isopropoxide (TIP) with tetramethylammonium hydroxide (TMAOH) or some alkylamines, while aqueous and alcoholic solutions containing titanium atoms stabilized chelating ligands were examined for comparison. The TiO2 films prepared from the TIP-TMAOH solution were already crystallized at 350°C to anatase form and those formed at 600°C had high transparency and refractive indices of 2.40. No carbon residue in the film prepared at 400°C was detected by XPS. The pure anatase form was sustained up to 850°C. Interestingly, it was found that the (004) preferentially oriented anatase films were obtained from TIP-lactic acid (LA) system until 700°C. The solutions containing citric acid (CA) or alkanolamines yielded anatase and rutile form fired at the temperatures equal to or higher than 600°C. Carbon residue was detected in the film fired at 400°C. The film thickness monotonically decreased from the upper to the bottom ends of the substrate. However, it was found that the thickness uniformity was drastically improved by an addition of sucrose to the aqueous solutions. The effects of the solution composition and polyhydroxy compounds on the crystal modifications of formed films and the film uniformity are discussed.  相似文献   

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