Investigation of the Sintering Mechanisms for (U,Am)O2 Pellets Obtained by CRMP Process

The use of CRMP (Calcined Resin Microsphere Pelletization) process for AmBB (Americium Bearing Blankets) fabrication is today a key research axis in americium transmutation domain, where its very high activity requires minimization of powder dissemination. In this aim, the use of oxide microspheres as compaction precursors is a promising clean alternative to powder metallurgy. Understanding the different steps of densification during CRMP pellet sintering thus appears as fundamental to obtain final materials with the specific features required for AmBB. The densification curve recorded in dynamic conditions shows different sintering steps. A first decrease of shrinkage rate happens at low temperature, around 1100 K. This phenomenon is not normally observed in the sintering of conventional powders. Chemical and microstructural studies were performed on (U,Am)O₂ and also on (Ce,Gd)O₂ surrogate compound to highlight the causes of this low-temperature sintering step. Multiscale reorganization finally appears as the sole explanation, through the sintering of nanometric aggregate present in the green pellet and related to the morphology of the starting microspheres employed as pelletization precursors.     

Two-step sintering behavior of sol–gel derived nanocrystalline corundum abrasive with MgO–CaO–SiO<Subscript>2</Subscript> additions

Nanocrystalline corundum abrasive with mean crystal size of less than 100 nm was synthesised by sol–gel process via two-step sintering technique. A remarkable suppression of grain growth was achieved by controlling sintering temperature and taking advantage of sintering aids (MgO–CaO–SiO₂) during the final stage of a two-step sintering process. The grain size of the high densification samples (>99% theoretical density) produced by two-step sintering method was about seven times less than that of the samples made by the conventional sintering technique. The microstructure of the samples was homogeneous without abnormal grain growth and the obtained corundum abrasive exhibited excellent abradability compared to conventional sintering methods.     

Influence of Ni nanoparticle addition and spark plasma sintering on the TiNiSn–Ni system: Structure,microstructure, and thermoelectric properties

The electronic and thermal properties of thermoelectric materials are highly dependent on their microstructure and therefore on the preparation conditions, including the initial synthesis and, if applicable, densification of the obtained powders. Introduction of secondary phases on the nano- and/or microscale is widely used to improve the thermoelectric figure of merit by reduction of the thermal conductivity. In order to understand the effect of the preparation technique on structure and properties, we have studied the thermoelectric properties of the well-known half-Heusler TiNiSn with addition of a small amount of nickel nanoparticles. The different parameters are the initial synthesis (levitation melting and microwave heating), the amount of nickel nanoparticles added and the exact pressing profile using spark plasma sintering. The resulting materials have been characterized by synchrotron X-ray diffraction and microprobe measurements and their thermoelectric properties are investigated. We found the lowest (lattice) thermal conductivity in samples with full-Heusler TiNi₂Sn and Ni₃Sn₄ as secondary phases.     

Secondary raw materials in cement industry

The sintering and hydration processes of a modified cement raw mix were examined using thermal analysis techniques. One reference and four modified mixtures, prepared by mixing the reference sample with 0.5, 1.0, 1.5 and 2.0 % w/w of a wolframite-stibnite mineral were studied. The clinkering reactions were recorded and the total enthalpy change during the sintering was determined by means of a differential scanning calorimetry. The combined water and the Ca(OH)₂ content in samples hydrated for 1 to 28 days were determined, using thermogravimetry. As it is concluded, the effect of the added mineral on the sintering and hydration reactions can be fully recorded and evaluated using thermal analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Modifying Additives on the Sintering and Properties of SiC/SiC<Subscript>w</Subscript> Ceramic Composite Material

Spark plasma sintering and hot compaction methods were used to obtain experimental samples of a composite material of the SiC?SiC_w system with various modifying additives (AlN, B₄C, HfB₂, Y₂O₃, Al₂O₃, Si₃N₄). The effect of the modifying additives on the sintering process, physicomechanical, and thermal properties of the ceramic composite material was examined. The introduction of the modifying additives lowered the sintering temperature of silicon carbide produced by the hot compaction method by 200°C and that formed with spark plasma spark sintering by 300?450°C as compared with the sintering temperature of silicon carbide without additives.     

