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1.
The influence of Al2O3 (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO3) was experimentally determined. The investigated compositions were Y0.125Mg0.875Si0.875B0.125O3 and Y0.125Mg0.725Ba0.15Si0.875B0.125O3. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 °C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 °C and 1100 °C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al2O3 in glass batches affected the thermal properties of the glasses and resulted in MgSiO3-based glass ceramics well sintered between 900 °C and 1100 °C. In the BaO-free MgSiO3 glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian. 相似文献
2.
PbO–SrO–Na2O–Nb2O5–SiO2 glass–ceramics were prepared via roll-quenching followed by controlled crystallization from 700 °C to 900 °C. The effects of PbO and SrO contents on crystallization and dielectric properties were investigated. The results show that Pb2Nb2O7, Sr2Nb2O7 and their solid solutions crystallize at 700 °C, NaNbO3 is the primary phase at 800 °C, Pb2Nb2O7 disappears and PbNb2O6 forms at 900 °C. The dielectric properties of the glass–ceramics formed through controlled crystallization has a strong dependence on the phase compositions that were developed during heat treatment. The highest dielectric constants (∼600) are found in samples with 6.0 mol% SrO annealed at 900 °C for 3 h. The dielectric–temperature characteristics of the samples show stability over the range from −60 °C to 180 °C, except the sample without SrO heated at 900 °C. 相似文献
3.
Boron suboxide (B6O) powder was synthesized at temperatures of about 1400 °C from the reaction of amorphous boron powder with boric acid. The synthesized B6O powders were hot pressed at temperatures up to 1900 °C and at pressures of 50 MPa. Additionally to pure B6O materials, composites with aluminium were prepared. The microstructure and properties of the sintered compacts were investigated. The addition of aluminium in the composites results in the formation of an additional aluminium borate phase. The composites showed a similar hardness (∼30 GPa) as the pure B6O samples but increased fracture toughness (∼3.5 MPa m1/2). 相似文献
4.
Mechanochemical synthesis was used to prepare BaTa2O6 powders from BaCO3 and Ta2O5 precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa2O6 and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa2O6 even after 1 h of milling time and single phase BaTa2O6 was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa2O6 at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa2O6 grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa2O6 grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination. 相似文献
5.
A series of high-temperature close coupled catalysts Pd/Ce-Zr-M/Al2O3 (M = Y, Ca or Ba) were prepared by ultrasonic-assisted successive impregnation. The catalysts were subjected to a series of characterization measurements. The results of activity evaluation show that Y is the best promoter for propane total oxidation, especially at the calcination temperature of 1100 °C. It is interesting that although the BET specific surface areas and the dispersion of Pd species decrease, the Y-promoted catalyst calcined at 1100 °C shows higher catalytic activity than the corresponding one calcined at 900 °C and better sulfur-resisting performance. The results of TEM, TPHD and CO chemisorption indicate that Y can remarkably increase the dispersion of Pd species. However, the dispersion is hard to be connected with the activity increase as the calcination temperature is elevated from 900 to 1100 °C. The change of active phases and the interaction between Pd species and the supports may account for the activity enhancement. Combined with XRD, H2-TPR and O2-TPD results, it is deduced that the coexistence of metallic Pd and PdO species in the catalysts calcined at 1100 °C may be also favorable to C3H8 oxidation. In a word, Pd/Ce-Zr-Y/Al2O3 is indeed a promising high-temperature close coupled catalyst applicable to high temperature. 相似文献
6.
The search for suitable additives for boron suboxide (B6O) materials which could improve densification, reduce sintering temperature and tailor the microstructure has been of great importance. In an earlier study it was shown that transition metal borides qualify as sintering aids for B6O, but partial segregations of the boride secondary phases were found. In this work, efforts have been made to understand the chemical interaction between the B6O and boride phase. A reaction couple of sintered B6O, nickel and green compact B6O were assembled and heat-treated at 1850 °C for 20 min. XRD and SEM examinations of the reaction zone show the formation of nickel boride, diffusing into the B6O matrix and a substantial grain growth of B6O at the interface. 相似文献
7.
