The effect of pre-annealing and post-annealing on the transparency of MgAl2O4, prepared by slip casting and spark plasma sintering (SPS)

The aim of this paper is to investigate the effect of slip casting process and the annealing before and after sintering to achieve a transparent MgAl₂O₄. To remove contaminants such as carbon from the structure of shaped spinel bodies, at first, the samples were annealed at temperature of 800?°C, 900?°C and 1000?°C for 2?h and then sintered at 1400?°C. By annealing the sample before sintering at 900?°C, the transmission increased (15% at IR region and 10% at visible region). Although by annealing the samples, the amount of carbon contamination reduced. Annealing the samples after sintering also had some desirable results. The samples annealed at temperature of 1200?°C for a time of 3, 5 and 10?h. The darkness of samples reduced due to the removal of carbon impurities and the sample was annealed at 1200?°C for 5?h had the most transparency in the visible and infrared regions.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.     

Al2O3-3YTZP-Graphene multilayers produced by tape casting and spark plasma sintering

This work aims to establish a colloidal route to obtain laminates of alumina–zirconia combining layers with and without graphene. Green tapes of alumina, alumina with 5 vol.% of 3Y-TZP and alumina with 5 vol.% of 3Y-TZP and graphene-oxide (2 vol.%) were obtained by aqueous tape casting. It is possible to design materials for different structural applications with a controlled microstructure with a high number of different layers. The tapes were punched into 20-mm discs, joined to form laminates alternating up to 18-layers, and sintered in one-step by spark plasma sintering (SPS) at 1400 °C. It has demonstrated that there is a significant graphite diffusion provoked by the required graphite holders into the SPS-furnace. Dense laminates with layer thicknesses ∼100 μm and good cohesion between layers were obtained. Nanoindentation results showed that hardness and elastic modulus values were higher than 27 GPa and 300 GPa, respectively, and similar for all layers.     

Al2O3-C质整体塞棒在浇铸钙处理钢中的应用

分析了Al2O3-C质整体塞棒在钙处理钢浇铸过程中的侵蚀机制和其他一些重要影响因素,针对Al2O3-C质整体塞棒的特性,从炼钢的工艺改进、设备保障、优化过程控制和耐火材料材质优化等方面采取了相应措施,并提出了改进意见,使Al2O3-C质整体塞棒在钙处理钢的使用寿命不断提升,达到了比较满意的效果。     

Preparation of porous Al2O3 ceramics by starch consolidation casting method

Porous alumina ceramics were fabricated by starch consolidation casting using corn starch as a curing agent while their microstructure, mechanical properties, pore size distribution, and corrosion resistance were examined. Results showed that the porous alumina ceramics with the flexural strength of about 44.31MPa, apparent porosity of about 47.67% and pore size distribution in the range of 1‐4 μm could be obtained with 3wt% SiO₂ and 3wt% MgO additives. Corrosion resistance results showed mass losses: hot H₂SO₄ solution and NaOH solution for 10 hours were 0.77% and 2.19%, which showed that these porous alumina ceramics may offer better corrosion resistance in acidic conditions.     

Furnace smelting and extractive metallurgy of red mud: Recovery of TiO2, Al2O3 and pig iron

Turkish red mud (bauxite waste) has been mixed with dolomite and coke, pelletized and sintered at 1100°C, and finally smelted at 1550°C to produce pig iron and a slag. The slag was leached with 30% H₂SO₄ at 90°C. The leachate was diluted, ferric iron was reduced with SO₂, and extracted with 5% D2EHPA solution in kerosene. Silica and Al₂O₃ were recovered from the remaining aqueous solution, while the organic extract was stripped with 10% Na₂CO₃ solution, finally hydrolysed and calcined to produce pigment-grade TiO₂. The titanium recovery efficiency on the basis of slag weight was 84·7%. The extractive separation of titanium from both valencies of iron was investigated as a function of pH and time. A stoichiometric flowsheet for the whole process has been developed. ©1997 SCI     

