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1.
Ceramic materials with a very low thermal expansion coefficient are synthesized by the sol-gel process. The binary gel is obtained by hydrolysis and polycondensation reactions of organometallic compounds of aluminium and titanium. The thermal evolution of the amorphous powder is followed by DTA and TGA measurements. Structural evolution is followed using X-ray diffraction. The crystallization of the TiO 2 rutile and Al 2O 3 corindon starts at 700 and 900°C respectively. The transformation of Al 2O 3 and TiO 2 into Al 2TiO 5 appears between 1200 and 1300°C. The densification of the powder is performed by the hot pressing process. The shrinkage of the powder was previously followed by dilatometric measurements. The physical properties of the final material are studied as a function of pressing parameters. 相似文献
2.
Gels with composition xZrO 2−(100− x)SiO 2, X = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors. Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment. Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia. 相似文献
3.
Alkoxide derived gels were prepared in the system Na 2O---B 2O 3---SiO 2. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram. Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na2O, B2O3 and SiO2 extracted from the attack gels were analyzed. The experimental results indicate that the amount of B2O3 has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B2O3 mol% the greater are the amounts of Na2O and B2O3 extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B2O3 and Na2O extracted slightly increases as the B2O3 mol% increases. Small amounts of extracted SiO2 were always observed. These results are complemented with other measurements so that an explanation of the controlling mechanism is given. 相似文献
4.
A glass rod of the PbO---K 2O---B 2O 3---SiO 2 system having a radial gradient of refractive index has been prepared by the sol-gel process using aqueous solutions of lead acetate and potassium nitrate as the sources for index-modifying cations. A gel prepared by gelatinizing a sol from the mixture of tetramethoxysilane, tetraethoxysilane, and the aqueous solutions of boric acid and lead acetate was placed in an aqueous solution of potassium nitrate to form the concentration gradient of modifier cations by diffusion through the micropores of the gel. The glass rod of about 7 mm in diameter obtained by drying and sintering the gel had a refractive index of parabolic profile changing from the center toward the perimeter with a maximum difference of about 0.04. 相似文献
5.
(30 − x/2)Li 2O·(70 − x/2)B 2O 3· xAl 2O 3( x = 0, 5 and 10) composite gels have been fabricated by the sol-gel method. LiOCH 3, B(OC 4H 9) 3, and Al(OC 4H 9) 3 were used as precursor for Li 2O, B 2O 3, and Al 2O 3, respectively. B(OC 4H 9) 3 and Al(OC 4H 9) 3 were hydrolyzed separately and then mixed. The crystallization behavior and structure of the gels upon thermal treatment temperatures between 150 and 550 °C are characterized on the basis of SEM, XRD and IR analyses. Xerogel with x = 0 exhibits non-crystal features, whereas crystalline phases are found in the xerogels with x = 5 and 10. The crystalline phases are not found with increasing heat treatment temperatures from 150 to 450 °C, but crystalline phases appear present at 550 °C. The xerogel with x = 0, subject to thermal treatment below 450 °C, is found to be still amorphous, and a 550 °C heat treatment leads its structure changing from glassy to crystalline. 相似文献
6.
Zr---Al metallo-organic compounds (zircoaluminates), having (CH 2) 4COOH, (CH 2) 12CH 3 and (CH 2) 2NH 2 as the organofunctional groups, were treated preliminary by (1) spray-drying, (2) gelation of addition of 10% NH 4OH aqueous solution followed by spray-drying and (3) rotary evaporation under a reduced pressure. After the treatment they were heated in air to prepare ZrO 2---Al 2O 3 composite powders. The IR and DTA profiles for the treated compounds indicated that the procedures modified the structures for the zircoaluminates. The stability of tetragonal ZrO 2 for the ZrO 2---Al 2O 3 composite powder were dependent on the modification in the structure for the zircoaluminates. Balloon shaped particles, 0.5–2 μm in diameter, were obtained through procedure (1) and spherical particles, 1–4 μm in diameter, through (2). Tetragonal ZrO 2 grains, 0.1–0.2 μm in diameter, were dispersed in the particles when heated at 1400°C. 相似文献
7.
Ta 2O 5, Ta-Nb-O, Zr-Al-Nb-O, and Zr-Al-O mixture films or solid solutions were grown on Si(1 0 0) substrates at 300 °C by atomic layer deposition. The equivalent oxide thickness of Ta 2O 5 based capacitors was between 1 and 3 nm. In Zr-Al-O films, the high permittivity of ZrO 2 was combined with high resistivity of Al 2O 3 layers. The permittivity, surface roughness and interface charge density increased with the Zr content and the equivalent oxide thickness was between 2.0 and 2.5 nm. In the Zr-Al-Nb-O films the equivalent oxide thickness remained at 1.8-2.0 nm. 相似文献
8.
