Ceramic Compacts Research Articles

Integration algorithms of elastoplasticity for ceramic powder compaction

Inelastic deformation of ceramic powders (and of a broad class of rock-like and granular materials), can be described with the yield function proposed by Bigoni and Piccolroaz (2004, Yield criteria for quasibrittle and frictional materials. Int. J. Solids and Structures, 41, 2855-2878). This yield function is not defined outside the yield locus, so that 'gradient-based' integration algorithms of elastoplasticity cannot be directly employed. Therefore, we propose two ad hoc algorithms: (i.) an explicit integration scheme based on a forward Euler technique with a 'centre-of-mass' return correction and (ii.) an implicit integration scheme based on a 'cutoff-substepping' return algorithm. Iso-error maps and comparisons of the results provided by the two algorithms with two exact solutions (the compaction of a ceramic powder against a rigid spherical cup and the expansion of a thick spherical shell made up of a green body), show that both the proposed algorithms perform correctly and accurately.

Structure and microwave dielectric properties of 5BaO–2V2O5 binary ceramic system

5BaO–2V2O5 ceramic has been prepared by conventional solid state reaction method. The X-ray diffraction and Laser Raman studies have been carried out to ascertain the phase purity and crystal structure of the prepared composition. The microwave dielectric properties of the well sintered ceramic compacts were measured by Hakki & Colemann post resonator and cavity perturbation techniques using a vector network analyzer. The 5BaO–2V2O5 ceramic exhibits a low dielectric constant of 12.1, relatively good quality factor of 26,790 GHz, and a small temperature variation of resonant frequency of 7 ppm/°C in the microwave frequency region.

Direct Characterizing of Densification Mechanisms during Spark Plasma Sintering

Spark plasma sintering (SPS) is a convenient and rapid means of producing dense ceramic compacts. However, the mechanisms responsible for rapid densification have not been identified satisfactorily, with different studies using an indirect approach yielding varied values for the densification parameters. This study involved SPS in high purity nanocrystalline alumina with temperatures ranging from 1173 to 1423 K and stresses from 25 to 100 MPa. A direct approach, with analyses at a constant density, revealed a stress exponent of ~1 and an inverse grain size dependence of ~3, consistent with Coble creep process. Whereas the direct approach gives a stress exponent of ~1, the indirect approach used previously gives stress exponents ranging from ~2.2 to 3.5 with the same data, thereby revealing potentially spurious values of the densification parameters from conventional indirect approaches to characterizing densification. The rapid densification during SPS is related to the finer grain sizes retained with the rapid heating rates and the imposed stress that enhances the driving force for densification.

Elastic Analysis on Homogenization Process in Ceramic Powder Compact during Repeated Cold Isostatic Pressing by Multiple Shell Model

Elastic Analysis on Homogenization Process in Ceramic Powder Compact during Repeated Cold Isostatic Pressing by Multiple Shell Model

Enhancing the contrast of low-density packing regions in images of ceramic powder compacts using a contrast agent for micro-X-ray computed tomography

Enhancing the contrast of low-density packing regions in images of ceramic powder compacts using a contrast agent for micro-X-ray computed tomography

Elastoplastic coupling to model cold ceramic powder compaction

The simulation of industrial processes involving cold compaction of powders allows for the optimization of the production of both traditional and advanced ceramics. The capabilities of a constitutive model previously proposed by the authors are explored to simulate simple forming processes, both in the small and in the large strain formulation. The model is based on the concept of elastoplastic coupling providing a relation between density changes and variation of elastic properties and has been tailored to describe the transition between a granular ceramic powder and a dense green body. Finite element simulations have been compared with experiments on an alumina ready-to-press powder and an aluminum silicate spray-dried granulate. The simulations show that it is possible to take into account friction at the die wall and to predict the state of residual stress, density distribution and elastic properties in the green body at the end of the forming process.

