Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al₂O₃ is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al₂O₃-Cr₂O₃-ZrO₂ composites were produced by hot-pressing in order to harden the Al₂O₃ matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO₂ inhibited the grain growth of Al₂O₃-Cr₂O₃ and the grain growth of ZrO₂ proceeded with increasing amounts of ZrO₂ in the Al₂O₃-Cr₂O₃-Zr₂ composite. The formation of solid solution Al₂O₃-Cr₂O₃ was also confirmed by XRD, and monoclinic ZrO₂ increased on addition of Cr₂O₃. Maximum hardness was at Al₂O₃-10wt% Cr₂O₃ with 10 vol% ZrO₂ and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

The structure of Al2O3-Cr2O3 powders condensed from a plasma

The structure of Al₂O₃-Cr₂O₃ powders prepared by plasma oxidation of the mixed halides has been examined by X-ray diffraction, electron microscopy and electron spin resonance. The powders consisted predominantly of faceted spherical particles of a well crystallized solid solution of Cr₂O₃ in θ-Al₂O₃, with diameters of the order of 0.1 μm. Some larger particles of α-Cr₂O₃ were present in powders containing 17.8 and 24 wt% Cr₂O₃. The maximum solid solubility of Cr₂O₃ in θ-Al₂O₃ observed was 18 wt%. It is suggested that nucleation of crystallization of liquid Al₂O₃-Cr₂O₃ droplets occurs as a structure based on cubic close packing of oxygen ions and that the presence of chromium results in ordering to the θ-Al₂O₃ form rather than the δ-Al₂O₃ form usually observed in alumina powders prepared by plasma methods.     

Phase formation processes in the NiO-CuO-Fe2O3-Cr2O3 system upon salt decomposition

This paper presents a comparative evaluation of phase formation in the NiO-CuO-Fe₂O₃-Cr₂O₃ system during salt decomposition reactions. A spinel phase has been shown to form in all of the materials studied. The structure of the compounds obtained has been investigated by X-ray diffraction and low-temperature nitrogen adsorption measurements. The results demonstrate that synthesis in the presence of an organic precursor allows fine-particle spinel materials to be obtained.     

Subsolidus,high temperature phase relations in the systems Al2O3-Cr2O3-ZrO2, MgO-Cr2O3-ZrO2 and MgO-Al2O3-ZrO2

In the temperature range 1600 to 1900° C, the system A₂O₃-Cr₂O₃-ZrO₂ is characterized by the coexistence of ZrO₂ (unstablilized) and an (Al, Cr)₂O₃ solid solution series. In the systems MgO-Cr₂O₃-ZrO₂ and MgO-Al₂O₃-ZrO₂ a nearly stoichiometric spinel coexists with both stabilized and unstabilized ZrO₂. At temperatures above 1600°C a new ternary Mg-Al-Zr oxide becomes stable in the MgO-rich part of the MgO-Al₂O₃-ZrO₂ system.     

Experimental determination of some periclase and spinel compositions in the system CaO-MgO-Al2O3-Cr2O3-Fe-O2-SiO2

The solubilities of Cr₂O₃, Fe₂O₃ and Al₂O₃ (as well as CaO and SiO₂) in periclase were determined from samples equilibrated in the temperature range 1550 to 1725° C, at two oxygen pressures (air and fO₂ about 10^–9atm). Most samples contained periclase, spinel, and liquid which is the mineralogy of most chrome-bearing refractories at manufacturing and service temperatures. In air a large observed change in the R₂O₃ (where R₂O₃ = Al₂O₃+Fe₂O₃+Cr₂O₃) content of periclase with increasing temperature results in a new example of retrograde solubility which causes crystalline periclase to grow in amount in some refractory compositions as the temperature increases. At low oxygen fugacity (log fO₂ –9) the iron oxide content (mainly FeO) of the periclase decreased (from 6.4 to 1.3wt%) as temperature increased. In air the iron oxide content (FeO + Fe₂O₃) decreased marginally (typically from 6.8 to 6.4 wt%) as temperature increased which contrasts with previous literature statements that there is an increase. At both oxygen pressures the Cr₂O₃ content increases markedly as temperature increases (up to 17 wt %), suggesting that Cr₂O₃ (and not Fe₂O₃ as the previous literature states) may be the cause of textural changes observed in commerical products subjected to temperature cycling. The presence of increasing CaO in the bulk samples leads to lower R₂O₃ contents in the periclase crystals. The data allow further more restricted choices to be made in refractory formulations towards the goal of matching the phase compositions to the properties desired at service temperatures. A few analyses of spinels show that iron levels decrease as temperatures increase and CaO levels up to 0.9 wt % were encountered.     