High thermal conductivity polylactic acid composite for 3D printing: Synergistic effect of graphene and alumina

With the continuous development of the electronics industry, in order to meet the requirements of electronic equipment to reduce the size and increase power consumption, the development of high thermal conductivity materials is crucial. In this study, thermally conductive polylactic acid (PLA) composites were prepared by constructing graphene and alumina (Al₂O₃) hybrid filler network, and it was further successfully used in additive manufacturing. Due to the synergistic effect of Al₂O₃ and graphene, the resulting composite achieved the thermal conductivity of 2.4 Wm^?1 K^?1 with 70 wt% Al₂O₃ and 1 wt% graphene, which are superior to data reported in the literature in the same filler condition. The Al₂O₃ and graphene hybrid filler network reduced the agglomeration of graphene and the thermal contact resistance between the fillers, thereby leading a faster cooling rate. Furthermore, the obtained thermally conductive PLA composite has good thermal stability at a normal temperature. The PLA composite powder obtained by the cryogenic pulverization can be used in the laser sintering additive manufacturing process to prepare a heat conductive material with a complicated shape.     

Sintering of silica fume in furnace atmospheres with controlled water vapour partial pressure

Sintering process of compacted silica fume samples with respect to various water vapour partial pressures (p _total=101 kPa) within the furnace atmosphere has been characterized by means of dilatometry and bulk density measurements. Dilatometric study shows the strong dependence of sintering process on the water vapour partial pressure. Water vapour intensifies the process and affects relative densities of the final product, as well.The work was supported by Grant No. 930 of the Ministry of Education and Science of the Slovak Republic.     

Effect of boric acid on properties of high‐alumina phosphate‐bonded plastic refractory materials

The effect of boric acid on the properties of high‐alumina phosphate‐bonded plastic refractory materials at medium temperature is investigated in this work. Powder X‐ray diffraction (XRD), thermogravimetric analysis/differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM) techniques are used to investigate the compositions and microstructures of the Al₂O₃–H₃BO₃ sintering products, in order to study the influence of the generated aluminum borate on the high‐aluminum refractories. Additionally, the effect of the addition of H₃BO₃ on the densification and mechanical strength of high‐aluminum phosphate‐bonded plastic refractories is studied by the permanent linear change, apparent porosity, cold compressive strength, flexural strength, and scanning electron microscopy pattern. The densification and mechanical strength of the refractories can be improved significantly by the optimal addition of H₃BO₃. However, excess H₃BO₃ will bring about a large amount of bound water into the refractories, and superabundant aluminum borate whiskers will be generated by the excess addition of H₃BO₃, both of them resulting in the reduction of the densification and mechanical strength of the refractory. In conclusion, the optimum dosages of H₃BO₃ in the powder system of high‐alumina phosphate‐bonded plastic refractories are 5, 4, and 3 wt%, sintered at 700, 900, and 1100 °C, respectively.     

Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A series of Pt/Al₂O₃ catalysts were prepared using a sol-gel method. The influence of several parameters used in the synthesis including: metal content, identity of the metal precursor, and the water/alkoxide ratio on the structural properties of the fresh (dried) and calcined samples were studied. It was found that the BET surface area decreased with an increase in the platinum content. A surface area of 500 m²/g was obtained following calcination at 773 K. The structure of fresh samples as determined by FTIR corresponded to that of a pseudoboehmite structure. Samples prepared using a water/alkoxide ratio (H₂O/ATB) of 9 showed a well-defined, uniform pore size distribution following calcination at 773 K. Metal dispersions comparable to those obtained using impregnation methods were obtained. Aging studies (calcination at 873 K for 24 h) performed on these catalysts, exhibited sintering behavior which were similar to Pt/Al₂O₃ catalysts prepared by other methods. The sample prepared using a H₂O/ATB ratio of 9 had the highest surface area and was more thermally resistant towards metal sintering. A bimodal metal particle size distribution was observed: some particles exhibited sintering while others of similar size showed a greater thermal stability to sintering. The sample having the largest surface area and the highest thermal stability following thermal treatment was a consequence of a more condensed structure and a higher pore roughness obtained after drying the gel. This enabled the formation of an alumina structure which was more amorphous and limited aggregation of platinum particles due to surface diffusion within the pore structure.     

PZT Aerogels: Sol-Gel Derived Piezoelectric 3-3 Composites

Ultrasonic transducers with low acoustic impedances are required for applications in gaseous media. In porous piezoelectrics the pore volume acts as a second medium of a solid-gas composite and thus lowers the overall acoustic impedance. Lead zirconate titanate (PbZr_.53Ti_.47O₃, PZT) aerogels were prepared by sol-gel processing and supercritical drying in isopropanol. Prefiring at 400°C in air and subsequent thermal treatment in saturated PbO-atmosphere results in phase transformation to PZT (perovskite phase), grain growth and aerogel densification. In this paper the densification of this new class of piezoelectric material at temperatures above 600°C is described. Porosities were measured as a function of sintering time and temperature. Changes in the microstructure were characterized by scanning electron microscopy. Dielectric measurements are presented.     