Mechanical and thermal characterization of a ceramic/glass composite seal for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) require seals that can function in harsh, elevated temperature environments. Comprehensive characterization and understanding of seals is needed for commercially viable SOFCs. The present research focuses on a novel ceramic/glass composite seal that is produced by roller compaction or tape casting of glass and ceramic powders and an organic binder. Upon heat treatment, micro-voids and surface anomalies are formed. Increased heating and cooling rates during the heat treatment resulted in more and larger voids. The first goal of the current research is to suggest an appropriate heating and cooling rate to minimize the formation of microstructural defects. After identifying an appropriate cure cycle, seals were thermally cycled and then characterized with laser dilatometry, X-ray diffraction, and sonic resonance. From these experiments the crystalline phases, thermal expansion, and elastic properties were determined. Subsequently compression testing with an acoustic emission (AE) sensor and post-test microstructural analysis were used to identify the formation of damage. By fully understanding the characteristics of this ceramic/glass composite seal, next generation seals can be fabricated for improved performance.

Green strength of binder-free ceramics

The Discrete Element Method (DEM) is used to investigate the compaction of ceramic powders by focusing on the role of primary particle and aggregate sizes. Hard aggregates of primary particles are represented as a collection of spherical particles bonded together by solid necks, which may break during compaction. Numerical simulations are performed to investigate the effect aggregate microstructure both on compaction and on the green strength of the ceramic compact. Tensile green strength originates only from van der Waals adhesion, without any action of a binder. It is shown that in such conditions, the green strength is inversely proportional to the size of the primary particles. The size of aggregates also plays a role, with smaller aggregates leading to larger green strength. The results of the simulations are compared to experimental data obtained on dioxide uranium aggregated powders, confirming the particle size effect.

Microwave dielectric properties of low temperature sinterable RE2Mo4O15 (RE = Nd, Sm) ceramics for LTCC applications

The present paper investigates the structure and microwave dielectric properties of novel low temperature sinterable RE2Mo4O15 (RE = Nd, Sm) ceramic system. Phase pure ceramics were prepared through solid state ceramic method. The powder X-ray diffraction patterns show that these materials have triclinic crystal structure with space group PĪ. The title compounds are well densified around 690 °C. The Scanning Electron Microscopic image reveals dense microstructure for the well sintered ceramic compacts. Nd2Mo4O15 ceramic exhibits a dielectric constant of 11.1, an unloaded quality factor of 5,123 at 12 GHz, and a temperature coefficient of resonant frequency (τf) of −44 ppm/ °C. Whereas Sm2Mo4O15 samples possess a dielectric constant of 10.7, unloaded quality factor of 5,468 at 11.6046 GHz together with a temperature coefficient of resonant frequency (τf) of −50 ppm/°C. Both the ceramic systems under study show good chemical compatibility with silver electrode.

Preparation of Gradient ZTA Ceramic by Centrifugal Slip Casting Method

Gradient Zirconia toughened alumina (ZTA) ceramics with high fracture strength and toughness were prepared using centrifugal slip casting method. Aqueous Al2O3-20vol% ZrO2 slurries with different solid contents were prepared and the rheological characteristic of the slurries were investigated. The effect of solid loading of slurries on green density difference of ZTA ceramic compacts was observed. Mass segregation of Al2O3 and ZrO2 particles were studied. Microstructure and bending property of the sintered ZTA ceramics were investigated. The results show that the segregation phenomenon of Al2O3-ZrO2 slurries comes form the difference of settling velocity of Al2O3 and ZrO2 particles. With the increase of solid content of slurries, the gradient distribution of green density becomes unconspicuous. After sintered at 1550○C for 2 h, the gradient ZTA ceramics centrifuged with 20vol% slurries have high sintered density (99.2% TD) and continuous gradient distribution of Al2O3 and ZrO2 particles. The fracture strength and toughness of gradient ZTA ceramic are 732MPa and 5.4MPam1/2, respectively.

Synthesis of β-Sialon Ceramics Using Si, Al and Al<sub>2</sub>O<sub>3</sub> Powders

Single-phase β-Sialon ceramic compacts with z=2 have been synthesized by reaction-bonded sintering using Si, Al and Al2O3 as raw materials. After post-sintering at 1550°C for 2h, only β-Sialon ceramic compacts with z=2 was obtained. When the temperature up to 1650°C and 1750°C, the decomposition reaction of samples was occurred.