氧化铝陶瓷凝胶注模成型工艺浆料流变性的研究

以氧化铝陶瓷为例,研究探讨了凝胶注模成型工艺中制备低粘度、高固相含量浓悬浮体的关键技术,分析讨论了氧化铝浆料的流变性特点及pH值、分散剂等因素对粘度的影响。实验表明:在pH=9,分散剂添加量为0.5%质量分数的Al2O3时,55%体积分数的氧化铝浆料的粘度为100mPa·s(D=77.48S-1)     

An approach for matrix densification based on particle packing and its effect on lightweight Al2O3-MgO castables

For lightweight refractory containing lightweight aggregates, the properties of the matrix are decisive to its performance. In the present work, Dinger–Funk equation was adopted to calculate the theoretical packing density of a castables matrix based on Stovall linear packing model and to design its particle size distribution. Four lightweight Al₂O₃-MgO castables with different particle size distribution (represented by q-value) were prepared and examined. Results show that a suitable q-value was needed to ensure acceptable properties including sintering characteristics, strength and slag resistance, which deteriorated distinctly at high q (>=0.31). For the sample with q=0.28, the matrix showed dense and uniform mirostructure, and the properties of castable reached a favourable compromise among sintering characteristics (apparent porosity=14.8%, bulk density=3.02 g cm⁻³, permanent linear change<0.6%), strength (cold modulus of rapture=12.4 MPa, cold crashing strength=155.5 MPa), and resistance against both slag corrosion (I_c=22.4%) and penetration (I_p=11.5%). The sample with q=0.25 showed the highest strength and resistance against slag corrosion (matrix dissolved in slag), but its slag penetration resistance was lower due to the existence of cracks between aggregates and matrix.     

New route for processing of multilayer Al2O3-Co3O4 materials through gelcasting

Multilayer dense ceramic materials composed of Al₂O₃ and Al₂O₃-Co₃O₄ layers have been obtained by gelcasting. The key stage in the process was the optimization of the polymerization idle time in order to ensure strong adhesion between layers without cracks and delamination in a green state and after sintering. The significant advantage of this method is occurence of strong connections between consituent layers due to the slight migration of the slurry to the gelled bottom part of the sample, what is not obseved in techniques based on lamination processes. The multilayer samples were composed of two Al₂O₃ layers and two Al₂O₃-Co₃O₄ layers arranged alternately. The rheological characterization of the slurries was done. The properties of the sintered multilayer bodies were examined in comparison to the single-layer alumina samples. Observations in SEM and ligth miscroscope were performed. The presence of the transition layer in the sintered bodies was observed.     

甲烷在Cr改性Pd/Al2O3催化剂上的催化燃烧性能

研究了Cr改性Pd/Al2O3催化剂上低浓度甲烷的催化燃烧反应,考察了载体Cr Al的制备方法和活性组分Pd的负载方法对催化剂催化活性的影响以及添加Ce对催化剂高温稳定性的影响。采用X射线衍射、程序升温还原等表征手段分析了催化剂结构和氧化还原性。结果表明,与采用沉淀法制备的载体P-Cr Al相比,采用浸渍法制备的载体I-Cr Al具有较高的比表面积和反应活性;用Na BH3对Pd负载过程进行还原处理能明显提高催化剂活性,其原因是还原过程加强了催化剂上Pd与载体Cr Al之间的作用,通过H2-TPR证明了其还原能力得到了增加;添加Ce缓解了Al2O3高温条件下的烧结,增强了催化剂的高温稳定性。     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

连铸用铝碳滑板的热应力仿真分析

使用ANSYS等有限元分析软件,针对钢带缩短量、滑板面压、紧固件强制位移等不同因素,通过施加适当的初始条件和边界条件,计算连铸用滑板在预热过程中的稳态热应力场及浇钢过程中的瞬态热应力场。并对不同因素影响下的热应力场结果进行比较,为改进实际生产中滑板的安装及使用方法提供理论依据。     

