Sintering of Silicon Nitride with YAIO3 Additive

Sintering of Si₃N₄ powder with the addition of a Y₂O₃+ Al₂O₃ mixture or YAlO₃ as sintering aids was investigated. Sintering was improved in the case of YAlO₃ additive compared to that for the Y₂O₃+ Al₂O₃ mixture. An initial delay in densification was most likely caused by heterogeneity of the liquid phase formed in the case of the separate oxide additions at temperatures above 1700°C.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

Determination of the Mechanical Response of Sintering Compacts by Cyclic Loading Dilatometry

Determination of the mechanical response of a powder compact during densification is critical in analyzing defect formation and macroscopic dimensional changes, particularly in systems where constrained sintering is involved. Cyclic loading dilatometry is proposed as a novel approach in evaluating the mechanical response of the sintering compact, including sintering pressure, elastic modulus, and viscosity. The advantage of this technique is that only one experiment is needed to determine the equilibrium elastic and viscous properties of a sintering material at any temperatures for any given heating schedule, and no interrupted tests are necessary. This methodology is demonstrated for sintering compacts of Al₂O₃, Ce-TZP, and their composite mixture. The loading dilatometric data showed that the compact behaved elastically prior to the onset of sintering and during the very initial stages of sintering, which was followed by a transition leading to a viscous behavior for the latter part of the densification. Application of different load levels, ranging from 0.25 to 1 MPa, showed that the compact viscosity is essentially linear within the applied stress range at temperatures greater than 1100°C.     

Effect of thermal insulation on microstructural homogeneity and onset temperature of flash sintered materials

The present work aimed to reduce the microstructure heterogeneity inherent to flash sintering by using alumina blankets as a thermal insulator around ZnO cylindrical samples during the sintering process, under different electric field conditions. Thermal insulation significantly reduced the flash onset temperature and the grain size heterogeneity. For higher electric fields, a temperature reduction as high as 480 °C was observed, which also led to lower densification. These findings were discussed in terms of changes in the heat loss dynamics coupled with the adsorbed water retention, both promoted by the applied thermal insulation. A model to estimate the temperature at stage III of flash sintering was proposed. The final temperature reached with thermal insulation did not differ significantly from the ones without it. Thus, thermal insulation could represent an alternative route to flash sinter materials with lower furnace temperatures with energy savings up to 78 % and a more homogeneous microstructure.     

Preparation of Ti-based laminated composites through spark plasma sintering with different carbon sources as the bonding layer

In the present study, woven carbon fabrics, graphene nano sheets (GNPs) and graphites were used as the bonding source between Ti sheets to produce Ti-TiC laminated composites through spark plasma sintering. In the case of carbon fibers, the woven fabrics were cut in a circular shape and inserted between the Ti sheets; on the other hand, in the case of graphite and GNPs, ethanol with carbon sources as the mixture was sprayed on the surface of the starting Ti sheets until reaching the specified increase in the weight of the coated Ti. After that, seven layers of Ti sheets (including six coated layers and one un-coated one) were inserted to the graphite die and the sintering process was performed with the initial and final applied pressure of 10 and 50 MPa, respectively, at the sintering temperature of 1250 °C. The XRD patterns taken from the cross-section of the prepared composites revealed the formation of the TiC crystalline phase as the product of the reaction between Ti and the carbon sources. The FESEM images also showed the proper bonding between the layers due to the formation of TiC and carbon diffusion into the Ti sheets. The fracture surfaces of the prepared sample demonstrated the almost ductile fracture for the Ti-laminated sample, as well as the Ti-GNPs laminated composite, while the brittle fracture was obtained for the composites with graphite and the carbon fabric as the bonding layers. The highest hardness of 912 ± 17 Hv was calculated for the cross-section of the Ti-CF laminated composite; also, the highest bending strength of 1215 ± 18 MPa was obtained for the Ti-GNPs laminated composite.     

铁矿石烧结中混合料特性对火焰烽面与烧结性能的影响机理研究

通过中试规模烧结杯试验和综合烧结模型，将返矿平衡和非平衡条件相结合，研究了混合料特性（烧结碱度、焦炭和水分添加量）对火焰烽面特性和烧结性能的影响机理。模拟研究涉及多种烧结条件下的125个烧结杯工况。为揭示火焰烽面区域中熔体生成与凝固行为，使用FactSage软件进行化学热力学模拟并建立了更完善的熔化和凝固子模型。模拟与试验研究表明，随着碱度和焦炭添加量的变化，火焰烽面速度、成品率、焦比和利用系数会出现局部最大或最小值。在本文烧结条件下，最大利用系数工况为碱度2.0、水分7.7%、焦炭6.4%。最小焦比工况为碱度2.0、水分6.5%、焦炭6.4%。最小焦比和最大利用系数的条件并不相同，而最终烧结操作取决于每个烧结厂的控制目标。     

Determination of porosity from shrinkage curves during sintering of granular materials

This paper investigates the implementation of an analytical model to determine the porosity of a granular material during reactive and nonreactive sintering. A graphical interpretation of this model is proposed to calculate the porosity by comparing the experimental shrinkage curve with the ideal one. For the nonreactive sintering, some examples have been taken from the literature to illustrate the application of this method for two granular materials (alumina and zircon). In the case of reactive sintering, we have used our experiments to study the sintering behavior of magnesium hydroxide. The shrinkage curve was determined by dilatometer and the porosity was measured by helium pycnometer. The comparison revealed that the porosity calculated from the model is fully consistent with the porosity measurements in the both cases.     

