Initial Densification Research Articles

Synthesis of Nanoparticles of Cu, Sb, Sn, SnSb and Cu<SUB>2</SUB>Sb by Densification and Atomization Process

Here we present a technique based on an initial densification of solid precursor materials using magnetic pulses followed by an atomization process via spark discharging. These two processes allow changing bulky micron sized materials into nanoparticles (5-60 nm). The resulting intermediates and nanomaterials have been characterized using electron microscopy (TEM, SEM) and X-ray diffraction to show the texture and structure evolution between the initial bulk phase and the final nanoparticles. In this paper we present the nanoparticle formation of certain metals (Cu, Sn, Sb), alloys and intermetallics (SnSb, Cu2Sb) starting with pure elemental powders.

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO 2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO 2–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.

Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO 3 ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO 2 and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.

Dilation Behavior of Hot Mix Asphalt Under Triaxial Loading

ABSTRACT Permanent deformation generation in asphalt pavements has been explained to occur as a result of the initial traffic densification with the subsequent plastic flow. Recent studies indicate that asphalt concrete exhibits dilation under traffic loading; the displaced volume of asphalt mixture, including the downward depression in the wheel paths and the upheaval beside the wheel paths, is strongly correlated to the magnitude of rutting. In this study, triaxial strength testing was performed for both dense-graded mixtures and stone mastics asphalt mixtures. The dilation phenomenon was characterized by using the Corelok apparatus to measure the volume changes of the tested specimens at selected axial strains. The test results were analyzed using Mohr-Coulomb failure theory and Critical State Soil Mechanics theory, which studies the coupling effects of the density, volume change, and shear strength of soils.

Thermal evolution of free volumes and of crystallization in amorphous Si–B–C–N ceramics

Abstract We report on the crystallization behavior of initially amorphous precursor-derived Si3B1C4.3N2 ceramics, making use of macroscopic and atomic scale investigation techniques. As derived from our kinetic studies upon heating, the following temperature sequence of solid state processes is observed with approximately the same kinetic time constants: after an initial densification in the amorphous state (1673K), crystallization occurs (1873K) with subsequent strain relaxation and disappearance of nanovoids (2073K). This behavior indicates an increasing activation enthalpy of the atomic mechanisms giving rise to these processes when the annealing temperature is increased. The nanovoids may be located in the turbostratic BNCx layers as derived from positron annihilation studies. For grain growth at high temperatures an activation energy of HG=3.9eV is derived.

Effects of CaCO 3 content on the densification of aluminum nitride

The effect of adding up to 13.4 wt.% CaCO 3 on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C. The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves. Two microstructural aspects determined the densification of aluminum nitride with CaCO 3: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO 3 caused the formation of higher melting point calcium aluminates (mainly CA 2) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO 3 amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C 12A 7 related eutectic temperature. Irrespective of the initial CaCO 3 content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions. The effect of this composition changing on densification is discussed. Large pores were formed in the sites originally occupied by large particles of CaCO 3 and retarded the bulk densification in samples with high additive contents.

Preparation of Dense Mullite Ceramics through Gelcasting of Alumina Slurry Dispersed in Silica Sol

A nano‐SiO2‐coated‐Al2O3 (NSCA) micro‐composite system has been derived from silica sol and alumina to prepare dense mullite ceramics by gelcasting technology. Concentrated NSCA suspension with high solid loading but low viscosity was successfully prepared, utilizing the stabilization mechanism of silica nanoparticle halos around alumina particles at pH 9.5. The defect‐free dried green body with bending strength of 5.85 MPa was obtained with only 7.7 wt.% of monomer and dense mullite ceramics was sintered at 1550°C for 2 h, through initial densification by transient viscous sintering, followed by a densification arrest and then a further densification by solid‐state diffusion at higher temperatures.

Characteristics and Sintering Behavior of Oxide Coated Iron Nanopowder Synthesized by Plasma Arc Discharge Process

This paper investigates the effect of the surface oxide layer of Fe nanopowder synthesized by plasma arc discharge (PAD) process on the densification process in hydrogen atmosphere. The densification process was divided into two steps: rapid densification at low temperature and densification retarding at high temperature. The reduction process of the oxide layer was quantitatively analyzed, and it was observed that the reduction process dominantly affected the initial densification process at lower temperature. The volume shrinkage due to the reduction of the oxide layer corresponded to 75% of the initial volume shrinkage at low sintering temperature. The volume shrinkage of the reduced oxide layer and the densification rate were discussed in terms of microstructural evolution.