Investigation on the sintering behaviors of low-temperature lignite ashes

To investigate the sintering behaviors of lignite ashes, the 450 °C Xiaolongtan (XLT) and Huolinhe (HLH) lignite ash samples were analyzed by press-drop sintering technique, scanning electron microscope, and X-ray diffraction. The result shows the sintering temperature of XLT ash is lower than that of HLH, as a result of that base/acid (B/A) ratio of XLT is higher than that of HLH. The sintering temperatures of two lignite ashes under reducing atmospheres (H₂ and CO) are lower than those under oxidizing atmospheres (CO₂ and O₂), which result from the effects of different iron states under different atmospheres. The sintering temperatures of two lignite ashes decrease with the increase in pressure. It decreases slightly in the range of low pressure, changes clearly in the range of 0.7–1.0 MPa, and changes slightly again with further increase in pressure. The sintering process of lignite ashes is proposed by the presentation of partial-melting phases, the generations of aggregates, and the densification of aggregates.     

The influence of different additives and the mode of their addition on the sintering behavior and the properties of semiconducting barium titanate ceramics

The influence of different additives (TiO₂, TiO₂/SiO₂, CaO/TiO₂/SiO₂, 2BaO/TiO₂/2SiO₂) and the mode of their incorporation (spray drying: series 1; mixed-oxide method: series 2) on the sintering behavior and the microstructural and electrical properties of n-doped BaTiO₃ ceramics has been investigated. The incorporation of the additives from their aqueous solution by spray drying produces a homogeneous distribution of the additive in the BaTiO₃ matrix powder by coating the BaTiO₃ particles. This homogeneous distribution significantly lowers the activation energy for the densification process compared with that of the powders of series 2. Dynamic and isothermal dilatometric measurements revealed that the sintering process is considered as a classical solid phase sintering followed by recrystallization of the BaTiO₃ matrix particles by a eutectic melt. The densification process is dominated by sliding processes. These sliding processes are caused by the amorphous layer of the additive and by the defect-rich grain boundary layers of the BaTiO₃ grains generated intermediately by diffusion processes and reactions of the matrix material with the additive forming the secondary phases Ba₂TiSi₂O₈ and Ba₄Ti₁₃O₃₀, respectively. In series 1, ceramics with a homogeneous microstructure and useful electrical properties (e.g. low resistivity at room temperature) were already produced at a sintering temperature of 1280°C (with SiO₂-containing additives) due to the homogeneous distribution of the additive. The ceramics of series 2 sintered at the same temperature could only be obtained in poor quality. At higher sintering temperatures the differences between the two series vanished.     

Pre-Eutectic Densification of Calcium Carbonate Doped with Lithium Carbonate

Pressureless sintering of CaCO₃ was carried out, with Li₂CO₃ (from 0.5 to 8 wt%) as an additive, under different pressures of CO₂. Densification occurs between 600 and 700°C. Sintering above the eutectic temperature (T>662°C) leads to the decomposition of calcium carbonate and the materials become expanded. At 620° under 1 kPa of CO₂, a relative density of 96% is reached. Li₂CO₃ enhances the densification process and grain growth of calcium carbonate. CO₂ pressure slows down densification and grain growth kinetics. These results are explained by the influence of carbonate and calcium ion vacancies on the sintering mechanisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anisotropic thermoelectric properties of Bi1.9Lu0.1Te2.7Se0.3 textured via spark plasma sintering

Spark plasma sintering method was applied to prepare bulk n-type Bi_1.9Lu_0.1Te_2.7Se_0.3 samples highly textured along the 001 direction parallel to the pressing direction. The texture development is confirmed by X-ray diffraction analysis and scanning electron microscopy. The grains in the textured samples form ordered lamellar structure and lamellar sheets lie in plane perpendicular to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction (∼50 nm against ∼400 nm). A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction within the 290 ÷ 650 K interval was found. The specific electrical resistivity increases and the thermal conductivity decreases for the parallel orientation as compared to these properties for the perpendicular orientation. The Seebeck coefficient for both orientations is almost equal. Increase of the electrical resistivity is stronger than decrease of the thermal conductivity resulting in almost three-fold enhancement of the thermoelectric figure-of-merit coefficient for the perpendicular orientation (∼0.68 against ∼0.24 at ∼420 K). The texturing effect can be attributed to (i) recovery of crystal structure anisotropy typical for the single crystal Bi₂Te₃-based alloys and (ii) grain boundary scattering of electrons and phonons. An onset of intrinsic conductivity observed above T_d ≈ 410 K results in appearance of maxima in the temperature dependences of the specific electrical resistivity, the Seebeck coefficient and the thermoelectric figure-of-merit coefficient and minimum in the temperature dependence of the total thermal conductivity. The intrinsic conductivity is harmful for the thermoelectric efficiency enhancement since thermal excitation of the electron-hole pairs reduces the Seebeck coefficient and increases the thermal conductivity.     