Influence of the Synthesis Method on the Mechanism of Formation and Dielectric Properties of Ba2Ti9O20

This paper presents the results of analyzing the influence of synthesis methods, namely, the solid-phase interaction and coprecipitation, on the mechanism of formation, the degree of sintering, and the dielectric properties of barium titanate Ba₂Ti₉O₂₀. The effect exerted by ZrO₂ and SrO additives on the solid-phase synthesis is investigated. It is demonstrated that the degree of sintering and the dielectric properties can be controlled by introducing additives and changing the method of preparing the initial mixture. It is established that the synthesis of Ba₂Ti₉O₂₀ samples from the initial mixture obtained through the coprecipitation from aqueous solutions of chlorides makes it possible to improve the dielectric properties of the samples and to decrease the temperature and duration of their sintering.     

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1− x )Pb(Mg_1/2Nb_2/3)O₃− x PbTiO₃ (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high-temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.     

Novel model for the sintering of ceramics with bimodal pore size distributions: Application to the sintering of lime

A mathematical model for the sintering of ceramics with bimodal pore size distributions at intermediate and final stages is developed. It considers the simultaneous effects of coarsening by surface diffusion, and densification by grain boundary diffusion and lattice diffusion. This model involves population balances for the pores in different zones determined by each porosimetry peak, and is able to predict the evolution of pore size distribution function, surface area, and porosity over time. The model is experimentally validated for the sintering of lime and it is reliable in predicting the so called “initial induction period” in sintering, which is due to a decrease in intra‐aggregate porosity offset by an increase inter‐aggregate porosity. In addition, a novel methodology for determination of mechanisms based on the analysis of the pore size distribution function is proposed, and with this, it was demonstrated that lattice diffusion is the controlling mechanism in the CaO sintering. © 2016 American Institute of Chemical Engineers AIChE J, 63: 893–902, 2017     

Heterogeneities and defects in powder compacts and sintered alumina bodies visualized by using the synchrotron X-ray CT

The uniaxial die compaction and sintering is an important industrial production method for metal and ceramic components. While submicron- and nano-sized powders are finding increasing use for making dense materials with finest grain sizes, the uniformity of microstructure, and then, the reliability of the products depend on the heterogeneity in powder packing structure. Here we applied the synchrotron X-ray multiscale-CT to characterize the complex domain structures, i.e., agglomerates, in powder compacts, and revealed how heterogeneous distribution of fine residual pores is formed by the differential sintering of agglomerates by using a submicron alumina powder as a model. A crack-like defect was identified as the largest defect. This defect was formed along the boundary between a plate-shaped agglomerate and its surroundings. A technique was also proposed to visualize the heterogeneous distribution of residual pores by using SEM. These characterization methods will open up new possibilities for the optimization of ceramic processing.     

Apparent Thermal Conductivity of a Heterogeneous Ceramic Body Under Isothermal Conditions

USING statistical arguments, Brown¹ derived expressions for the apparent permittivity of a binary mixture of solid phases in terms of the volume fractions and the permittivitits of each phase. The purpose of the present note is to direct the attention of experimentalists working on heat-transfer phenomena in ceramics to the fact that simple extensions of Brown's results will yield equations for the apparent thermal conductivity of a heterogeneow ceramic body at constant temperature, whether the heterogeneity arises from the existence of isometric pores in an otherwise homogeneous solid or as a result of two solid phases being present. It is demonstrated below that the averaging formula for the apparent thermal conductivity of a porous ceramic body derived by Loeb,² and employed by Francl and Kingery, ² is a consequence of terminating series expansions for the thermal conductivity after the fist term. Brown's generalized result is given by where, for the dielectric case, KO is the ratio of the polarization to the Lorentz field intensity and K is the ratio of averaged values of these quantities, A is the shape factor for the particles of the minor phase in a binary mixture, and VA is the volume fraction of that phase.     

Possibility of electric-pulse sintering of powder nanostructural composites

The activities described in the paper are aimed at solving the problem of forming various articles from homogeneous metallic powders of copper and molybdenum, from a mixture of copper, titanium, and boron powders, and from a mixture of the copper powder with uniformly distributed particles of titanium diboride (up to 40 vol.%). The idea is to obtain compacts with the minimumchanges in the initial structure of the material and with high resistance to electric erosion under conditions of high currents. A method of electric-pulse sintering is used, where the compacting process is ensured with one capacitor discharge with a current density of ⩾10³ A/mm² and sintering time of ⩽10⁻⁴ sec. Conditions of sintering of powders pre-compressed by a pressure of several kilobars are studied. The density, microstructure, and phase composition of the sintered samples are examined. The erosion resistance of a Cu/TiB₂ porous nanostructural composite is tested on a model erosive plasma accelerator.     

