Kazunari Shinagawa

Ferroelectric mesocrystals of bismuth sodium titanate: formation mechanism, nanostructure, and application to piezoelectric materials.

Ferroelectric mesocrystals of Bi0.5Na0.5TiO3 (BNT) with [100]-crystal-axis orientation were successfully prepared using a topotactic structural transformation process from a layered titanate H1.07Ti1.73O4·nH2O (HTO). The formation reactions of BNT mesocrystals in HTO-Bi2O3-Na2CO3 and HTO-TiO2-Bi2O3-Na2CO3 reaction systems and their nanostructures were studied by XRD, FE-SEM, TEM, SAED, and EDS, and the reaction mechanisms were given. The BNT mesocrystals are formed by a topotactic structural transformation mechanism in the HTO-Bi2O3-Na2CO3 reaction system and by a combination mechanism of the topotactic structural transformation and epitaxial crystal growth in the HTO-TiO2-Bi2O3-Na2CO3 reaction system, respectively. The BNT mesocrystals prepared by these methods are constructed from [100]-oriented BNT nanocrystals. Furthermore, these reaction systems were successfully applied to the fabrication of [100]-oriented BNT ferroelectric ceramic materials. A BNT ceramic material with a high degree of orientation, high relative density, and small grain size was achieved.

Particle size distribution effects in an FEM model of sintering porous ceramics

A numerical simulation is presented on the sintering of porous alumina structures prepared by a controlled sedimentation technique. By forming this functionally gradient material with a very broad powder size distribution, the samples were able to remain flat through sintering. This experimental result is reflected in the present simulation results, which incorporated particle size distribution effects. In general, sintering functionally gradient ceramics can often introduce defects. Despite these common problems, the asymmetric structures considered in this paper featured a vertical functionality of continuously overlapping broad powder size distributions in the structure. This arrangement served to homogenize sintering rates. Modelling presented in connection with this shows that such structures can be readily sintered without warpage or cracking. To demonstrate these effects, a finite element method numerical simulation was developed to model the sintering characteristics of porous asymmetric ceramic structures by incorporating the powder particle size distribution into the model as a field variable. This work presents novel advances in the sintering model such that the contributions to the desired product properties attributable to particle size distribution effects can be demonstrated. These additions to the model produce numerical results which properly match observed structural profiles of physical samples.

An experimental and numerical study of particle size distribution effects on the sintering of porous ceramics

A single-step processing method has been established to prepare asymmetric porous alumina microstructures by a controlled sedimentation technique. Fine powder from an aqueous suspension is consolidated over a casting slab. Metastable surface chemical control of the suspension properties was able to induce a highly porous flat disc structure with a continuously increasing mean pore size from top to bottom. Formation of this gradient structure was facilitated by using a powder with a very broad particle size distribution. These structures can be used as either ultrafiltration media or as substrates for inorganic membrane making. Sintering can readily introduce defects into functionally gradient ceramics. Despite these problems, the asymmetric structures considered in this paper can be readily sintered without warpage or cracking. In this regard, a finite element method numerical simulation had been developed to model the sintering characteristics of functionally gradient ceramic structures. The key for being able to predict a non-warped structure was the incorporation into the model of the powder particle size distribution as a field variable. Across the vertical section of the structure, the distributions were broad and overlapping, all with a significant fines tail. These characteristics accelerate and homogenize local sintering rates, such that the net result is a non-warped fused structure. This paper presents recent advances with the simulation, where sample geometry, porosity and particle size distribution evolutions were traced alongside measurements made on physical specimens. In general the model corresponded well with the experimental observations. The correct accounting of observed trends lends confidence to the underlying sintering mechanisms incorporated into the model.

Stress analysis of powder compacts with graded structures in sintering process

During sintering of metal/ceramic functionally grade materials, cracks are often formed on the surface on the top ceramic layer due to the internal stress produced by mismatch shrinkage and warpage. The ways to reduce the internal stress are examined by using the finite element method as well as the plate theory for sintering. Uniaxial pressing, which gives the counter moment against the warping, can decrease the bending stress but only in the middle of the surface. Thinning the top layer is found to be effective in reducing the tensile stress on the surface when the sintering properties of some layers are modified appropriately. The suppression of the surface cracking in the improved graded powder compacts is confirmed by experiment.

A constitutive model for sintering of mixed powder compacts

Variations in the sintering rate of metal/ceramic powder mixtures are examined. Alumina and stainless steel powders are mixed in different ratios with a hinder and compacted by CIPing and fired. The change in microstructure and the shrinkage of the specimens are examined. The effects of mixing ratio, difference in powder size and pore size distribution on the densification are discussed. The shrinkage behavior of the mixed powders is modelled using a constitutive equation for sintering of each component powder. The interaction between a metal and a ceramic powder region is taken into consideration to the model. The model provides a good fit of the experimental data.