Kinetics of densification of powder compacts during the initial stage of sintering with constant rates of heating. A thermal analysis approach. Part I. Theoretical considerations

Densification of powder compacts during sintering (initial stage) under constant heating rate has been studied and methods of analysing the densification kinetic data have been suggested. p]Green compacts can broadly be envisaged to consists of two phases, viz. porosity and solid material. Annihilation of the pores causes densification of the compacts during sintering. So, the kinetics of the initial stage of densification may very well be represented by a densification parameter (α),where

Here, v_s and v_p,respectively, denote the pore volumes of the sintered and the green compact. Continuous and in situ recording of the linear shrinkage (ΔL) of the compacts during the sintering may be done with a dilatometer and using this value of ΔL,the value of α at any temperature (T) may be calculated.These kinetic data (α vs. T)may then be subjected to analysis using the well-known methods of thermal analysis, and the so-called “derived activation energy (E)” of densification may be evaluated therefrom. The exact form of g(α)—governing rate equation—is ascertained by a trial-and-error method.These methods were used to analyse the densification kinetic data of haematite, copper and silver powders (with particles of irregular shapes and sizes). The results of such analyses will appear on other parts of this paper.     

High-temperature dilatometer with pyrometer measuring system and rate-controlled sintering capability

An optical pyrometer is used to measure and, in conjunction with temperature programmer and controller, control the temperature of the NETSZSCH Dilatometer DIL 402 E/7 up to 2400°C. This instrument is thus suitable to investigate sintering of technical ceramic materials such as SSiC and ZrO₂. Measurements carried out on these materials containing organic additives show that the sintering range of SSiC starts at 1800°C—although its final density is not reached at 2400°C at a heating rate of 20 deg·min^?1—and that the densification of ZrO₂ occurs between 1000° and 1800°C. Using rate controlled sintering (RCS) the sintering process can be extended on a time scale, but the same densities are obtained at the same temperatures when comparing the measurements with and without RCS.     

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of -Al₂O₃ spinel and mullite, or simple -Al₂O₃ spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min^-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of -Al₂O₃ spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.     

Pd/CZ三效催化剂中钯的团聚现象研究

通过浸渍法制备了一系列负载0.5%（重量百分比）Pd的氧化铈-氧化锆（NDK-84,由日本新日本电工株式会社提供）催化剂材料,并通过全面的表征手段,包括扫描电子显微镜（SEM）、透射电子显微镜（TEM）、高分辨透射电子显微镜（HR-TEM）、元素分布、X-射线衍射（XRD）,氮气吸脱附测试与比表面积和孔径分布分析（BET）、X射线光电子能谱（XPS）等,研究了不同Pd前驱体和不同热老化处理条件、H₂还原条件对Pd在铈锆固溶体上的分散、生长与烧结行为的影响,并评估了它们的三效催化活性.结果表明,热老化处理过程与还原过程显著影响了Pd在氧化物载体表面上的分散,因此导致不同的催化活性.     

Modeling and optimization of densification of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powder prepared by a sol–gel method using response surface methodology

Response surface methodology (RSM) based on central composite design (CCD) was successfully applied to the optimization and modeling of densification of nanocrystalline Al₂O₃ powder prepared by sol–gel method. The effects of three operating variables, sintering temperature, calcination temperature and milling time on the densification of nanocrystalline Al₂O₃ were systematically evaluated. A quadratic model for densification was proposed. Analysis of variance (ANOVA) indicated that the proposed quadratic model could be used to navigate the design space. The simulated values obtained from the statistical model were in conformity with the experimental results within an average error of ±1.5%. The optimum operating conditions for densification were found to be 1,579 °C of sintering temperature, 909 °C of calcination temperature and 117 min of milling time. The obtained density under the optimum conditions determined by RSM was 98.5%. The results confirmed that RSM based on central composite design was an accurate and reliable method to optimize the densification conditions of nanocrystalline Al₂O₃ powder.     

Dilatometry as a tool to study a new synthesis for calcium hexaluminate

By using a wet chemical route, pure calcium hexaluminate (CA₆) was yielded, significantly lowering the reaction temperature and shortening the synthesis time if compared to usual industrial procedures. owever, dilatometric studies performed on compacts made of the as-prepared powder, just after pre-heating at 450°C, has shown a superposition between sintering shrinkage and expansion related to CA₂ formation, an intermediate phase formed during calcination and phase evolution to CA₆. oupling of such opposite phenomena led to microcracking of the material, mainly if the heating rates (10°C min^-1) were high. However, lower heating rates (1-5°C min^-1) could quite avoid microcracking but also limit densification. This revised version was published online in July 2006 with corrections to the Cover Date.