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1.
A homogeneous α-Al2O3 crystal membrane was fabricated by the sol–gel technique on 316L porous stainless steel (PSS) substrate with an average pore
size of 1.0 μm. The preparation process was optimized by carefully choosing the binder, the concentrations of the casting
solutions and the sintering temperatures of the membranes. Compared to methylcellulose and polyethylene glycol 20000, polyvinyl
alcohol 1750 was found to be the most effective binder to fabricate a homogeneously structured Al2O3 membrane without defects. The concentration to prepare an uniform coverage membrane with a thickness of ~10 μm was 0.032 mol/L.
When sintered at 1000 °C, γ-Al2O3 membrane with ~3 μm grains was obtained. When sintered at 1200 °C, γ-Al2O3 completely transformed into α-Al2O3 and the grains grew to ~5 μm. Accordingly, the process was applied to a bigger pore-sized PSS with an average pore size of
1.5 μm to fabricate an α-Al2O3 intermediate layer to initially modify its surface. A single α-Al2O3 crystal layer with a thickness of ~5 μm and an average pore size of 0.7 μm was achieved. Subsequently, TiO2, SiO2, and TiO2–SiO2 hybrid membranes were tried on the modified PSS. Defect-free microfiltration membranes with average pore sizes of ~0.3 μm
were readily fabricated. The results indicate that the sol–gel method is promising to initially modify the PSS substrates
and the sol–gel-derived α-Al2O3 crystal layer is an appropriate intermediate layer to modify the PSS and to support smaller grain-sized top membranes. 相似文献
2.
A powder compact of α-SiC and α-Al2O3 was infiltrated with a liquid precursor of SiO2, which on subsequent heat treatment at 1500 °C produced a mullite bonded porous SiC ceramics. Results showed that infiltration
rate could be estimated by using weight gain measurements and theoretical analysis. The bond phase was composed of needle-shaped
mullite which was observed to be grown from a siliceous melt formed during the process of oxide bonding. The porous SiC ceramics
exhibited a density and porosity of 2 g cm−3 and 30 vol%, respectively, and also a pore size distribution in a range of 2–15 μm with an average pore size of 5 μm. No
appreciable degradation of room temperature flexural strength (51 MPa) was observed at high temperatures (1100 °C). 相似文献
3.
T. S. Lin D. C. Jia P. G. He M. R. Wang 《International Journal of Thermophysics》2009,30(5):1568-1577
Geopolymers with different content of α-Al2O3 particle filler were prepared. The thermo-mechanical and microstructural characterization of the obtained geopolymers were
systematically studied by flexural strength and thermal shrinkage measurements, TG-DTA (thermogravimetry and differential
thermal analysis), XRD (X-ray diffractometry), and SEM (scanning electron microscopy). The results show that the addition
of α-Al2O3 particle filler not only increases the onset crystalline temperature but also reduces the crystalline velocity of the geopolymers.
The thermal shrinkage of the geopolymers increases with increasing heat treatment temperatures due to the water loss and densification.
The flexural strength of the geopolymers increases with the increase of heat treatment temperatures from RT to 1200 °C, and
shows a sharp increase in the range from 600 °C to 800 °C due to crystallization and solidification. The increase in content
of α-Al2O3 particle filler can clearly reduce the thermal shrinkage and maintain a higher porosity at high temperatures. However, it
has no distinct influence on the flexural strength after heat treatment. This is mainly attributed to the higher thermal resistance
and strength of α-Al2O3. 相似文献
4.
The effect of SrO addition on the thermal stabilization of transition aluminas with the aim of producing membrane layers (supported and unsupported) has been investigated. Al2O3–x wt.% SrO composite powders (x = 1, 3, 5, 8) were synthesized by co-precipitation of the hydroxides from solutions of AlCl3 and Sr(NO3)2 salts using NH4OH as a precipitating agent. Optimum SrO dopant concentration regarding the transition aluminas stabilization effect was determined to be 5 wt.% based on XRD analysis. STA analysis showed a 30 °C shift versus higher temperatures in the transformation of final transitional alumina (θ-Al2O3) to stable alpha phase due to addition of 5 wt.% SrO. The mechanism of transition aluminas thermal stabilization as a result of SrO addition is thoroughly discussed. Unsupported alumina membranes were prepared by drying boehmite sols at 600, 800, 1000 and 1100 °C. The effect of calcination temperature on surface area, pore size distribution of unsupported membranes containing 5 wt.% SrO has been investigated. The microstructure of unsupported and supported membranes revealed quite different. Smaller grains in the supported layers were attributed to the interaction between support and membrane. 相似文献
5.
Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes 总被引:1,自引:0,他引:1
Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity. 相似文献
6.
