Combustion Synthesis of Metal Chromite Powders

Fine-particle metal chromites (MCr₂O₄, where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn) have been prepared by the combustion of aqueous solutions containing the respective metal nitrate, chromium(III) nitrate, and urea in stoichiometric amounts. The mixtures, when rapidly heated to 350°C, ignite and yield voluminous chromites with surface areas ranging from 5 to 25 m²/g. MgCr₂O₄, sintered in air at 1500°C for 5 h, has a density of 4.0 g/cm³.     

Aqueous Combustion Synthesis of Strontium-Doped Lanthanum Chromite Ceramics

An aqueous combustion synthesis is used to produce powders of La_0.8Sr_0.2CrO₃ perovskite. It is shown that interaction between chromium nitrate and glycine controls the process. In addition, it is suggested that glycine reacts with products of nitrate decomposition to yield an intermediate compound, which decomposes exothermically providing high-temperature conditions for complex oxide formation. It is remarkable that although reaction temperature is high (up to 800°C) and characteristic time is small (∼1 s) for synthesis under the self-propagating high-temperature mode, the produced perovskites have high specific surface area (∼40 m²/g) and well-defined crystalline structure. As a result, ceramics sintered by using these powders are dense (∼96% of theoretical) and possess high electronic and low ionic conductivities, important for interconnect applications in solid oxide fuel cells.     

Chemical Preparation of Pure and Strontium- and/or Magnesium-Doped Lanthanum Gallate Powders

Powder compositions of LaGaO₃, La_0.9Sr_0.1GaO_2.95, and La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 were prepared via a Pechini-type process that uses citric acid and ethylene glycol. The calcination behavior of the precursor powders of the above-mentioned phases was studied in the temperature range of 200°–1400°C in an air atmosphere. Characterization of the powder samples were performed using several processes, including X-ray diffractometry, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy, and carbon and nitrogen analyses.     

Bonding of Zirconia and Lanthanum Chromite by Co-firing

Zirconia has been bonded to lanthanum chromite by co-firing. The ceramic bodies are joined during sintering with an applied pressure of less than 0.1 MPa. This technique facilitates the formation of a junction phase which involves the impurity silica. During firing at 1500°C, silica is driven from zirconia and lanthanum chromite to the junction region, where it combines with calcia as well as other components to form a liquid phase. The liquid phase relaxes stresses due to differential shrinkage of the two bodies and, on cooling, solidifies, thus bonding lanthanum chromite to zirconia.     

Tape Casting of Lanthanum Chromite

The effects of process additives, ball milling, and solids loading were evaluated for tape casting suspensions of glycinenitrate-synthesized La_0.7Ca_0.31CrO₃ powder. An optimized formulation was obtained based on rheological characterization, electrokinetic sonic amplitude measurements, qualitative examination of green tapes, and the sintered microstructure. The tape casting formulation incorporated 66:34 methyl ethyl ketone/ethyl alcohol solvent, an aliphatic phosphate ester dispersant, and 80 wt% (35 vol%) solids. The best binder/plasticizer system was 12 wt% (15 vol%) poly(isobutyl methacrylate) and 5 wt% (6.3 vol%) benzyl butyl phthalate plasticizer (binder:plasticizer = 2.3). Cast tapes were sintered at 1300°C for 2 h, producing a bulk density of 96.2% theoretical, with linear shrinkage of 22% and an approximate grain size of 1.3 μm.     

Synthesis and Characterization of Lanthanum Zirconate Nanocrystals Doped with Iron Ions by a Salt-Assistant Combustion Method

Nanocrystalline (La _x Fe_1−x )₂Zr₂O₇ series solid solutions were prepared by a convenient salt-assisted combustion process using glycine as fuel. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results showed the La ion can be partially replaced by Fe ion. The partial substituted products were still single-phase solid solutions and the crystal form remained unchanged. TEM images reveal that the products are composed of well-dispersed square-shaped nanocrystals. The method provides a convenient and low-cost route for the synthesis of nanostructures of oxide materials. The catalytic property of (La _x Fe_1−x )₂Zr₂O₇ can be measured by the change of catalytic degradation rate of Methyl orange. The results show that the catalytic property of La _x Fe_2−x Zr₂O₇ is significant, and it has good application prospects as photocatalyst.     

Dispersing Behavior of Hydroxyapatite Powders Produced by Wet-Chemical Synthesis

Three hydroxyapatite powders with different surface properties were produced by wet-chemical synthesis and characterized. The electrokinetic properties of powders dispersed in water were investigated by electroacoustic spectroscopy measurements. The different surface reactivity (pH_iep and ζ potential versus pH curves) was related to the interplay of dissolution and adsorption of Ca²⁺ ions. With a view toward the preparation of porous bodies by sponge impregnation, the behavior of powder suspensions was studied. Four deflocculants were tested, and the optimum dispersing conditions for each powder were found. Anionic polyelectrolytes resulted in the best effective dispersing agent, with different optimum amounts added to the suspensions.     