H.Y WangE Ruckenstein 《Carbon》2002,40(11):1911-1917
The carbon formation during methane decomposition was investigated at 900 °C over the 48 wt% Co-MgO catalysts as a function of the calcination temperature Tc used in their preparation. Examination of the carbonaceous deposits by transmission electron microscopy revealed three kinds of structures: shapeless tangles, shell-like materials, and carbon filaments. In another set of experiments, the structural characteristics of the calcined catalysts were investigated using temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Co3O4, Co2MgO4, and (Co, Mg)O (solid solution of CoO and MgO) were identified for Tc≤700 °C, Co3O4 and (Co, Mg)O for Tc=800 °C and only (Co, Mg)O for Tc=900 °C. It was found that the metal particles originated from the reduction of the solid solution favored the formation of filamentous carbon. A possible explanation is proposed. 相似文献
8.
One-step mechanochemical process followed by thermal treatment has been used to produce calcium phosphate-based composite nanopowders. Effects of milling and subsequent heat treatment on the phase transition as well as structural features were investigated. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The results revealed that the dominant phases after mechanical activation were hydroxyapatite, anatase (TiO2), and periclase (MgO), while after thermal annealing process at 700 °C hydroxyapatite along with geikielite (MgTiO3) and periclase (MgO) were the major phases. In addition, decomposition of hydroxyapatite to tricalcium phosphate (β-TCP) occurred after heat treatment in the range 900–1100 °C which resulted in the formation of tricalcium phosphate-based composite nanopowders. Evaluation of structural features of the samples calculated by X-ray diffraction profiles analysis indicated that the average crystallite size of hydroxyapatite after 10 h of milling and subsequent heat treatment at 700 °C were about 21 and 34 nm, respectively. TEM and SEM studies exhibited that the considerable morphological changes at temperatures ≥900 °C had to be ascribed not only to grain growth, but also for the transformation of hydroxyapatite to β-TCP. 相似文献
9.
A gradient self-healing coating consisting of three layers, SiC-B4C/SiC/SiO2, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B4C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO2 as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B4C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions. 相似文献
10.
J. MürbeJ. Töpfer 《Journal of the European Ceramic Society》2012,32(5):1091-1098
Nanocrystalline Ni-Cu-Zn ferrite powders Ni0.20Cu0.20Zn0.62Fe1.98O3.99 were prepared by thermal decomposition of an oxalate precursor. The particle size is 6 nm and 350 nm, respectively, for powders obtained through calcinations at 350 °C or 750 °C. The shrinkage behavior significantly changes with particle size; the temperature of maximum shrinkage rate is TMSR = 700 °C for particles of 6 nm size and increases to TMSR = 880 °C for particles 350 nm in size. Dense samples with a permeability of μ = 780 are obtained by sintering at 900 °C for 2 h. Mixtures of nanocrystalline and sub-micron powders allow tailoring of the shrinkage behavior. A maximum permeability of μ = 840 is obtained after sintering of a 1:1-mixture at 900 °C. This demonstrates the potential of nanocrystalline ferrites for co-firing without additives at 900 °C and integration of ferrite inductors into LTCC modules. 相似文献
11.
Mathias Herrmann Akhilesh Kumar Swarnakar Maik Thiele Omer Van der Biest Iakovos Sigalas 《Journal of the European Ceramic Society》2011,31(13):2387-2392
B6O is a potential superhard material with a hardness of 45 GPa measured on single crystals. Recently it was found that different oxides can be utilized as an effective sintering additive which allows densification under low pressures. In this work the effect of addition of Y2O3/Al2O3 on high temperature properties was investigated using impulse excitation technique (IET), hardness measurements and dilatometric measurements. The IET technique reveals the softening of the residual B2O3 in the materials without additives at 450 °C; in the materials with Y2O3/Al2O3 the softening is observed at only about 800 °C. This data agrees with the values found for different borate glasses.The materials showed no pronounced reduction of hardness at these temperatures. This is additional evidence, supporting previous observations that the material consists of pure grain boundaries between B6O grains. Hardness values (HV5) of up to 17 GPa at 1000 °C were observed. 相似文献
12.