Densificação rápida de cerâmicas de SnO2

Os pós à base de óxido de estanho são conhecidos por apresentarem baixa densificação mesmo a temperaturas de sinterização acima de 1500 °C. A introdução de diferentes íons metálicos como Mn2+, Fe3+ e Cu2+ induzem a redução do volume de poros e crescimento de grãos durante a sinterização. Pós à base de SnO2 foram preparados pela rota química derivada da patente de Pechini, contendo diferentes concentrações de Mg2+ ou Fe3+. Todas as amostras apresentaram uma alta taxa de densificação nos momentos iniciais, quando sinterizadas por "fast firing". As amostras contendo 5% em mol de ferro sinterizadas durante 30 s a 1200 °C apresentaram densidades superiores às amostras sinterizadas por aquecimento em taxas normais (10 °C/min) por 4 h a 1200 °C. O fenômeno de eliminação rápida de poros pode ser explicado, levando em conta o papel da superfície no fenômeno de sinterização. A saturação da superfície pelo aditivo foi confirmada por medidas de mobilidade eletroforética dinâmica com base na literatura e em dados experimentais.

Determination of the Local Stress-Strain Response of Foams

A new specimen for determining the local stress-strain response of foams under uniaxial compression is presented along with the corresponding theory. The drawback of using conventional cubic specimens is that average stresses and strains are calculated, which introduces size effects due to structural and material inhomogeneities of commercial foams. Under compression, foam cubes develop unstable regimes, which involve localized deformation. The instabilities cause difficulties in establishing the correct stress-strain response of the material. Tapering specimens can enable controlled motion of the boundary separating the unstable and stable regimes. This concept is exploited in the present paper in experiments on closed-cell aluminum foam trapezoids. A crushing front propagates down the tapered specimen during compression, while the deformed region develops a new lateral shape. The experimental results are used along with several assumptions to extract a more representative stress-strain response of foam. The response is characterized by the initial plateau stress, shape exponent and densification strain. The significant effect of the variable Poisson’s ratio during crushing is also introduced. The results provide a basis for developing local constitutive behavior of foams.

Relation between aggregation and phase separation: three-dimensional Monte Carlo simulations.

We study phase separation of particles in solution using Monte Carlo simulations of reversible aggregation on a cubic lattice. Two stages of the phase separation can be clearly distinguished: initial random aggregation and subsequent densification. Step one leads to a distribution of fractal aggregates close to the binodal and to a temporary gel for large attractive interaction. Step two leads to isolated spherical dense domains close to the binodal and branched wormlike strands for large attractive interactions. The transition between the two types of structure is gradual and there is no clear feature that shows the existence of a spinodal. The first stage of the phase separation is metastable very close to the binodal or at very large interaction energy. In the latter case, the second step can be viewed as an aging process of the gel formed in the first step.

Interpretation of experimental results from moderate-power in-pile testing of a Pu–Er–Zr–oxide inert matrix fuel

Pu–Er–Zr oxide as an inert matrix fuel (IMF) could be an attractive option for a once-through LWR strategy aimed at reducing the currently growing plutonium stockpiles. A basic question related to the practical introduction of such an IMF into a current-day LWR, without affecting any significant change in fuel element design and core performance, is the irradiation behaviour of the proposed IMF. The present paper reports first results from a corresponding validation experiment which has been launched recently, viz. the comparative in-pile testing of specially fabricated IMF and PuO 2/UO 2 mixed-oxide (MOX) fuel rodlets in the Halden reactor. The maximum linear power rating was limited to about 25 kW/m during the currently reported initial testing phase, as compared to the design value of 35 kW/m to be aimed at during subsequent cycles at Halden. While fuel centre-line temperatures were found to be within the expected range for both fuel types, the variation of pressure in the rodlets has clearly indicated significant densification in the IMF specimens, with fission gas release being not detectable at the low burnups achieved to date. A possible sintering model for the IMF is proposed to explain the experimental results obtained during this moderate-power testing phase, viz. the apparent “geometrical stability” of the IMF in spite of its strong initial densification.

