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Powder Compacts (powder + compact)
Selected AbstractsSimulation of the Stress-Assisted Densification Behavior of a Powder Compact: Effect of Constitutive LawsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Héctor Camacho-Montes The densification of powders with linear and nonlinear viscous behavior (Scherer and Riedel models) and with power-law-deformation (Khun,McMeeking) behavior was studied under hot pressing and sintering forging conditions. Several numerical experiments, designated cases in this work, were performed to study the effect of (i) the uniaxial stress exerted by the piston and (ii) the rate of the uniaxial stress. The stress state was calculated using the finite-element program ANSYS for each case. Considering the mesoscopic behavior of the powders, densification rates were obtained. The similarities and differences between predictions from the three constitutive models are highlighted. The relationship between the constitutive behavior and the most effective stress state is one of the focuses of this study. For example, we show that under constant stress loading, hot pressing more effectively promotes densification than sinter forging for constitutive behaviors that do not follow the power-law creep. In general, as expected, the increase of uniaxial applied stress and piston velocity favored densification. However, the increase in densification depends strongly on the constitutive law. [source] Evolution of Thermophysical Characteristics in Tin Oxide: From Pressed Powder Compact to Fired Porous BodyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Céline Poulier The thermal response of a ceramic body during firing will be controlled by its thermophysical characteristics. Laser flash measurements have been used to investigate the thermal conductivity of pressed powder compacts of pure tin oxide as a function of temperature. After an initial decrease up to 150°C attributed to removal of surface species, the thermal conductivity increases progressively with thermal treatment up to 1000°C. Because there is almost no variation in the pore volume fraction, the behavior is explained by grain boundary formation and neck growth, which increase the contact area between particles. Supporting evidence is given by an increase in mechanical strength. Finally, thermal treatment above 1000°C increases the thermal conductivity due to a reduction in the number of grain boundaries per unit length of heat path. [source] Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder CompactsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2003Samuel M. Salamone The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa2Cu3O6+x and composite powder compacts (YBa2Cu3O6+x powder with 10 vol% dense inclusions of YBa2Cu3O6+x) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured. [source] Stress Development Due to Capillary Condensation in Powder Compacts: A Two-Dimensional Model StudyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2000Stefan Lampenscherf A model experiment is presented to investigate the relationship between the humidity-dependent liquid distribution and the macroscopic stress in a partially wet powder compact. Therefore, films of monosized spherical particles were cast on silicon substrates. Using environmental SEM the geometry of the liquid necks trapped between particles was imaged as a function of relative humidity. Simultaneously the macroscopic stress in the substrate adhered particle film was measured by capacitive deflection measurement. The experimentally found humidity dependence of the liquid neck size and the macroscopic film stress are compared with model predictions. The circle,circle approximation is used to predict the size of the liquid necks between touching particles as a function of the capillary pressure. Using the modified Kelvin relation between capillary pressure and relative humidity, we consider the effect of an additional solute which may be present in the capillary liquid. The results of the stress measurement are compared with the model predictions for a film of touching particles in hexagonal symmetry. The contribution of the capillary interaction to the adhesion force between neighboring particles is calculated using the integrated Laplace equation. The resulting film stress can be approximated relating this capillary force to an effective cross section per particle. The experimentally found humidity dependence of the liquid neck size is in good agreement with the model predictions for finite solute concentration. The film stress corresponds to the model predictions only for large relative humidities and shows an unexpected increase at small values. As is shown with an atomic force microscope, the real structure of the particle,particle contact area changes during the wet/dry cycle. A solution/reprecipitation process causes surface heterogeneities and solid bridging between the particles. It is claimed that the existence of a finite contact zone between the particles gives rise to the unexpected increase of the stress at small relative humidities. [source] Effect of Nitrogen Atmosphere on the Densification of a 3-mol%-Yttria-Doped ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002Yeong-Kyeun Paek The densification behavior of a 3-mol%-Y2O3 -doped ZrO2 (3Y-ZrO2) has been investigated under N2 and O2 atmospheres. Powder compacts have been sintered at 1550° and 1400°C for various times. The density of the specimen sintered at 1550°C is higher in N2 than in O2, while the contrary result is obtained in the case of the specimen sintered at 1400°C. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO2 matrix. Because nitrogen solubility into the ZrO2 increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO2 and its role on the densification behavior of 3Y-ZrO2. [source] Shear Modulus of Polycrystalline Rhenium Diboride Determined from Surface Brillouin SpectroscopyADVANCED MATERIALS, Issue 42 2009Sergey N. Tkachev Surface Brillouin specotroscopy of a densified powder compact of rhenium diboride (see scanning electron microscopy image) results in a determined shear modulus of 223,GPa. This value is comparable to the shear modulus of c-BC2N and provides evidence for the superhard nature of ReB2. [source] Formation and Densification Behavior of MgAl2O4 Spinel: The Influence of Processing ParametersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2008Ibram Ganesh Different types of dense stoichiometric and nonstoichiometric magnesium aluminate (MgAl2O4) spinel (MAS) ceramics were prepared following a conventional double-stage firing process using different commercially available alumina and magnesia raw materials. Stoichiometric, magnesia-rich, and alumina-rich spinels were sintered at 1500°,1800°C for 1,2.5 h. The influence of the different processing parameters (average particle size, degree of spinel phase, green density, mass of the powder compact, sintering temperature, holding time at the peak temperature, and starting composition) on the densification behavior of MAS was assessed by measuring the bulk density, apparent porosity, and water absorption capacity, and microstructural observations. Most of the MAS compositions tested exhibited excellent sintering properties. [source] Abnormal Grain Growth Occurring at the Surface of a Sintered BaTiO3 SpecimenJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2004Jae-Sun Chun When a BaTiO3 powder compact was sintered in air, the abnormal grain growth (AGG) was observed to occur at the surface of the specimen. The BaO evaporation from the surface and consequent formation of eutectic liquid is suggested to be the cause of AGG. When the evaporation of BaO during sintering was prevented by embedding the specimen in the same powder, no AGG was observed to occur. [source] Stress Development Due to Capillary Condensation in Powder Compacts: A Two-Dimensional Model StudyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2000Stefan Lampenscherf A model experiment is presented to investigate the relationship between the humidity-dependent liquid distribution and the macroscopic stress in a partially wet powder compact. Therefore, films of monosized spherical particles were cast on silicon substrates. Using environmental SEM the geometry of the liquid necks trapped between particles was imaged as a function of relative humidity. Simultaneously the macroscopic stress in the substrate adhered particle film was measured by capacitive deflection measurement. The experimentally found humidity dependence of the liquid neck size and the macroscopic film stress are compared with model predictions. The circle,circle approximation is used to predict the size of the liquid necks between touching particles as a function of the capillary pressure. Using the modified Kelvin relation between capillary pressure and relative humidity, we consider the effect of an additional solute which may be present in the capillary liquid. The results of the stress measurement are compared with the model predictions for a film of touching particles in hexagonal symmetry. The contribution of the capillary interaction to the adhesion force between neighboring particles is calculated using the integrated Laplace equation. The resulting film stress can be approximated relating this capillary force to an effective cross section per particle. The experimentally found humidity dependence of the liquid neck size is in good agreement with the model predictions for finite solute concentration. The film stress corresponds to the model predictions only for large relative humidities and shows an unexpected increase at small values. As is shown with an atomic force microscope, the real structure of the particle,particle contact area changes during the wet/dry cycle. A solution/reprecipitation process causes surface heterogeneities and solid bridging between the particles. It is claimed that the existence of a finite contact zone between the particles gives rise to the unexpected increase of the stress at small relative humidities. [source] Particle Rearrangement and Pore Space Coarsening During Solid-State SinteringJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2009Hans Eckart Exner Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) pores in the late sintering stages are well established. Porosity is interconnected up to very high densities. Coarsening of the continuous pore space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the interconnected pore space and of closed pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions. [source] Three-Dimensional Solar Cell Finite-Element Sintering SimulationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2009Gordon R. Brown The sintering process is ubiquitous in manufacturing, but the design-oriented modeling of sintering has presented considerable challenges. This type of modeling is necessary to be able to predict deformation and thus design appropriate powder compacts so that after they are sintered, the desired dimensions will be achieved. Currently this is done through a costly and time-consuming trial and error process. In our research, an application of the Skorohod,Olevsky viscous sintering constitutive equation in a finite-element (FE) model is developed and used to model solar cell manufacturing. Experimental measurements are used to determine the properties of the solar cell materials, and these are used to calculate the parameters for the FE model. Simulation results are compared with experimental data and analysis has been made to evaluate the adequacy and usefulness of this approach. [source] Evolution of Thermophysical Characteristics in Tin Oxide: From Pressed Powder Compact to Fired Porous BodyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Céline Poulier The thermal response of a ceramic body during