Porous Ti3AlC2 ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH2 at 700 °C, which leads to the conversion of TiH2 to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti3AlC2 at 1300 °C. 相似文献
13.
Analyses made on the world's biomass energy potential show that biomass energy is the most abundant sustainable renewable energy. The available technical biomass energy potential surpasses the total world's consumption levels of petroleum oils, coal and natural gas. In order to achieve a sustainable harnessing of the biomass energy potential and to increase its contribution to the world's primary energy consumption, there is therefore a need to develop and sustain contemporary technologies that increase the biomass-to-energy conversion. One such technology is the high temperature air/steam gasification (HTAG) of biomass. In this paper we present findings of gasification experimental studies that were conducted using coffee husks under high temperature conditions. The experiments were performed using a batch facility, which was maintained at three different gasification temperatures of 900 °C, 800 °C, and 700 °C. The study findings exhibited the positive influence of high temperature on increasing the gasification process. Chars left while gasifying at 800 °C and 700 °C were respectively 1.5 and 2.4 times that for the case of 900 °C. Furthermore, increased gasification temperature led to a linear increment of CO concentration in the syngas for all gasification conditions. The effect was more pronounced for the generally poorly performing gasification conditions of N2 and 2% oxygen concentration. When gasification temperature was increased from 700 °C to 900 °C the CO yield for the 2% O2 concentration increased by 6 times and that of N2 condition by 2.5 times. The respective increment for the 3% and 4% O2 conditions were only twofold. This study estimated the kinetic parameters for the coffee husks thermal degradation that exhibited a reaction mechanism of zero order with apparent activation energy of 161 kJ/mol and frequency factor of 3.89 × 104/min. 相似文献
14.
Kiyoshi Itatani Kenta TsuchiyaYoshio Sakka Ian J. DaviesSeiichiro Koda 《Journal of the European Ceramic Society》2011,31(14):2641-2648
High-density and fine-grained transparent hydroxyapatite (Ca10(PO4)6(OH)2: HAp) ceramics with B2O3 and Na2O addition were fabricated using pressureless sintering and pulse-current pressure sintering between 1000 and 1100 °C; the superplastic deformation of these HAp specimens was evaluated. The relative density of pure HAp compacts pulse-current pressure sintered at 1000 °C for 10 min under a pressure of 50 MPa attained 99.9% and exhibited translucency. The tensile elongation of the pure HAp specimen, which was measured at 1000 °C under a strain rate of 1.48 × 10−4 s−1, was as high as 364%. The relative density of HAp compacts with 3.0 mol.% B2O3 addition pulse-current pressure sintered under the same conditions as those of pure HAp compacts was 98.9%, whereas the grain size was as low as 0.24 μm. The elongation of HAp specimens, measured at a test temperature of 1000 °C under a strain rate of 1.48 × 10−4 s−1, was as high as 578%. 相似文献
15.
Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF3 and HA–PSZ composite containing 5 wt% PSZ without YF3 were sintered in air at 900 °C, 1100 °C and 1300 °C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF3 showed desirable thermal stability below 1300 °C and besides various amounts of CaZrO3, any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 °C, composites with YF3 showed higher thermal stability than the composites without YF3. On the other hand, pure HA started to decompose and TCP was observed at 1300 °C. Composites with YF3 showed improved thermal stability than the composite containing 5 wt% PSZ without YF3 and pure HA at lower sintering temperatures such as 900 °C and 1100 °C. However, it was observed that the increasing amount of YF3 addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF3 showed the highest relative density among all of the composites with or without YF3. 相似文献
16.
The effect of BaCu(B2O5) (BCB) addition on the sintering temperature and microwave dielectric properties of 2.5ZnO-0.2SnO2-4.8TiO2-2.5Nb2O5 (ZSTN) has been investigated by the solid-state ceramic route. X-ray diffraction and scanning electron microscopy techniques were used to analysis the structure and microstructure. The microwave dielectric properties were measured by the resonance method. It was found that the addition of BCB can effectively lower the sintering temperature from 1100 °C to 900 °C, and improves the microwave dielectric properties of ZSTN ceramics. The BCB doped ZSTN ceramics can be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application. 相似文献
17.