Thermal formation of corundum from aluminium hydroxides prepared from various aluminium salts 总被引:2,自引:0,他引:2
J. Temuujin Ts Jadambaa K. J. D. Mackenzie P. Angerer F. Porte F. Riley 《Bulletin of Materials Science》2000,23(4):301-304
Aluminium hydroxides have been precipitated from various aluminium salts and the differences in their thermal behaviour have
been investigated. Pseudoboehmite derived from the nitrate, sulfate and chloride all form γ-Al2O3 at ∼ 400°C but the formation of α-A12O3 at 1200°C occurs more readily in the material derived from the sulfate. This contains a higher concentration of anionic impurities
related to differences in the solubility of the original aluminium salts. The sulfate is retained in the gel to higher temperatures
at which its eventual decomposition may lead to the formation of a reactive pore structure which facilitates the nucleation
of α-A12O3. 相似文献
7.
A. N. Guryanov M. V. Yashkov A. N. Abramov E. D. Shatailo M. A. Melkumov M. F. Torsunov M. N. Kachenyuk 《Inorganic Materials》2012,48(4):350-354
Al2O3:Cr3+ and Al2O3:Ti3+ particles with an average size of ∼1 and 0.7 μm, respectively, have been prepared through mechanical grinding of bulk crystals.
Using purpose-designed accessories made from chromium and titanium, we were able to prevent Fe contamination. We studied the
reaction between α-Al2O3 and SiO2 particles during high-temperature sintering. The results demonstrate that, in this system at temperatures above 1300°C, corundum
particles dissolve in silica to form mullite. The reaction temperature and rate depend on the particle size composition of
the Al2O3. The heating rate is shown to influence the dissolution rate of corundum particles. Increasing the heating rate from 7 to
15°C/min shifts the dissolution range of alumina particles from 1500 to 1630°C. 相似文献
8.
Ramji Lal N. M. Gokhale R. Krishnan P. Ramakrishnan 《Journal of Materials Science》1989,24(8):2911-2916
PZT powders of the composition Pb0.94Sr0.06 (Zr0.53Ti0.47)O3, prepared by spray drying and calcining techniques, were processed to sintered ceramics by conventional cold pressing and
sintering at various temperatures and periods between 1000 to 1250°C for 0.5 to 12h. Sintered ceramics were evaluated for
their microstructure and electromechanical properties. Highly dense ceramics having bulk density of the order of 97% of the
theoretical value could be obtained after sintering at a considerably lower temperature of 1000°C in comparison to the 1300°C
generally required for powders prepared by conventional ceramic processing. However, the increase in sintering temperature
of reactive spray-dried powders causes the entrapment of closed pores as a result of exaggerated grain growth and subsequent
pore coarsening thereby leading to a decrease in the bulk density of the ceramics. It has been observed that minor variations
in the sintering parameters influence the porosity, grain size and electromechanical properties. Values of the dielectric
constant, piezoelectric strain coefficient and electromechanic coupling factor increase with the increase in grain size and
decrease with the increase in porosity of the sintered ceramic whereas the dielectric dissipation factor decreases with the
increase in sintering temperature. 相似文献
9.
Raghunath P. Rana Swadesh K. Pratihar Santanu Bhattacharyya 《Journal of Materials Science》2006,41(21):7025-7032
Al2O3–ZrO2 composites containing nominally equal volume fraction of Al2O3 and ZrO2 have been synthesized through combined gel-precipitation technique. Subsequently the gels were subjected to three different post gel processing treatments like ultrasonication, ultrasonication followed by water washing and ultrasonication followed by alcohol washing. It was observed that while in unwashed samples crystallization took place at low temperature, crystallization was delayed in the washed gels. The phase transition of ZrO2 in the calcined gels followed the sequence; amorphous → cubic ZrO2 → tetragonal ZrO2 → monoclinic ZrO2. On the other hand, phase transition in alumina followed the sequence amorphous to γ-Al2O3, the transition taking place at 650 °C. No α-Al2O3 could be detected even after calcination at 950 °C. However, all the sintered samples had α-Al2O3. In spite of high linear shrinkage (19–21%) during sintering, the sintered sample had density of only above 70% for all the four varieties of the powders. However, in spite of the low sintered density of the pellets, 31% tetragonal zirconia could be retained after sintering at 1400 °C and it reduced to about 16% at 1600 °C. 相似文献
10.