Microstructure and Sintering of Ceramic Green Compacts Described in a Constitutive Model.

The effect of forming pressure on the internal structure and the sintering behavior of green compacts made from ceramic granules was investigated with the aim of describing those by a constitutive model. Alumina granules were compacted by cold isostatic pressing (CIP) at three different pressures of 98, 196 and 294MPa. Examined were the initial pore structure, shrinkage and densification of the granular compacts during sintering at four temperatures in the range between 1100 and 1400°C. Three distinct regions in the microstructure with fractured, unfractured granules and crack-like cavities were identified as the three peaks observed in the pore size distribution. The ratio of the respective pore volumes in these three regions was independent of the forming pressure within the range under examination, while the pore size decreased as the pressure increased. The sintering rate of the compacts formed at 98MPa was similar to that at higher pressures, because the dominant process responsible for the densification during sintering is the shrinkage of small pores in a dense region. However, the final density after sintering of the compacts formed at 98MPa was somewhat lower than that of the compacts formed at higher pressures. This stems from large crack-like pores around the unfractured granules remained even at 1400°C. The shrinkage behavior was modelled with a set of constitutive equations by taking account of the pore size distribution of granular compacts. The sintering behavior related to the forming pressure could be well described by the present model.

Synthesis of Nanocrystalline Lithium Zinc Ferrites Using Polyacrylic Acid, and Their Initial Densification

A chemical synthesis technique that uses polyacrylic acid (PAA) to prepare nanocrystalline Li0.3Zn0.4Fe2.3O4 was investigated. Phase‐pure spinel that had an average particle size of ∼13 nm was synthesized at a low temperature (450°C) for 30 min. The average particle size was dependent on the firing temperature and amounts of PAA and Bi2O3. The combustion reaction of PAA was believed to be the main reason for the low‐temperature synthesis of nanocrystalline lithium zinc (LiZn) ferrites. In addition, a chelating tendency was observed, via infrared (IR) spectroscopy, in the dried gel precipitates that were prepared at a low pH (∼2). The initial densification behavior of the nanopowder compacts was studied using linear‐shrinkage data. Densification of the nanopowder without Bi2O3 started at low temperatures (∼400°C). The addition of Bi2O3 to the nanopowder noticeably did not contribute to reducing the initial densification temperature. This result was supported by the activation‐energy calculation for the initial densification. Compared to an activation energy of 280 kJ/mol for grain‐boundary diffusion of the nanopowder without Bi2O3, a higher value (430 kJ/mol) was obtained in the case of the 3 wt% Bi2O3‐containing nanopowder.

A finite element model for consolidation of vacuum plasma spray-formed metal matrix composites

Monotapes consisting of Ti–6Al–4V vacuum plasma-sprayed onto SiC fibres were consolidated by uniaxial vacuum hot pressing. This consolidation process was modelled by computational analysis of a representative unit cell. To describe the initial deformation and densification behaviour of the monotape surface asperities, a yield criterion was derived using a continuum mechanics approach for porous metals and a contact mechanics approach for rough surfaces. Yield criteria for porous metals were applied to the subsequent consolidation of the surface and the remaining matrix. Mechanism-based constitutive equations which consider work hardening, strain rate and grain growth effects were used to describe the high-temperature deformation behaviour of the matrix. The density distributions in the monotape unit cell were calculated and compared with experimental data. Consolidation maps, which may be used to select appropriate processing parameters, were constructed from the finite element analysis. The effect of fibre spacing on consolidation is also predicted.

Anisotropic Grain Growth in Diphasic‐Gel‐Derived Titania‐Doped Mullite

Densification and anisotropic grain growth in diphasic‐gel‐derived, titania‐doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania‐doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final‐stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania‐doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania‐doped diphasic mullite gels followed the empirical equation Gn ‐ G0n=Kt, with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.