Reaction between Calcium-Doped Lanthanum Chromite and Silica

Calcium-doped lanthanum chromite reacted with silica to form silicates and chromium oxide at 1473-1773 K. A reaction mechanism was proposed at each temperature. At 1473 K, a reaction zone was observed in two areas: one was the bulk reaction area where silicates and chromium oxide were formed near the reaction interface; the other was the grain boundary reaction area where chromium oxide and calcium silicate spots were formed far from the reaction interface. Above 1673 K, silicates in the La₂O-CaO-SiO, system and chromium oxide were formed simultaneously in the reaction zone with no grain boundary reaction. No layered reaction products were observed in the reaction zone of the lanthanum chromite side. Formation of melt silicates above 1723 K at the reaction interface resulted in strong bonding between calcium-doped lanthanum chromite and silica.     

LaCrO_3导电陶瓷的研制

LaCrO3导电陶瓷是一种能在氧化气氛中使用的高温发热体新材料。本文作者按照实际使用的样品尺寸研究了含Ca 2 的LaCrO3导电陶瓷的低温烧结性和导电性关系。结果表明 ,Ca 2 含量 <1 2 %时 ,低温烧结的LaCrO3烧结体电阻率明显降低 ,当Ca 2 含量 >1 2 %时 ,烧结体电阻率升高。在采用注浆成形工艺中 ,找到了顺利脱模的添加剂和操作方法 ,从而解决了生产大尺寸样品的注浆成形问题。     

Synthesis and Characterization of Lanthanum Stannate Nanoparticles

Phase-pure lanthanum stannate (La₂Sn₂O₇) nanoparticles with a pyrochlore structure were produced by a simple coprecipitation method for the first time. The optical properties of La₂Sn₂O₇ synthesized by different conditions were investigated. They had different photoluminescence intensities around 400 nm at the same excitation wavelength. The size of La₂Sn₂O₇ nanoparticles synthesized by the reverse-drop method was smaller than those synthesized by the nature-drop method. The nanoparticles became large rapidly with the dilution of the original reagent solution.     

Synthesis and Characterization of Barium Lanthanum Titanates

Ternary compounds in the system BaO—TiO₂—La₂O₃ were prepared by the solid-state reaction technique at temperatures between 1300° and 1400°C using precursor oxides as the starting materials. In an alternative processing technique, BaTiO₃ was reacted with appropriate proportions of prefabricated lanthanum titanates at 1350°C to obtain the compounds. Two compounds were identified in the TiO₂-rich region of the system. The X-ray powder diffraction pattern of a compound with a chemical composition BaLa₂Ti₃O₁₀ (BaO·La₂O₃·3TiO₂) is indexed on the basis of an orthorhombic unit cell with a = 7.665 × 10⁻¹ nm, b = 28.524 × 10⁻¹ nm, and c = 3.876 × 10⁻¹ nm. The other compound, which has a chemical composition Ba₄La₈Ti₁₇O₅₀ (BaO·La₂O₃·4.25TiO₂) occurs in a narrow homogeneity range within the system. The X-ray powder diffraction pattern of the compound is indexed on the basis of an orthorhombic unit cell with a = 12.317 × 10⁻¹ nm, b = 22.394 × 10⁻¹ nm, and c = 3.881 × 10⁻¹ nm. Both the compounds are compatible with BaTiO₃ and form pseudobinary joins with BaTiO₃ in the system BaO—TiO₂—La₂O₃.     

Mechanical Properties of Solid-State-Synthesized Strontium- and Magnesium-Doped Lanthanum Gallate

This paper has examined the room-temperature mechanical properties of La_0.8Sr_0.2Ga_{1− x} Mg _x O_3−δ ( x = 0.1–0.2) that was synthesized by using a solid-state technique. The modulus of rupture (MOR) was measured over the range of x = 0.1–0.2 and decreased as the B-site dopant content increased. At x = 0.15, for example, the MOR was 139 ± 17 MPa, whereas at x = 0.2, an MOR value of 113 ± 8 MPa was observed. This decrease is postulated as being caused by the increased lattice stress due to the increased magnesium concentration on the B-site. The fracture toughness of the sample decreased as the magnesium content increased; at x = 0.15, a fracture toughness of 1.63 ± 0.2 MPam^1/2 was observed, whereas at x = 0.2, a value of 1.28 MPam^1/2 was obtained.     

Sol-Gel Synthesis of Lanthanum Cobaltite Powders with Added Strontium and Calcium

Glass and Ceramics - Powders with the composition La1–xSr(Ca)õCoO3 (x = 0.2) were obtained using the sol-gel method. A microwave setup (radiation power 90 W, frequency 2450 MHz) and a...     

Mechanical Properties of Magnesia-Doped Lanthanum Chromite versus Temperature

Magnesia-doped lanthanum chromite is a potential material for use in solid oxide fuel cells as an interconnector due to its resistance to oxidation and reduction. The strength and toughness of La(Cr _0.9 Mg _0.1 )O₃ were measured from 25° to 1000^deg;C in the as-fired reduced state and after oxidation. The as-fired samples showed a peak in toughness of approximately 3.9 MPa.m^1/2 at 125^deg;C which decreased to approximately 1.4 MPa.m^1/2 at 600^deg;C and 2.8 MPa.m^1/2 at room temperature. This peak in toughness is hypothesized to be due to the rhombohedral-to-orthorhombic phase transition by a toughening mechanism that is currently being investigated. The strength was also affected by the phase transition for both the as-fired reduced and the oxidized samples.     