Analytical transmission electron microscopy of La2O3-doped Y2O3

Analytical electron microscopy has been used to study the precipitation reactions in sintered samples of 9 mol% La₂O₃-Y₂O₃ samples upquenched from the single phase cubic region into the cubic and hexagonal phase field. Samples annealed just inside the two-phase cubic-cubic and hexagonal solvus exhibited predominantly grain boundary precipitation. Small La₂O₃ rich second phases formed within the first ten minutes and developed into strained, facetted precipitates after 300 min. Intergranular and intragranular precipitation occurred in samples annealed further into the two-phase field. Strained, lathlike La₂O₃-rich monoclinic precipitates, exhibiting a preferrred orientation in the matrix, appeared as the dominant morphology for long times at temperature. Chemical microanalyses of the strained structures obtained in samples annealed for 300 min revealed La₂O₃ matrix concentrations in agreement with phase diagram predictions. However, the La₂O₃ concentrations in the second-phase precipitates were found to be far in excess of the cubic and hexagonal-hexagonal solvus. This discrepancy is believed to arise from a re-equilibration of the second phase in the cubic and monoclinic phase field during quenching.     

La2O3在MgO-Al2O3-SiO2-TiO2微晶玻璃中的作用

在ＭｇＯ－Ａｌ_２Ｏ_３－ＳｉＯ_２－ＴｉＯ_２玻璃中添加不同数量的氧化镧,采用差热分析、Ｘ射线衍射及电子显微镜等技术研究了氧化镧对玻璃析晶过程与力学性能的影响．氧化镧的加入使玻璃中析出α－堇青石相的温度降低;同时避免了高膨胀方石英相的析出．随着氧化镧加入量的增加,玻璃整体析晶能力下降,微晶玻璃中晶相含量减少,晶粒尺寸增大,微晶玻璃的弹性模量与硬度减小,断裂韧性增加,体现出大尺寸长柱状金红石晶粒的增韧作用．     

Glass Formation in the BaO–La2O3–B2O3 System

The BaO · B₂O₃–La₂O₃ · B₂O₃ and 3BaO · B₂O₃–La₂O₃ · 3B₂O₃ joins of the ternary system BaO–La₂O₃–B₂O₃ are studied using x-ray diffraction and density measurements. The boundary of the glass-forming region is shown to pass near ternary eutectic compositions. The existence of the compound 3BaO · La₂O₃ · 2B₂O₃ is confirmed.     

Electrochromic and optical study of atmospheric pressure chemical vapour deposition MoO3-Cr2O3 films

Electrochromism (EC) is a phenomenon in which materials are able to change their optical properties in a reversible and persistent way under the action of a voltage pulse. The studied MoO3-Cr2O3 films are obtained by atmospheric pressure CVD. Mixing MoO3 films with Cr2O3 is expected to enhance optical transparency and to modulate electrochromic properties of MoO3 films. In the present work, the study is focused on the morphological, structural and optical properties of MoO3-Cr2O3 films as a function of annealing temperatures. Raman spectroscopy and optical spectrophotometry are used for the film characterization. The mixed oxide films obtained on ordinary glass substrates show transmittance values in the range of 70-80%. Surface morphology is analyzed by SEM and AFM methods. The microanalysis of MoO3-Cr2O3 films reveals uniform distribution of the elements, which is a sign of homogeneous structure.     

La_2O_3-Li_2O-B_2O_3三元系玻璃网络结构的研究

本文用拉曼及红外光谱和差热曲线研究了Ｌａ２Ｏ３－Ｌｉ２Ｏ－Ｂ２Ｏ３三元系玻璃的微观结构，着重研究了锂和镧的氧化物的引入对硼的配位数及玻璃网络结构的影响，发现Ｌｉ２Ｏ和Ｌａ２Ｏ３的作用相似，它们的增加都会使玻璃网络的基本结构基团四硼酸盐基团→二硼酸盐基团→偏硼酸盐基团演变。     

Rare-earth La2O3 modification of laser-clad coatings

The effect of rare-earth La2O3 on laser-clad coatings has been studied. For this purpose, nickel-based alloy powders with different contents of La2O3 were laser-clad on to a steel substrate. The clad coatings were examined and tested for microstructural features, microhardness, inclusions and phase composition. The results are compared with those for coatings without La2O3. The comparison indicates that additions of La2O3 refine the microstructure of the clad, and the coating increases its microhardness. Moreover, the wear resistance and corrosion resistance of the clad coatings are also enhanced.     

层状钙钛矿La2KMn2O7和La3KMn3O10的制备及磁、电性质的研究

采用溶胶-凝胶法制备了La2KMn2O7和La3KMn3O10两种后状钙钛矿锰氧化物多晶样品，研究了乙酸(CH3COOH)超声处理后对La2KMn2O7和La3KMn3O10结构和磁性的影响。结果表明超声取代可以提高样品的结晶度，对样品的结构及磁性能也有较大影响。本文对上述现象的可能原因也进行了初步讨论，并讨论了样品的磁电阻效应。     

Dielectric breakdown study of thin La2O3 films

The dielectric breakdown in La₂O₃ films of thickness 40–400 Å incorporated in capacitors is reported. The experimental results were analysed in the light of Forlani and Minnaja's theory of ionization avalanche breakdown. The dielectric breakdown strength was found to be a power function of thickness, varying as d^?0.66, in essential agreement with the theory of Forlani and Minnaja. Furthermore, these films are amorphous and have high breakdown fields (about 10 MV cm^?1).