firing will be controlled by its thermophysical characteristics. Laser flash measurements have been used to investigate the thermal conductivity of pressed powder compacts of pure tin oxide as a function of temperature. After an initial decrease up to 150°C attributed to removal of surface species, the thermal conductivity increases progressively with thermal treatment up to 1000°C. Because there is almost no variation in the pore volume fraction, the behavior is explained by grain boundary formation and neck growth, which increase the contact area between particles. Supporting evidence is given by an increase in mechanical strength. Finally, thermal treatment above 1000°C increases the thermal conductivity due to a reduction in the number of grain boundaries per unit length of heat path. [source] Multi-Scale Study of Sintering: A ReviewJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2006Eugene A. Olevsky An integrated approach, combining the continuum theory of sintering with a kinetic Monte-Carlo (KMC) model-based mesostructure evolution simulation is reviewed. The effective sintering stress and the normalized bulk viscosity are derived from mesoscale simulations. A KMC model is presented to simulate microstructural evolution during sintering of complex microstructures taking into consideration grain growth, pore migration, and densification. The results of these simulations are used to generate sintering stress and normalized bulk viscosity for use in continuum level simulation of sintering. The advantage of these simulations is that they can be employed to generate more accurate constitutive parameters based on most general assumptions regarding mesostructure geometry and transport mechanisms of sintering. These constitutive parameters are used as input data for the continuum simulation of the sintering of powder bilayers. Two types of bilayered structures are considered: layers of the same particle material but with different initial porosity, and layers of two different materials. The simulation results are verified by comparing them with shrinkage and warping during the sintering of bilayer ZnO powder compacts. [source] Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder CompactsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2003Samuel M. Salamone The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa2Cu3O6+x and composite powder compacts (YBa2Cu3O6+x powder with 10 vol% dense inclusions of YBa2Cu3O6+x) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured. [source] High-Strength Porous Silicon Carbide Ceramics by an Oxidation-Bonding TechniqueJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2002Jihong She Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO2 glass. Because of the crystallization of amorphous SiO2 glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 ,m SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ,31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance. [source] Ink-Jet Printing of Binders for Ceramic ComponentsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002Jooho Moon Layered manufacturing methods for fabricating ceramic components can involve selective deposition of binder using ink-jet printing. Selection of a proper binder plays a critical role in fabricating parts with good surface finish, dimensional accuracy, and high resolution. Several polymeric solution-phase binders were investigated in terms of their physical properties, printing performance, and binder-powder bed interaction. It was observed that the molecular weight should be <15 000 for the binder to be penetrated into dense powder compacts. Binder infiltration kinetics and printed line width were also significantly influenced by powder-bed characteristics, such as surface roughness and pore size, as well as the physical properties of the binder, such as viscosity and surface tension. [source] Sintering of Partially Stabilized Zirconia by Microwave Heating Using ZnO,MnO2,Al2O3 Plates in a Domestic Microwave OvenJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2000Satoru Fujitsu Partially stabilized zirconia (PSZ) powders were fully densified by microwave heating using a domestic microwave oven. Pressed powder compacts of PSZ were sandwiched between two ZnO,MnO2,Al2O3 ceramic plates and put into the microwave oven. In the first step, PSZ green pellets were heated by self-heating of ZnO,MnO2,Al2O3 ceramics (1000°C). In the second step, the heated PSZ pellets absorbed microwave energy and self-heated up to a higher temperature (1250°C), leading to densification. The density of PSZ obtained by heating in the microwave oven for 16 min was 5.7 g/cm3, which was approximately equal to the density of bodies sintered at 1300°C for 4 h or 1400°C for 16 min by the conventional method. The average grain size of the sample obtained by this method was larger than the average grain size of samples sintered by the conventional method with a similar heating process. [source] Effects of Calcination Temperature and A/B Ratio on the Dielectric Properties of (Ba,Ca)(Ti,Zr,Mn)O3 for Multilayer Ceramic Capacitors with Nickel ElectrodesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2000Wen-Hsi Lee The electrical performance of multilayer ceramic capacitors (MLCCs) with Ni inner electrodes, made from (Ba,Ca)(Ti,Zr,Mn)O3 (BCTZM), is closely related to the calcination temperature and the A/B ratio of the powder. For materials showing A/B = 1.000, the lifetime, the breakdown voltage, and the RC increase with higher calcination temperatures. No significant effect of the calcination temperature on RC and lifetime was found for materials showing A/B = 0.991. The isoelectric point of BCTZM is shifted toward higher pH values when the calcination temperature is decreased. The above results are attributed to the colloidal stability of aqueous BCTZM suspensions and the resulting green density of powder compacts. [source] | ||||||||||