The vacuum hot pressing behavior of silicon carbide fibers coated with nanocrystalline Ti–6Al–4V

The vacuum hot pressing (VHP) of silicon carbide monofilaments coated with nanocrystalline Ti–6Al–4V has been studied. During consolidation, very high densification rates were observed, even at temperatures and pressures well below those normally used for processing conventional Ti–6Al–4V. From the cross-sections of partially consolidated specimens, the evolution of coated fiber–fiber contacts and pore shapes were determined. The pores were found to be cusp-shaped throughout the consolidation process. Columns of coated fibers were observed to form along the loading direction and resulted in regions of locally high fiber volume fraction. Simulations of the VHP experiments were performed using a model which incorporated time and temperature dependent microstructure relationships. In the model, the initial densification was based upon a recent micromechanical contact analysis for a metal coated fiber. Final stage densification was analyzed by modifying the Qian et al. strain rate potential for a power law creeping body containing isolated cusp-shaped pores. Overall, the simulations compared well with the experimental density data, although the load supported by the regions of locally high fiber volume fraction resulted in the model slightly overestimating the observed densification time response.

Densification and Crystallization Kinetics of Cordierite Glass-Ceramic Coatings on Rigid Substrates

ABSTRACTWe report our findings on the sintering kinetics of slip-cast glass-ceramic coatings on rigid substrates at temperatures between 900°C and 1000°C; the glass-ceramic crystallizes into cordierite at sintering temperatures above 900°C. Changes in coating thickness during sintering were monitored in situ using an optical setup, and the crystallization of the coatings was determined by x-ray diffraction. The coatings went through a rapid shrinkage due to initial densification, followed by an expansion stage caused by the transformation of the glass into a crystalline phase with a lower density. The rate and extent of the expansion were more noticeable at sintering temperatures above 900°C. We also found that wetting between the glass-ceramic and the substrate was important to the sintering kinetics and sintered microstructure of the coatings. For coatings formed on silicon substrate, growth of large pores was observed at the glass-ceramic/silicon interface. Using wafer-curvature measurement technique, we found a significant tensile stress build-up in the coatings during the initial stage of sintering. We believe that the combination of poor wetting and the tensile stress build-up led to the observed desintering phenomenon at the glass-ceramic/silicon interface.

Sintering, Microstructure, and Properties of Submicrometre Cemented Carbides

The effect of initial particle size on the sintering behaviour, microstructure, and physical and mechanical properties of submicrometre grain sized hardmetals manufactured from WC powders of different particle sizes (0·3 – 0·8 μ m) has been studied. The composition of the powders was WC–6Co with additions of VC and Cr3C2 grain growth inhibitors. The sintering was carried out in vacuum followed by hipping at 1380°C for 1 h. Ultrafine hardmetals exhibit substantially improved mechanical properties with a good combination of hardness, fracture toughness, and transverse rupture strength when hipping is applied after sintering. The finer the initial WC powder, the higher is the degree of shrinkage during the initial solid stage of densification. The analysis of sintering kinetics using dilatometric techniques showed that initial densification occurs by an interfacial diffusion mechanism in ultrafine powders as well as in regular powders, while the activation energy of the fine powders is considerably lower. The microstructure of sintered hardmetals is characterised by very fine grain size. A high volume fraction of direct WC–WC low angle and high angle grain boundaries with no Co interlayer was observed by scanning transmission electron microscopy/energy dispersive spectroscopy techniques. PM/0733

Constitutive Law for the Densification of Fused Silica, with Applications in Polishing and Microgrinding

We discuss a constitutive model describing the permanent densification of fused silica under large applied pressures and shear stresses. The constitutive law is assumed to be rate‐independent and uses a yield function coupling hydrostatic pressure and shear stress, a flow rule describing the evolution of permanent strains after initial densification, and a hardening rule describing the dependence of the incremental densification on the levels of applied stresses. Normality, or lack thereof, of the permanent strain increments to the current yield surface in stress space allows for various relative contributions of densification and shear flow in the ensuing deformation. The constitutive law accounts for multiaxial states of stress, since during polishing and grinding operations complex stress states, with large shear components due to friction and abrasion, occur in a thin surface layer due to the action of abrasive particles. We apply the constitutive law in estimating the extent of the densified layer during the mechanical interaction of an abrasive grain and a flat surface under polishing and grinding conditions. The grain is assumed to be spherical and in Hertz contact with the surface, or sharp and in point contact. The effect on the densified depth of stress relaxation due to densification is discussed.