A three-dimensional computer study of gravity induced skeletal structure evolution during liquid phase sintering

Abstract In this paper we will investigate numerically gravity induced skeletal structure evolution during liquid phase sintering. Applying three-dimensional domain methodology, solid skeleton evolution will be introduced by the definition of skeleton units determined by the equilibrium dihedral angle and the formation of large solid skeletons arranged in a long chain of connected solid phase domains. The settling procedure will be simulated by using two general submodels: for free settling, in which solid phase domains fall under gravity over domains that have already settled, and for extended settling, in which settled domains continue their motion until they reach a position of local equilibrium. The same submodels will be applied for free settling and extended settling of solid skeletons. It will be assumed that under gravity conditions, Stokes’s law settling usually dominates microstructure formation, where the settling procedure will be simulated by computation of the settling time and average migration distance during a defined time interval. Thus gravity induced skeleton structure evolution will be simulated by simultaneous computation of the displacement of the center of mass.

Effect of Si Content on Fracture Behaviour Change by Strain Rate in Si Steels

Fracture behaviour transitions due to change in the strain rate in steels with various Si content ranging from 2% to 5 wt% were studied. Room-temperature tensile tests were conducted over wide range of strain rates ranging from 10-3 s-1 to 103 s-1. Concerning of the steels with low Si content (no more than 3%), the nominal stress - nominal strain curves represented both uniform and local elongations at all strain rates. On the other hand, in 4% Si steel at a strain rate higher than 101 s-1, the tensile sample broke down without local elongation (necking). The stress at breaking was found to be nearly equal to its work hardening rate. The strain rate at which fracture behaviour transition took place in 5% Si steel (10-1s-1) was lower than that in 4% Si steel. TEM observations clarified the existence of deformation twins in the sample that fractured without necking. These results indicated that Si addition is subject to the brittle fractures and that the fracture mechanism transition is closely related with the deformation twinning behaviour.

Effects of Inhomogenization on Sintering Behavior of Ni/Al2O3 Powder Mixtures

Aiming at the modification of the sinterability of metal/ceramic powder mixtures, the influence of the homogeneity in microstructure on the sintering behavior is examined. Ni/Al2O3 specimens, with ratios ranging from 0/100 to 100/0, are prepared by compaction through two different procedures of mixing and granulating, to change the degree of particle dispersion. The sintering stress and the viscosity of the specimens are measured by sinter-compression tests to consider the sinterability from the viewpoint of both driving force and resistance. The sintering rate of the inhomogeneous compacts, with agglomeration of powder particles, is larger than that of the homogeneous compacts. This is because the viscosity becomes low, but the sintering stress does not change much by inhomogenizing the microstructure.

A Mathematical Approach to Ostwald Ripening Due to Diffusion and Deformation in Liquid Bridge

From many experiments with mixtures of small and large grains, it can be concluded that during liquid phase sintering, smaller grains partially dissolve and a solid phase precipitates on the larger grains and grain coarsening occurs. The growth rate can be controlled either by the solid-liquid phase boundary reaction or by diffusion through the liquid phase. The microstructure may change either by larger grains growing during the Ostwald ripening process or by shape accommodation. In this study, two-dimensional mathematical approach for simulation of grain coarsening by grain boundary migration based on a physical and corresponding numerical modeling of liquid phase sintering will be considered. A combined mathematical method of analyzing viscous deformation and solute diffusion in liquid bridge between two grains with different sizes will be proposed. The viscous FE method will be used for calculating meniscus of the liquid bridge, with the interfacial tensions taken into consideration. The FE method for diffusion will be also implemented by using the same mesh as the deformation analysis. [Projekat Ministarstva nauke Republike Srbije, br. OI172057]

Fracture Behavior Transition by Change of Strain Rate in Dislocation-Induced Si Steels

The fracture behavior transition due to the change of strain rate in 5%Si magnetic steel with dislocation microstructures was studied. The Si steel was multi-passed rolled at 800°C to a various reductions up to 50%. The room temperature tensile deformation was conducted at various strain rates from 10-5/s to 100/s. All rolled steels were fractured in ductile manners with local elongation (necking) at slower strain rate. When strain rate was faster, the local elongation disappeared and the fracture manner was turned to brittle. The strain rate at which fracture mechanism changed from ductile to brittle increased with the increasing of the reduction. On the other hand, the almost fully recrystallized Si steel was fractured in the brittle manner at any strain rate and the transition strain rate was not found. The fractured tensile specimen with no local elongations contains deformation twins; whereas these deformation twins were not observed in the fractured specimen with local elongations. This result indicates that dislocation structure evolved during rolling suppressed the twinning and that the dislocation structure is effective for the enhancement of toughness in Si steel.