Kinetics and Mechanism of a Sintering Process for Macroporous Alumina Ceramics by Extrusion

The paper describes an evaluation of kinetic parameters for sintering from the analysis of stepwise isothermal dilatometry data for the pore-forming process of macroporous alpha-Al₂O₃ ceramics fabricated by extrusion. The experimental results demonstrated that isothermal shrinkage data for the sintering process can fit the following rate equation: d Y /d t = nK ( T ) Y (1 - Y )[(1 - Y )/ Y ]^{1/ n} , where Y is the fraction of volume shrinkage. Kinetic parameters, i.e., the average exponent n and the activation energy E for the pore-forming process, were calculated to be 0.232 and 415.5 kJ/mol, respectively, in the temperature range of 1200-1400°C. The results also show the equivalence between isothermal sintering and stepwise sintering in alumina. In the latter case the temperature is increased in steps over a period of time. The equivalence is derived from understanding how the sintering rate changes with porosity. The sintering mechanism in different temperature ranges is discussed, with reference to microstructure development. The bend strengths of porous ceramics with different porosities are also presented.     

Pyroplastic deformation of porcelain stoneware tiles: Wet vs. dry processing

Several batches of porcelain stoneware were formulated by partial replacement of coarse Na and Na-K feldspars (standard batch) with quartz in two different grain sizes, micronized K-feldspar or a mixture of these components. Everyone was processed (laboratory scale) by wet and dry routes to compare: sintering curve, microstructure, phase composition and viscosity of the liquid phase at high temperature. Pyroplasticity index was determined by 3-point bending test. Results indicate that finer the quartz particles, more they dissolved in liquid phase, increasing viscosity and reducing deformation. Micronized K-feldspar increased the sintering kinetics (causing deformation). However, when K-feldspar was combined with quartz, densification rate was improved without compromising integrity of tile. About dry route, microstructural heterogeneity turned as fundamental to prevent Pyroplastic deformation. As conclusion, pyroplastic deformation occurs by different mechanisms in samples prepared by the two processing routes and bodies prepared via dry route are less likely to undergo pyroplastic deformation.     

Reaction Processing of Al2O3 Composites Containing Iron and Iron Aluminides

Different Fe-Al₂O₃ and FeAl-Al₂O₃ composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al₂O₃, Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al₂O₃ matrix, whereas aluminide-containing Al₂O₃ composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.     

Inhomogeneity-Packing Density Relations in Binary hwders

A theoretical procedure for determining the specific volume of an inhomogeneous binary mixture having a large radius ratio is presented. The procedure is based on the hypothesis that the Furnas model holds on a local level. The methodology developed is demonstrated using two hypothetical compositional distribution functions. In one case the compositional distribution describes changes that occur from a completely unmixed to a completely mixed state. The second utilizes the normal distribution for describing smaller compositional variations.     

Carborundum-graphite crucibles for melting copper-base alloys

Conclusions Carborundum-graphite crucibles holding 500 kg of liquid metal worked reliably in a high-frequency furnace for 35 – 40 melts in the case of the copper-chromium hardener, and up to 75 melts in the case of the BKh-08 alloy.On account of the low specific resist nce of the carborundum-graphite crucibles, the furnace can be easily adjusted, and the melting of the metal speeded up; there is good sintering of the electro-corundum rammed mixture, which can act as the melting receptable if the crucible is damaged.Carborundum-graphite crucibles are quite suitable for melting copper-base alloys with a silicon content up to 0.2 – 0.3%.     

高掺量粉煤灰烧结制品可行性研究

通过XRD,SEM等方法较系统地研究了高掺量粉煤灰-粘土混合料的烧成过程,从坯料成型、制品物理力学性能、烧结活性及显微结构4方面探讨了大掺量粉煤灰作制品的可行性,并探讨其烧结机理.     

低温烧成高纯Al_2O_3多孔陶瓷膜支撑体的制备

为了降低高纯Al2O3(α-Al2O3质量含量≥99%)多孔陶瓷膜支撑体烧成温度,以粒径为30μm的α-Al2O3为原料,分别添加TiO2和TiO2/Cu(NO3)2为烧结助剂,通过干压成型和挤出成型制备片状和管式多孔支撑体。Al2O3-TiO2和Al2O3-TiO2-CuO体系分别在高温下出现的液相低共熔物促进了多孔支撑体的烧结。当氧化铝支撑体中添加0.5%(摩尔分数,下同)TiO2+0.5%Cu(NO3)2后,在1600℃的烧成即可获得机械性能高、渗透性能好和耐酸碱腐蚀性能优异的管式支撑体。在压力为0.1MPa时,支撑体的水渗透通量为12.1m3/(m2·h),弯曲强度为44.5MPa。经过80℃,含10%(质量分数,下同)HNO3的溶液腐蚀800h及80℃,含10%NaOH的溶液1200h后,支撑体的质量损失率分别为1%和0.35%。