Thermal behavior and characteristics of fired geopolymers produced from local Cameroonian metakaolin
The thermal behavior along with certain characteristics of geopolymers produced from local Cameroonian metakaolin and heated up to 1000 °C were examined. Geopolymers fired up to 900 °C had the same physical aspect as initial ones and those fired at 1000 °C warped, were glazed and blistered. The TG showed elimination of water according to two stages. The dilatometric curves of preheated samples showed shrinkage between 90 and 250 °C followed by expansion and sintering. The samples heated up to 700 °C were amorphous and new crystalline phases appeared around 900 °C. The microstructure of geopolymers heated between 300 and 900 °C showed progressive disruption and the linear shrinkage increased. The water absorption of the samples fired up to 700 °C increased slightly and tremendously around 900 °C. A drastic decrease of compressive strength was observed with the samples fired between 300 and 900 °C. Hence, the characteristics of geopolymers lessened with elimination of the water which forms hydration spheres around the compensating cations (Na+) opposed to tetrahedral groups AlO4− along with transformation of amorphous phase. 相似文献
18.
Li2TiO3 ceramic powders have been developed by a solid state reaction method and those have been sintered at four different temperatures (600 °C, 700 °C, 800 °C and 900 °C) towards the optimization of sintering temperature that has been found to be at 800 °C based on the nature of the XRD profiles. The sample sintered at 800 °C has shown a good crystallinity situation from its XRD peaks and the sample is found to be in monoclinic structure which is in accordance with the reported data of JCPDS 33-0831. The SEM images for samples sintered at 600 °C, 700 °C, 800 and 900 °C, EDAX peaks, FTIR profile have been measured for the temperature optimized (800 °C) sample for understanding the structural details of Li2TiO3 ceramic powders. Besides these, dielectric constant, dielectric loss and a.c. conductivities have been measured for the temperature optimized sample. In order to strengthen the observations made in the XRD profiles at four different temperatures, Raman spectra of those four sintered ceramic powders have also been studied. In respect of the thermal properties, only for the as synthesized (precursor) sample, simultaneous measurement of TG-DTA profiles has been carried out for analysis. 相似文献
19.
A. Christogerou T. Kavas Y. Pontikes S. Koyas Y. Tabak G.N. Angelopoulos 《Ceramics International》2009
Colemanite, ulexite and tincal are the main boron ores found in Turkey and account to 63% of the world estimated reserves. The production route of boron products results in significant amounts of different types of Boron Wastes, BW. Their open field disposal raises substantial environmental concerns in fear of leaching and groundwater pollution. The heavy clay ceramic industry can potentially absorb substantial quantities of BW. In the present study, BW produced from Kirka borax plants in Turkey, with 12.6 wt.% B2O3 was introduced in 0 wt.%, 5 wt.% and 15 wt.% in a heavy clay body mixture. Four peak temperatures, 800 °C, 850 °C, 900 °C and 950 °C, were examined for the dry pressed samples. The thermal behaviour analyzed by dilatometry and TGA shows that major loss in weight starts about 600 °C and continues to 700 °C approximately. For 5 wt.% BW addition and firing at 900–950 °C, the sintered bodies present comparable or improved physical and mechanical properties with respect to the reference formulation. The microstructure was analyzed by SEM whereas the main crystalline phases were identified by XRD. Samples fired at 900 °C with 5 wt.% BW present comparable properties with the reference ones. At 950 °C, the obtained properties were improved. Deformation occurs for samples with 15 wt.% BW when fired at >900 °C. 相似文献
20.
Sr3Al2O6 was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr3Al2O6 were investigated. Increasing the CA/M promoted the formation of Sr3Al2O6. Single-phase and well-crystallized Sr3Al2O6 was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr3Al2O6 nanoparticles with a diameter of about 50-70 nm were obtained. 相似文献