Study on Mass Segregation Phenomenon of Al<sub>2</sub>O<sub>3 </sub>Ceramic Slurries during Centrifugal Slip Casting

Mass segregation of Al2O3 slurries during centrifugal slip casing has important effect on microstructure and mechanical property of sintered Al2O3 ceramic, so the study on effect of centrifugal process parameters on mass segregation of slurries has significant meaning. Al2O3 ceramic slurries with different solid content varying from 30vol% to 50vol% were prepared. The effect of centrifugal time, centrifugal acceleration and solid content of slurries on green density uniformity of Al2O3 ceramic compacts was studied. The microstructure of centrifuged compacts was observed using scanning electron microscopy (SSX-505). The results show with the increase of centrifugal acceleration and solid content of slurries, the mass segregation of Al2O3 particles with different size increases and decreases, respectively. However, centrifugal time has no obvious effect on green density uniformity. SEM observation shows the green compacts have uniform microstructure when centrifuged at 2860g for 2h with 50vol% slurries.

High Velocity Compaction: Comparison with Conventional Compaction for New Press Development in Hot Cell Pellet Manufacturing

Within the framework of the technological developments for fuel fabrication in hot cell, the compaction of ceramic powders using both static and dynamic processes has been studied as regards the manufacturing of nearly fully dense ceramics with unique microstructures from precursor ceramic powders. High Velocity Compaction (HVC) is a dynamic compaction without shock wave. The objective of this study [24,[25] was to compare the results of shaping between a conventional and a HVC process, both with hydraulic systems providing the impaction. Various inactive alumina powders were tested. Their characteristics after pressing were assessed to show the differences between green and sintered pellets.

Spark plasma sintering of sol–gel derived 45S5 Bioglass®-ceramics: Mechanical properties and biocompatibility evaluation

This work aims to find an efficient sintering technique and optimal sintering conditions of a novel sol–gel derived Bioglass®-ceramic powder so as to achieve much improved mechanical properties compared to conventional Bioglass®. To this end, the spark plasma sintering (SPS) technique was for the first time used to densify the sol–gel derived Bioglass®-ceramic powder. It was found that the sol–gel derived Bioglass®-ceramics sintered with the SPS technique at 950°C for 15min had a high Young's modulus value of ~110GPa, which was comparable to that of compact bone and significantly higher than the maximal value achieved by the conventional heat treatment. Moreover, the Bioglass®-ceramic compacts sintered with SPS released alkaline ions slowly and as a result, these highly densified Bioglass®-ceramics exhibited better cytocompatibility at the early stage of cell culture testing, compared to the conventional Bioglass®. Hence, the SPS technique is recommended to be used in the process of sol–gel derived Bioglass®-ceramics and its tissue engineering scaffolds.

Field assisted sintering of nanocrystalline titanium nitride powder

The effect of field assisted sintering (FAS) on the compaction of TiN ceramics is examined using a nanosized TiN powder. Microstructural evolution at different stages of FAS is evaluated using electron microscopy. Coarse spherical particles (1–5 μm) are found at 700°C. These particles apparently result from excessive heat release on interparticle contacts when an electrical current passes through agglomerates at certain stages. The specific surface area and pore size distribution in sintered samples are examined with static volumetric absorption. The compaction and microstructural evolution are discussed in terms of electric field effects.

Injection molding of surface modified powders with high solid loadings: A case for fabrication of translucent alumina ceramics

Solid loading is a critical key to the fabrication of ceramic compacts with high densities via ceramic injection molding. As reported in most previous work, solid loading of ultra-fine alumina feedstock system could be achieved only up to ∼58 vol% with stearic acid (SA) as the surface modification agent. In present work, different from the traditional work in which SA has been introduced just in the powder blending process, we have successfully prepared the feedstock with a much higher solid loading up to ∼64 vol% by a prior ball milling treatment of ceramic powders with a small amount of SA before the traditional blending process. It can be attributed to that SA can be coated homogeneously around the powder surfaces by a chemical reaction induced by ball milling treatment. Highly translucent Al 2O 3 ceramics have been fabricated, which suggests an alternative route for fabrication of translucent ceramics with high quality.