Preparation and evaluation of a nanocomposite of polythiophene with Al2O3

The polymerization of thiophene (TP), in bulk and in solution in CHCl₃ by FeCl₃ resulted in the formation of a polymer which was characterized by FTIR as polythiophene (PTP). High yield was realized in the latter case. The polymerization of TP with FeCl₃ and nanodimensional Al₂O₃ resulted in the formation of a nanocomposite which was partly dispersible in aqueous and non‐aqueous media. The dispersibility appeared to be higher when the polymerization was conducted in a suspension containing a higher amount of Al₂O₃. Scanning electron micrographs showed globular particles and the presence of clusters of composite particles. Transmission electron micrographic (TEM) analyses revealed the particle size of the composite to be in the range 22–74 nm. Thermal analyses (TG/DTA) revealed the outstanding stability of PTP–Al₂O₃ composites compared to that of PTP. The conductivity of PTP and of PTP–Al₂O₃ composite was of the order of 10^?3 S cm^?1 for samples doped with I₂. © 2003 Society of Chemical Industry     

Al2O3 Nanoparticulate LZS Glass–Ceramic Matrix Composites for Production of Multilayered Materials

This work reports the processing steps of Al₂O₃ (1–5 vol%) nanoparticulate (d_V.50 = 13 nm) LZS glass–ceramic matrix (19.58Li₂O·11.10ZrO₂·69.32SiO₂, mol%, d_v.50 = 3.5 μm) composites for production of multilayered materials with thermal expansion gradients obtained by tape casting. Suspensions were prepared in water to solids contents ranging from 40 to 47 vol% using ammonium polyacrylate as a deflocculant, and an acrylic copolymer and polyvinyl alcohol as binders. Optimum performance was achieved by sonication and controlling the rheological properties for every step of the process. To prepare the composites, different concentrations (1, 2.5 and 5 vol%) of nanoalumina were added to fresh, as‐prepared LZS suspensions, by changing the solid contents as required to maintain similar viscosities. Green tapes with high uniformity, without macroscopic defects and easy to handle were sintered to relative densities between 89% and 94%. Dense and homogeneous laminates with gradual composition with increasing concentrations of alumina were obtained.     

Activity and stability of Cu/ZnO/Al2O3 catalyst promoted with B2O3 for methanol synthesis

The addition of B₂O₃ to a Cu/ZnO/Al₂O₃ catalyst increased the activity of the catalyst for methanol synthesis after an induction period during the reaction. The stability of the B₂O₃-containing Cu/ZnO/Al₂O₃ catalyst was greatly improved by the addition of a small amount of colloidal silica to the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

不同粒径Pd/Al2O3催化乙炔加氢反应微观动力学分析

采用等量浸渍法制备了α-Al₂O₃负载的系列Pd催化剂,运用BET、XRD、ICP-AES、CO化学吸附、TEM等手段对催化剂进行了表征;根据部分析因实验设计方案进行动力学实验,采用微观反应动力学方法模拟和分析了所获稳定期本征动力学实验结果。结果发现,制备所得催化剂Pd颗粒的平均粒径分别为1.6、3.4、5.5 nm,CO化学吸附所测定达到活性稳定期后的催化剂表面Pd原子数与Hardeveld模型计算的Pd（111）表面原子数一致;模拟结果表明该微观动力学模型可以很好地模拟不同粒径催化剂上的动力学结果,在所研究范围内表面最丰物种为C₂H₄^*和C₂H₃^*,通过微观与宏观动力学的特征判断3种催化剂上乙炔加氢的速率控制步骤为乙烯基加氢生成乙烯。     

Effect of substituting Na2O for SiO2 on the non-isothermal crystallization behavior of CaO-BaO-Al2O3 based mold fluxes for casting high Al steels

The non-isothermal crystallization behaviors of CaO-BaO-Al₂O₃-based mold fluxes in which SiO₂ had been replaced by Na₂O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction. The initial crystallization temperatures, crystalline phases generated at different cooling rates, crystallization rates, Ozawa indices N_O (as determined using Mo's equation) and effective crystallization activation energies (as determined using the Friedman equation) were evaluated. The initial crystallization temperature increased along with the Na₂O content in the flux. In addition, the precipitation of CaF₂ crystals was inhibited while the growth of CaAl₂O₄ crystals was enhanced at Na₂O concentrations over 4?wt%. Increasing the cooling rate inhibited the precipitation of MgAl₆O₁₀, and of CaSiO₃, MgAl₂SiO₆ and NaAlSi₃O₈, respectively, in fluxes containing 0, 4 and 8?wt% Na₂O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na₂O level was raised, as a direct result of changes in their N_O values. The NS1 flux consistently had the lowest N_O value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the N_O values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na₂O levels, and this was evidently an important factor affecting the initial crystallization temperature.     

