Electric Field Assisted Sintering of Cubic Zirconia at 390°C

Using the flash sintering technique, cubic yttria‐stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm. Furthermore, flash sintering experiments performed with electric fields between 60 and 2250 V/cm were used to show that the relationship of the temperature at the onset of flash sintering (T_Onset) and the applied field (E) follows the power relationship T_Onset (K) = 2440 E^?1/5.85(V/cm). Using this relationship, and considering the sintering of the sample in the absence of an electric field, the critical electric field to enter the flash sintering regime is shown to be 24.5 V/cm. For electric fields between this critical electric field and 2250 V/cm, the onset of flash sintering occurs in the same range of critical volumetric power dissipation, between 1 and 10 mW/mm³, suggesting this is a material property. Despite the volumetric power dissipation being the critical value for the onset of flash sintering behavior, the current density achieved during sintering appears to be more critical for densification rather than maximizing power dissipation.     

Intergranular amorphous films formed by DC electric field in pure zirconia

It is difficult to obtain pure ZrO₂ sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO₂ can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO₂ flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.     

Molecular Weight Polydispersity Effects on Diffusion at Polymer/Polymer Interfaces

The effect of molecular weight distribution (MWD) on diffusion at symmetric polymer/polymer interfaces is investigated by rheological tools. A new model allowing the determination of a self‐diffusion coefficient of polydisperse polymer systems is presented. The model is based on the double reptation theory and Doi and Edwards' molecular dynamics applied to A/A polymers brought into intimate contact in the molten state. The material parameters for the model are obtained from linear oscillatory shear experiments, in which the dynamic shear modulus is measured in parallel plate geometry under a small amplitude of deformation as a function of time and frequency for a sandwich‐like assembly. The experiments were conducted on polystyrene (PS) blends with constant weight average molecular weight (M_w) but with variable number average molecular weights (M_n). The measured self‐diffusion coefficients showed that the presence of short molecules in the blend increases the mean value of the self‐diffusion coefficient and the magnitude of such increase can be quantitatively evaluated by the proposed model.     

A metallurgical approach to the pre-yield and yield behavior of glassy polymers

This paper discusses some new mechanical and small angle neutron scattering (SANS) data on glassy polymers, both thermoplastics and thermoset resins, from the point of view of dislocation-like defects introduced in the molecular chain arrangement by deformation. In the pre-yield stage, a new parameter, the work-hardening rate K is introduced and its measurement is defined. Experiments are reported which show that K can be used as a very sensitive probe for microstructural changes during physical aging or curing. In one hand, the theory of yielding is revisited to make clear how dislocations and their propagation in polymers depend on specific features like entanglements and chain stiffness. On this basis, experimental internal stresses and activation volumes at yield (i.e., the temperature slope of yield stress) are accounted for. On the other hand, SANS data provide us with experimental evidence at the scale of 10 to 20 Å of the dislocation nature of the molecular “shear defects” introduced in the polymer by deformation. Finally, temperature is known to have a pronounced influence on yield processes. It is shown that two distinct deformation modes exist below and above a critical temperature T_c. Above T_c, a dislocation climb, which probably involves β-processes, gives rise to a “diffusional” deformation mode where chains within a (diffuse) shear band are no longer oriented. A tentative formalization of this behavior, and its relation to the small strain creep of polymers, are then presented.     

Fabrication of embedded fine line inductors by constrained sintering and formulation of photoimageable conductive paste

Abstract

For miniaturisation and precision of electronic device, the new technologies such as photoimageable thick film process were combined with conventional thick film process, and constrained sintering with near zero shrinkage in the x and y direction has been proposed. In this research, photoimageable conductive paste for forming embedded components via constrained sintering by low temperature cofired ceramic (LTCC) technology was formulated. Afterwards, by optimising paste formulation, formation process of fine line and sintering method, miniaturised LTCC components especially embedded fine line inductors were fabricated and their properties such as line resolution, surface morphology and yield were investigated. As a result, embedded fine line inductors formed by constrained sintering with fine line resolution of 20 μm and yield over 90% were acquired.     