Nanoindentation hardness of hot-pressed boron suboxide

Abstract The existence of the indentation size effect implies the absence of a single hardness value for the material under investigation especially at low applied loads. In this paper we present an investigation of the indentation size dependence behaviour of nanoindentation hardness in boron suboxide ceramic compacts prepared by uniaxial hot-pressing. Berkovich nanohardness indentations were conducted and analyzed accordingly. In addition to the ordinary Oliver and Pharr method of nanoindentation data analysis, a quantitative approach for the loading curve analysis is proposed. Using the proposed approach, the description and characterization of the observed indentation size effect through the application of the Meyer’s law, and the classical and the modified proportional specimen resistance models as well as the multi-fractal scaling law was conducted and is reported. The load-independent hardness values deduced from our quantitative approach are comparable to the results calculated with conventional methods, especially with the multi-fractal scaling law.

Piezoelectric Behavior Based on Mixing-Doped in Lead Zirconate Titanate Ceramics and Application

In this investigation, we extend our previous works to improve piezoelectric properties of Pb(Zr,Ti)O3 ceramics. Modified lead zirconate titanate (PZT) piezoceramics with a composition Zr/Ti=53/47 containing a trace of mixing dopants were prepared by conventional ceramic technology sintering powder compacts. Replacement of (Zr, Ti)+4 by Nb+5, Pb+2 by Sb+3 and Mn+4 in PZT perovskite type solid solutions was accomplished by the creation of cation and anion vacancies. Modified ceramics were explored as a function of firing temperature to acquire exceedingly good piezoelectric characterizations. From the analysis results, calcined at 850°C for 2 h, and then sintered at 1280°C for 2 h, PZT piezoceramic showed the larger dielectric constant er 2013, mechanical quality factor Qm 120 and maximum electromechanical coupling factor kp 0.67. Besides, the bulk ceramic grains distribution were found to be uniform. Furthermore, the sample was found to possess piezoelectric properties, the resonance frequency being about 200 KHz suitable for acoustic sensor.

Organic-Inorganic Conversion Process for Material Creation -Formation and Function of Characteristic Nanostructures-

In the case that an organometallic compound or an organic metal complex is used as the raw material in an “organic–inorganic conversion” process, the desired shape and characteristic nanostructure can be formed. This nanostructure can provide unique functionality and lead to the creation of original materials. From a fibrous form of polycarbosilane, which is an organosilicon polymer, made by melt spinning in an inert gas, continuous inorganic SiC fibers can be obtained. These fibers are not easily oxidized and their tensile strength does not decrease even in a high temperature in air. They can therefore be used in fiber-reinforced composite materials. Polycarbosilane is also an excellent binder for ceramic powder that is difficult to sinter. In addition, it is an excellent impregnation agent for ceramic compacts. Thus, preparation of ceramic compacts having high mechanical strength and oxidation resistance is possible using polycarbosilane. On the other hand, when bis(acetylacetonato)zinc; (Zn(acac)2), which is an organic metal complex, is made into a fibrous form by sublimation and pyrolysis with superheated steam, inorganic ZnO fibers can be obtained. Such fibers exhibit visible-light photocatalytic ability and effectively decompose volatile organic gases. [DOI: 10.1380/ejssnt.2011.181]

Preparation, characterization and dielectric properties of Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] ceramics

The ceramic compositions Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] were prepared through conventional solid state ceramic route. A detailed study has been carried out to correlate the structure of Ba3−xSrxLiM3Ti5O21[M=Nb and Ta, x = 0 to 3] with respect to their dielectric properties. The structure and microstructure of ceramic samples were studied using powder X-ray diffractometer and Scanning Electron Microscopic techniques. The dielectric properties of the sintered ceramic compacts have been studied. The Ba-rich compositions were identified as promising candidates for high frequency applications whereas the Sr-rich compositions were excellent ionic conductors and can be commercially exploited for applications in solid-state batteries.