The rate of reaction of the compacted powders of CuO-η-Al2O3 and ZnO-η-Al2O3 systems was measured in air at 800 to 900° C and the effect of the mixing procedure (dry and wet mixing) and compaction pressure
(0 to 8.3×108 Pa) of reactant oxides on the fraction of reaction completed (α) was investigated. In the reaction of the CuO-η-Al2O3 system, the α-values obtained for the sample prepared by wet mixing in ethanol were higher than those for the sample prepared
by dry mixing in air and were not influenced by the compaction pressure, whereas in the case of dry mixing they varied with
the compaction pressure and had a maximum value at 2.1×108 Pa. On the other hand, in the reaction of the ZnO-η-Al2O3 system the α-values for the sample obtained by wet mixing were lower than the values obtained by dry mixing, in contrast
to the results in CuO-η-Al2O3 system, and the α-values for the samples prepared by both dry and wet mixing were not influenced by the compaction pressure.
The effect of mixing procedure and compaction pressure of reactant powders on the α-values was found to be explained on the
basis of the aggregate size of CuO and ZnO dispersed in the matrix of η-Al2O3 fine powder. 相似文献
11.
Keiichi Tsugeki Takafumi Kato Yoshihiro Koyanagi Katsuki Kusakabe Shigeharu Morooka 《Journal of Materials Science》1993,28(12):3168-3172
α-Al2O3 particles whose primary size is ca. 450 nm are smoothly fluidized by forming agglomerates of ca. 200 μm and are coated with
TiN crystallites generated by a chemical vapour deposition (CVD) reaction of TiCl4 and NH3 at 973 K. The α-Al2O3-TiN composite particles and those obtained by mechanically mixing constituent particles are sintered at 1873 K in a nitrogen
atmosphere, and the electroconductivity of sintered bodies is measured at 298 K. The electroconductivity of the CVD-hybridized
composites is higher than that of mechanically mixed ones. This shows the effectiveness of fluidized-bed CVD processing in
the preparation of composite ceramic particles. 相似文献
12.
Jinwei Liu Yadong Li Wenbao Wang Hongtao Wang Feng Zhang Guilin Ma 《Journal of Materials Science》2010,45(21):5860-5864
Dense ceramic samples of BaCe1−x
Ca
x
O3−α (x = 0.05, 0.10, 0.15, 0.20) have been synthesized by heat treating the precursor prepared from a microemulsion route under
lower calcining and sintering temperatures than traditional high-temperature solid-state reaction. The samples exhibited single
phase of orthorhombic perovskite. It was found that the samples were almost pure proton conductors in wet hydrogen at 300–600 °C.
Ammonia was synthesized successfully from nitrogen and hydrogen gases at atmospheric pressure using BaCe0.9Ca0.1O3−α as an electrolyte of ammonia synthesis reactor. 相似文献
13.
The effect of sintering temperature on the development of grain boundary traps in zinc oxide based varistors 总被引:3,自引:0,他引:3
A series of zinc oxide based varistors containing 0.5 wt% Bi2O3 and 0.5 wt% Mn2O3 was prepared by a conventional mixed oxide route and sintered at temperatures between 950° and 1300°C. All samples showed
varistor behaviour, although as the sintering temperature was increased from 950°C to 1300°C, the non-linearity coefficient,
α, decreased from 22 to 3.
Deep level transient spectroscopy of the varistors showed that the main active electron trap migrated to shallower levels
within the bandgap as the sintering temperature increased. At the lowest sintering temperature, where α attained the highest
values, a second, shallower trap was also activated. 相似文献
14.
The reaction products of an allophane heated with carbon at 850–1600 °C in the stream of nitrogen for a given time were characterized
by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850–1300 °C, β-Si3N4 and α-Al2O3 at 1300–1500 °C, and SiC-AlN-Fe2Si at temperatures higher than 1500 °C. SiC-AlN-Fe2Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 °C
with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of
small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites
obtained is also discussed. 相似文献
15.
B. S. Karnik M. J. Baumann S. J. Masten S. H. Davies 《Journal of Materials Science》2006,41(20):6861-6870
Alumina–zirconia–titania (AZT) ceramic membranes coated with iron oxide nanoparticles have been shown to improve water quality
by significantly reducing the concentration of disinfection by-product precursors, and in the case of membrane filtration
combined with ozonation, to reduce ozonation by-products such as aldehydes, ketones and ketoacids. Commercially available
ceramic membranes with a nominal molecular weight cut-off of 5 kilodaltons (kD) were coated 20, 30, 40 or 45 times with sol
suspension processed Fe2O3 nanoparticles having an average diameter of 4–6 nm. These coated membranes were sintered in air at 900 °C for 30 min. The
effects of sintering and coating layer thickness on the microstructure of the ceramic membranes were characterized using atomic
force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). AFM images show
a decreasing roughness after iron oxide coating with an average surface roughness of ∼161 nm for the uncoated and ∼130 nm
for the coated membranes. SEM showed that as the coating thickness increased, the microstructure of the coating changed from
a fine grained (average grain size of ∼27 nm) morphology at 20 coating layers to a coarse grained (average grain size of ∼66 nm)
morphology at 40 coating layers with a corresponding increase in the average pore size from ∼57 nm to ∼120 nm. Optimum water
quality was achieved at 40 layers, which corresponds to a surface coating morphology consisting of a uniform, coarse-grained
structure with open, nano-sized interconnected pores. 相似文献
16.
Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods 总被引:2,自引:0,他引:2
The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from -to -alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C. 相似文献
17.
A comprehensive analysis of size and strain broadened profile shapes in X-ray diffraction line broadening analysis is presented.
Both size and strain broadened profiles were assumed to be Voigtian and the derived microstructural parameters (size and strain)
were found to be in close agreement with those calculated from model independent Warren-Averbach method. The method is applied
to three different alumina samples viz. micron size α-alumina (α-Al2O3) prepared by the combustion of aluminium nitrate and urea mixture, annealed samples and commercial α-Al2O3 sample. It is likely from the present analysis that a significant Gaussian size contribution is related to narrow size distribution
observed from the analysis. It has been concluded that present Voigtian analysis is more reliable and may largely replace
the earlier simplified integral breadth methods of analysis often used in line broadening analysis. 相似文献
18.
TEM characterization of iron-oxide-coated ceramic membranes 总被引:1,自引:0,他引:1
B. S. Karnik M. J. Baumann L. M. Corneal S. J. Masten S. H. Davies 《Journal of Materials Science》2009,44(15):4148-4154
Commercially available porous alumina–zirconia–titania ceramic (AZTC) membranes having a titania surface coating were characterized
using transmission electron microscopy (TEM), X-ray diffraction (XRD), and the Brunauer–Emmett–Teller (BET) method. TEM photomicrographs
showed the as-received AZTC membrane to be a multi-layered structure consisting of a porous alumina–zirconia–titania core
having ultrafine pore sizes, coated by an additional layer of nanoporous titania. Electron diffraction studies revealed an
amorphous surface titania layer while the underlying AZTC membrane was crystalline. The AZTC membranes were coated 20, 30,
40, 45, or 60 times with iron oxide (Fe2O3) nanoparticles, after which the membranes were sintered in air at 900 °C for 30 min. TEM revealed a relatively uniform nanoporous
Fe2O3 coating on the sintered, coated membranes, where the Fe2O3 coating thickness increased with increasing number of layers. Electron diffraction patterns showed the Fe2O3 coating to be crystalline in nature. This was confirmed by the XRD results showing the structure to be α-Fe2O3, while the AZTC membrane was a mixture of the anatase and rutile phase of TiO2 as well as ZrO2 and corundum, Al2O3. The average pore size of the underlying AZTC membrane increased after the Fe2O3-coated membrane was sintered. The nanoporosity in the sintered Fe2O3 coating increased until 40 layers, beyond which no significant increases in the average pore size were observed. The iron-oxide-coated
membrane improved catalytic properties when used in combination with ozone to treat water. The optimal benefit, in terms of
water treatment efficacy, was found at 40 layers of Fe2O3. 相似文献
19.
Soumen Pal A. K. Bandyopadhyay S. Mukherjee B. N. Samaddar P. G. Pal 《Bulletin of Materials Science》2011,34(4):865-872
Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO-nAl2O3, n = 1) MgAl2O4 spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor
of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical
activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl2O4 — γ-Al2O3 solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures,
due to large agglomerate size, MgO can not properly interact with the exsolved α-Al2O3 from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and
attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in
attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary)
is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel,
can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited
precursor is most effective in terms of nano size (< 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C. 相似文献
20.
Basavaraj Angadi V. M. Jali M. T. Lagare N. S. Kini A. M. Umarji 《Bulletin of Materials Science》2002,25(3):191-196
The low thermal expansion ceramic system, Ca1-xSr{x}Zr4P6O24, for the compositions with x = 0, 0.25, 0.50, 0.75 and 1 was synthesized by solid-state reaction. The sintering characteristics
were ascertained by bulk density measurements. The fracture surface microstructure examined by scanning electron microscopy
showed the average grain size of 2.47 μm for all the compositions. The thermal expansion data for these ceramic systems over
the temperature range 25–800°C is reported. The sinterability of various solid solutions and the hysteresis in dilatometric
behaviour are shown to be related to the crystallographic thermal expansion anisotropy. A steady increase in the amount of
porosity and critical grain size with increase in x is suggested to explain the observed decrease in the hysteresis. 相似文献