Mechanical modelling of microwave sintering and experimental validation on an alumina powder

Microwave sintering (MW) allows fast heating (≤30 min) and densification of ceramic materials, like alumina Al₂O₃. In order to predict the final material properties (density, size and grain size) the mechanical SOVS (Skorohold Olevsky Viscous Sintering) model is adapted and validated for conventional sintering of alumina. The model is implemented on ABAQUS with UMAT subroutine. Secondly, the SOVS model is modified for the microwave sintering by adapting the shear viscosity Arrhenius type law. Pre-exponential and exponential coefficients are modified for MW sintering. The calculated relative densities are compared to experimental results from conventional and microwave sintering and the relative difference remains under 3%. The coefficients identified for the MW sintering reveal a decrease in the shear viscosity by around 10 and an increase by up to 50 times in the grain boundaries diffusion coefficient.     

Hot‐Pressing Kinetics and Densification Mechanisms of Boron Carbide

The hot‐pressing kinetics of boron carbide at different stages in the hot‐pressing process was investigated. Based general densification equation and pore‐dragged creep model, the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature, sintering pressure and dwell time. Stress exponent of n ≈ 3 at the initial dwell stage suggests the plastic deformation may dominates the densification. The further TEM observations and the calculation based on effective stress and plastic yield stress also indicate that plastic deformation may occur and account for the large increase in density at the initial stage of sintering. Calculated grain size exponent of m ≈ 3 suggests that the grain‐boundary diffusion dominates the densification at the final stage. During the final stage of sintering, grain growth may be determined by evaporation/condensation and grain‐boundary migration.     

Macromolecular motions and hydrodynamic radius variation in dilute solutions under shear action

Understanding the dynamics of single polymer chains and rheological mechanism in dilute polymer solutions under shear stress is essential for fields such as the petroleum and food industries, biomedical materials and drug delivery. Here we present an experimental method for measuring the viscosity of polymer solutions and studying the variation of single polymer chain conformation and the mechanism of molecular motions according to the relationship between the intrinsic viscosity, [η], and the shear rate. Of striking interest is that we find that [η] changing with the shear rate presents three stages which may explain the nature of the viscoelastic performance of polymer solutions and the isolated molecular motions. The significance of these results is the finding of the polymer chain deformation to match the pore throat which has enormous potential implications in drug delivery, genetics and biomedicine © 2014 Society of Chemical Industry.     

Porous Ni/ZrO2 Cermet from Highly Concentrated Composite Colloid

Rheology and shaping of concentrated cermet suspensions consisting of nickel (Ni) and yttria‐stabilized zirconia (YSZ) nanoparticles in water have been examined over a broad range of volumetric solids concentration (? = 0.1–0.4) and Ni fraction (f_Ni = 0.15–0.45). Preferential adsorption of pyrogallol‐poly(ethylene glycol) polymer (i.e., Gallol‐PEG) on surface of the Ni and YSZ particles imparts steric hindrance between the suspending particles so that fluidity can be obtained under shear stress. The cermet suspensions exhibit shear‐thinning flow behavior under steady‐shear measurement over shear rates of 10⁰–10³ s^?1. Yield stress and yield strain of the suspensions appear to vary pronouncedly with ? and f_Ni under oscillatory shear over a shear‐strain range of 10^?1–10³%. With the Gallol‐PEG adsorption, an apparent viscosity less than 6 × 10^?1 Pa.s at a shear rate of 10² s^?1 has been obtained for the highly concentrated composite suspension with ? of 0.40 and f_Ni of 0.25. A high solids concentration effectively prohibits phase segregation during wet‐shaping processes. Uniform green compacts have been obtained from slip casting of the concentrated cermet mixture (? = 0.30) without use of binder and are then fired at 1200°C under reducing atmosphere to form porous Ni/YSZ compacts. Relative sintered density increases from 65% to 75% of the theoretical value when f_Ni was increased from 0.15 to 0.45, due mainly to the lower sintering temperature required for the Ni phase.     