YAG超细粉体的制备与表征

以Y2O3、Nd2O3、Al(NO3)3·9H2O、(NH4)2SO4为原料,分别采用碳酸氢铵和碳酸铵两种沉淀剂,通过共沉淀法制备了YAG超细粉体.采用热重-差热分析(TG-DTA)与红外光谱(IR)对前驱体进行分析研究.对不同温度下煅烧的粉体进行X射线衍射(XRD)与透射电镜(TEM)分析.研究结果表明:采用碳酸氢铵作为沉淀剂得到的前驱体经1000 ℃煅烧可获得纯YAG相,颗粒尺寸为60～150 nm;而采用碳酸铵作为沉淀剂得到的前驱体经900 ℃煅烧即可获得纯YAG相,颗粒尺寸较小,约为40～50 nm,具有良好的分散性.     

Characterization of Calcium Lanthanum Oxalate Powders Coprecipitated from Different Binary Solvent Mixtures

White amorphous powders of well-mixed calcium and lanthanum oxalates were prepared by coprecipitation of calcium and lanthanum nitrates with 1 M oxalic acid. SEM observation showed particle shapes of cubes, flakes, and agglomerates when water-organic solvent mixtures were employed as the precipitation media. The measured specific surface area and the median particle diameter ( D ₅₀) ranged between 1.5–76 m²/g and 0.9–14 μm, respectively, as a result of the divergent particle morphology. The morphology changes were attributed to a change of precipitation rate in different solvent mixtures. An attempt was made to explain the rate changes by the solvation energy change of ions which resulted in interfacial energy change within the depositing surface region.     

ZA—5型氨合成催化剂的研究：基本物化特性与表征

我国Ｆｅ１－ｘＯ基催化剂体系的创立为熔铁催化剂的发展开辟了一条新途径。经多年努力,在Ｆｅ１－ｘＯ催化剂体系和Ａ３０１型催化剂基础上又开发成功低温高活性的ＺＡ－５型催化剂,运用ＸＲＤ、ＸＰＳ、ＳＥＭ、Ｍｏｓｓｂａｕｅｒ、Ｎ２ＴＰＤ、ＴＧ－ＤＴＧ和ＢＥＴ及化学吸附等方法研究了ＺＡ－５型催化剂的化学组成、晶体结构、表面组成和表面结构等基本物化特性。     

Strain and Fracture of a Ceramic Based on Lanthanum Chromite

The bending behavior of a ceramic — a lanthanum chromite-based solid solution La_{1 – x} Ca_xCrO₃ (x = 0.025) doped with 5 – 40 wt.% Y₂O₃ — is studied in the temperature range of 20 – 1400°C. At 20°C the composition with 5 – 10 wt.% Y₂O₃ is strained inelastically and exhibits relatively superior mechanical properties in comparison to compositions containing 20 – 40 wt.% Y₂O₃. These compositions are prone to brittle fracture within the linear elastic strain. Based on the strain and fracture characteristics of lanthanum chromite at 20°C, its mechanical behavior at high temperatures is predicted.     

Synthesis of Mullite Powders Through a Suspension Combustion Process

This work reports on the synthesis of mullite powders by a suspension combustion process. Aluminum nitrate, as a source of Al, and a colloidal silica suspension, as a source of Si, are used as reagents, and urea serves as the fuel. This colloidal suspension allows a complete mixing of the reactants, thus promoting a core-shell reaction to produce the mullite. The powders obtained need to be treated at a high temperature to form the mullite. However, when ammonium nitrate is also added as a combustion aid, the mullite phase is directly obtained without any further treatment. A chemical reaction involving all these compounds is proposed, and the thermodynamic parameters are calculated in order to predict the temperature reached during the combustion process. The addition of ammonium nitrate reduces the temperature of the reaction by ca. 150°C. Once the conditions for the combustion reaction are optimized, the Al/Si ratio is modified in an attempt to obtain monophasic mullite.     

Solvothermal Synthesis and Characterization of Barium Titanate Powders

Cubic BaTiO₃ powders have been synthesized from gel powders after thermal treatment in an alcohol solution and characterized using X-ray diffractometry, transmission electron microscopy, and other techniques in detail. The borderline reaction conditions, such as the reaction temperature and time, for the synthesis of crystalline BaTiO₃ powder in different solvents are established. The single-phase BaTiO₃ powder has low agglomeration and seems to have a regular morphology. The formation of BaTiO₃ powders via solvothermal reaction is more difficult, in comparison to hydrothermal processing. The crystalline powders, which have a small particle size (∼20–60 nm) and narrow particle-size distribution, can be synthesized in methanol, ethanol, or n -propanol systems. Unlike the hydrothermal reaction, tetragonal-phase BaTiO₃ powders cannot be prepared via the solvothermal reaction, even with alkali catalysts.