Glasses in the system Na 2O/B 2O 3/Al 2O 3/In 2O 3 were melted and subsequently tempered in the range from 500 to 700 °C. Depending on the chemical composition, various crystalline phases were observed. From samples without Al 2O 3, In 2O 3 could not be crystallized from homogeneous glasses, because either spontaneous In 2O 3 crystallization occurred during cooling, or other phases such as NaInO 2 were formed during tempering. The addition of alumina, however, controlled the crystallization of In 2O 3. Depending on the crystallization temperature applied, the crystallite sizes were in the range from 13 to 53 nm. The glass matrix can be dissolved by soaking the powdered glass in water. This procedure can be used to prepare nano-crystalline In 2O 3-powders. 相似文献
9.
Internal photoemission of electrons was used to determine the band alignment in metal (Mg, Al, Ni, Cu, Au)-oxide-silicon structures with Al 2O 3- and ZrO 2-based insulators. For Al 2O 3- and ZrO 2 layers grown on Si by atomic layer deposition the barrier height between the Si valence band and the oxide conduction band was found to be 3.25 and 3.1 eV, respectively. Thermal oxidation of the Si/oxide stacks results in a barrier height increase to ≈4 eV for both cases due to formation of a silicate interlayer. However, there is a significant sub-threshold electron emission both from silicon and metals, indicating a high density of states in the band gap of the insulators. These states largely determine the electron transport across metal oxides and may also account for a significant dipole component of the potential barrier at the metal/ZrO 2 and metal/Al 2O 3 interfaces. 相似文献
10.
The effect of Y 2O 3 addition on the phase transition and growth of yttria-stabilized zirconia (YSZ) nanocrystallites prepared by a sol-gel process with various mixtures of ZrOCl 2 · 8H 2O and Y(NO 3) 3 · 6H 2O ethanol-water solutions at low temperatures has been studied. DTA/TGA, XRD, SEM, TEM and ED have been utilized to characterize the YSZ nanocrystallites. The crystallization temperature of 3YSZ, in which Y 2O 3/(Y 2O 3 + ZrO 2) = 0.03, gel powders estimated by DTA/TG is about 427 °C. When 3YSZ and 5YSZ gels are calcined at 500-700 °C, their crystal structures as composed of coexisting tetragonal and monoclinic ZrO 2, and tetragonal phase decreases with calcination temperature increasing from 500 to 700 °C. Pure cubic ZrO 2 is obtained when added Y 2O 3 is greater than 8 mol%. A nanocrystallite size distribution between 10 and 20 nm is obtained in TEM observations. 相似文献
11.
Optical characteristics of silica glass optical fibers containing Co 2+ doped ZnO-Al 2O 3-SiO 2 (ZAS) glass-ceramics prepared by slurry-doping method were investigated. The absorption and emission bands of the fibers were found to be originated from the tetrahedral Co 2+ in ZnAl 2O 4 crystals in ZAS glass-ceramics particles embedded in the core of the fibers. The crystal field strength of the Co 2+ ions in the optical fiber was found to be smaller than that of the Co 2+ ions in the bulk ZAS glass-ceramics. 相似文献
12.
By rapid quenching in a twin roller apparatus, glass was found to occur widely in the systems of Li 2O with Al 2O 3, Ga 2O 3, Bi 2O 3 and in mixed systems. Examination of the resulting flakes by X-ray powder diffraction, differential thermal analysis, and capacitance data revealed the occurrence of glass, glass transitions, crystallization exotherms and the nature of some of the crystallization paths.The log ionic conductivity of the glasses was found to follow a linear relationship with the Li concentration. Evidence was observed for three new metastable crystalline phases, one in the Li 2OAl 2O 3 system and two in the Li 2OBi 2O 3 system. The latter system also showed evidence for the occurrence of two glasses at almost all compositions. 相似文献
13.
Although shale glasses exhibit surface devitrification when heat treated, the addition of Cr 2O 3 to the glass enables volume crystallization to occur. The mechanisms and kinetics of crystallization for both processes are presented and found to correlate well with existing theories; the Swift equation for surface crystallization and the Johnson-Mehl equation for volume crystallization. Phase separation in the Cr 2O 3-containing glasses promotes the formation of an iron-chrome spinel as nuclei and leads to the development of a fine-grained glass-ceramic. 相似文献
14.