Influence of Al2O3 support on the activity of Ag/Al2O3 catalysts for SCR of NO with decane

The role of the Al₂O₃ support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated. A series of Ag/Al₂O₃ catalysts were prepared by impregnation method and characterized by N₂ pore size distribution, XRD, UV–Vis, in-situ FT-IR and acidity measurement by NH₃ and pyridine adsorption. The catalytic activity differences of Ag/Al₂O₃ are correlated with different properties of Al₂O₃ supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al₂O₃ (Ag/Al₂O₃ (SG), showed higher NO _x conversion (65% at 400 °C), compared with the respective catalysts made from commercial Al₂O₃ (Ag/Al₂O₃ (GB), Ag/Al₂O₃ (ALO), (∼26 and 7% at 400 °C). The higher surface area, acidity and pore size distribution in sol–gel prepared Al₂O₃ (SG) results in higher NO and hydrocarbon conversion. Based on the UV–vis characterization, the activity of NO reduction is correlated to the presence of Ag_n^δ+ clusters and acidity of Al₂O₃ support was found to be one of the important parameter in promoting the formation and stabilization of Ag_n^δ+ clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al₂O₃ (SG) is confirmed, which could also contribute to low temperature hydrocarbon activation and improve NO conversion. In situ FT-IR measurements revealed the higher rate of –CN and –NCO intermediate species formation over 4wt% Ag/Al₂O₃ (SG). We conclude that the physico–chemical properties of Al₂O₃ play a crucial role in NO _x conversion over Ag/Al₂O₃ catalysts. Thus, the activity of the Ag/Al₂O₃ catalyst can be tailored by using a proper type of Al₂O₃ support.     

Highly stable Fe/γ‐Al2O3 catalyst for catalytic wet peroxide oxidation

BACKGROUND: A highly stable Fe/γ‐Al₂O₃ catalyst for catalytic wet peroxide oxidation has been studied using phenol as target pollutant. The catalyst was prepared by incipient wetness impregnation of γ‐Al₂O₃ with an aqueous solution of Fe(NO₃)₃· 9H₂O. The influence of pH, temperature, catalyst and H₂O₂ doses, as well as the initial phenol concentration has been analyzed. RESULTS: The reaction temperature and initial pH significantly affect both phenol conversion and total organic carbon removal. Working at 50 °C, an initial pH of 3, 100 mg L^?1 of phenol, a dose of H₂O₂ corresponding to the stoichiometric amount and 1250 mg L^?1 of catalyst, complete phenol conversion and a total organic carbon removal efficiency close to 80% were achieved. When the initial phenol concentration was increased to 1500 mg L^?1, a decreased efficiency in total organic carbon removal was observed with increased leaching of iron that can be related to a higher concentration of oxalic acid, as by‐product from catalytic wet peroxide oxidation of phenol. CONCLUSION: A laboratory synthesized γ‐Al₂O₃ supported Fe has shown potential application in catalytic wet peroxide oxidation of phenolic wastewaters. The catalyst showed remarkable stability in long‐term continuous experiments with limited Fe leaching, < 3% of the initial loading. Copyright © 2010 Society of Chemical Industry     

Long-term stability of the V2O5/Al2O3 catalyst for the selective reduction of nitrogen oxides

Changes of the V₂O₅/Al₃O₃ catalyst aged for up to 10 years under real conditions of the selective catalytic reduction of NO _x by ammonia (SCR) at the tail gases of the nitric acid plant were characterized by⁵¹V NMR spectroscopy, porosimetry, temperature programmed reduction (TPR) and catalytic activity measurements. The catalytic activity and the redox properties of the catalyst were found intact. Only small variations of the ratio of the octahedral and tetrahedral vanadia species were documented by⁵¹V NMR on aged catalyst.