A small-angle X-ray diffraction study of the supercritical region of the B 2O 3PbOAl 2O 3 immiscibility surface is presented. This is the first time that diffraction methods have been applied to the study of critical opalescence in a system of oxides at high temperatures. Debye's theory is shown to be applicable and, from the spectra correlation lengths and the range of molecular interactions l is calculated. The topography of the supercritical region is determined and the relationship between l and the main interatomic distances in the melt discussed. 相似文献
15.
超声空化作用已经被证明是制备纳米粉体有效技术,可以防止团聚体生成。本文以柠檬酸为络合剂,在超声波中制得溶胶,用凝胶自蔓延法合成出了纳米Al2O3粉体。通过X射线衍射仪(XRD)、扫描电镜(SEM)和透射电镜(TEM)分析,探讨了不同比例金属离子与柠檬酸对纳米粉体的形成和粉末晶粒尺寸大小的影响。研究结果表明,超声空化可以减少团聚,金属离子与柠檬酸比例为0.6∶1干凝胶在600℃燃烧,保温60 min,可以得到团聚少、颗粒尺寸为20~30 nm的纳米Al2O3粉体。 相似文献
16.
Reduction in the temperature coefficient of the optical path length, d S/d T of Li 2O-Al 2O 3-SiO 2 glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The d S/d T value of 2.6 mol% B 2O 3-doped glass-ceramic was 12.5 × 10 −6/°C, which was 0.9 × 10 −6/°C smaller than that of B 2O 3-free glass-ceramic. On the other hand, reduction in d S/d T through B 2O 3 doping was not confirmed in precursor glasses. Results showed that reduction in d S/d T of the glass-ceramic through B 2O 3 doping is caused by the reduction in ?. The reduction in ? from B 2O 3 doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases d S/d T. The d S/d T value of B 2O 3-doped glass-ceramic crystallized under 196 MPa was 11.7 × 10 −6/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass. 相似文献
17.
Differences in strength connected with colour variations in NiO-containing Mg-alumosilicate glass ceramics give rise to a remission spectroscopic analysis of the Ni-on. In the base glass, Ni 11 occupies tetrahedral and octahedral positions of the dark green NiAl spinel. An increase of the Mg-component and a rise of the melting intensity promote the octahedral nickel coordination and along with it the formation of the strength-reducing, light green Ni-forsterite. The demand for a strong final product is thus met by the technologically desirable shortening of the melting time. 相似文献
18.
The preparation of large bulk oxide eutectics with homogeneous and dense structure in nano-scale by melt growth method is a difficult challenge. Fully dense, homogeneous and crack-free ternary nanostructured Al 2O 3/YAG/ZrO 2 hypereutectic plate with large surface is successfully obtained by laser remelting. The hypereutectic in selected composition presents an ultra-fine eutectic-like microstructure consisting of alternating interpenetrating Al 2O 3, YAG and ZrO 2 lamellae with mean interphase spacing of about 150 nm, which is much smaller than the ternary eutectic composition grown at the same growth conditions. With the increase of laser scanning rate, the lamellar spacing is rapidly decreased. The minimum value obtained is 50 nm. The analysis indicates that the strong faceted growth behavior and cooperative branching of the component phases related with high entropies of fusion and large kinetic undercooling during laser rapid solidification are the primary formation reasons for the irregular eutectic growth morphology. Furthermore, the unique cellular microstructure with complex structure is also observed at high growth rate, and their formation mechanism and effect of the composition on the microstructure are discussed. 相似文献
19.
The colorless and transparent glasses in the Al 2O 3---B 2O 3---SiO 3 system with high B 2O 3 and SiO 2 content were prepared from gels at low temperature. Their IR spectra not only revealed the evolution of the gel to glass conversion, but also showed that the formation of mixed bonds in the glasses obtained did not show any effect due to the B 2O 3 content. The accuracy of the glass composition is dependent upon the SiO 2/B 2O 3 molar ratio. The higher the ratio, the less the deviation of the analyzed compositions of the resulting glasses from their original calculated values. It is obvious that the higher the ratio, the lower the thermal expansion coefficient and the higher the transformation temperature of the glass, and the temperature at which the thermal contraction reaches an equilibrium is higher. 相似文献
20.
The immiscibility boundary and the critical point of SiO 2-Nd 2O 3 system glass were determined as a function of Al 2O 3 addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al 2O 3. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd 2O 3 in SiO 2. It is concluded that the Al 2O 3 addition to Nd 2O 3 containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al 2O 3 addition diminishes the concentration fluctuation of Nd 2O 3 in silica glass. 相似文献
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