Nonisothermal sintering of Cr2AlC powder

This work studies the sintering behavior of ultrafine powder of Cr₂AlC compound during nonisothermal heating using a dilatometer. Sample exhibits relatively slower densification under pressureless condition where relative density increased from about 61% (green compact) to about 73% during heating up to 1350°C. Axial shrinkage data were analyzed through the volume diffusion and grain boundary diffusion‐based sintering models. Estimated activation energy of sintering (352 kJ/mol) for volume diffusion mechanism, was found to be closely matching with the reported activation energy of chromium diffusivity, indicating volume diffusion as a controlling mechanism and chromium as a major diffuser in Cr₂AlC.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Constrained Sintering of a Low‐Fire,Polycrystalline Bi2(Zn1/3Nb2/3)2O7 Dielectric

The densification behavior of a low‐fire, polycrystalline Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) dielectric under constrained sintering at 800°C–900°C is investigated. Although the constrained densification is retarded in relative to free sintering, a high sintered density of >95% is obtained at 900°C. No significant anisotropy with similar grain sizes is developed under free and constrained sintering. The densification behavior and stress development during constrained sintering of BZN is thus analyzed by using the well‐known isotropic constitutive laws.     

Fabrication of dense and defect-free diffusion barrier layer via constrained sintering for solid oxide fuel cells

To enable the development of next-generation solid oxide fuel cells (SOFCs), the fabrication of dense and defect-free diffusion barrier layers via constrained sintering has been a significant challenge. Here, we present a double layer technique that enables complete densification of a defect-free gadolinia-doped ceria diffusion barrier layer. In this approach, top and bottom layers were individually designed to perform unique functions based on systematic analysis of constrained sintering. The top layer, which contains 1 wt% CuO as a sintering aid, provides sufficient sintering driving force via liquid-phase sintering to allow complete densification of the film, while the bottom layer without a sintering aid prevents detrimental chemical reactions and regulates the global sintering rate to eliminate macro-defects. Such fabrication of dense diffusion barrier layers via a standard ceramic processing route would allow the use of novel cathode materials in practical SOFC manufacturing. Furthermore, the strategy presented in this study could be exploited in various multi-layer ceramic applications involving constrained sintering.     

Grain growth and pore coarsening in dense nano‐crystalline UO2+x fuel pellets

Dense nano‐sized UO_2+x pellets are synthesized by spark plasma sintering with controlled stoichiometries (UO_2.03 and UO_2.11) and grain sizes (~100 nm), and subsequently isothermally annealed to study their effects on grain growth kinetics and microstructure stability. The grain growth kinetics is determined and analyzed focusing on the interaction between grain boundary migration, pore growth, and coalescence. Grains grow much bigger in nano‐sized UO_2.11 than UO_2.03 upon thermal annealing, consistent with the fact that hyper‐stoichiometric UO_2+x is beneficial for sintering due to enhanced U ion diffusion from excessive O ion interstitials. The activation energies of the grain growth for UO_2.03 and UO_2.11 are determined as ~1.0 and ~2.0 eV, respectively. As compared with the micrometer‐sized UO₂ in which volumetric diffusion dominates the grain coarsening with an activation energy of ~3.0 eV, the enhanced grain growth kinetics in nano‐sized UO_2+x suggests that grain boundary diffusion controls grain growth. The higher activation energy of more hyper‐stoichiometric nano‐sized UO_2.11 may be attributed to the excessive O interstitials pinning grain boundary migration.     

Fracture initiation in bi-material joints subject to combined tension and shear loading

Abstract

Linear elastic solution of the stress field near an interface corner of bi-material joints is of the form Hr^λ?1, where r is the radial distance from the corner, H is the stress intensity factor and λ–1 is the order of the singularity. Finite element analysis is used to determine the magnitude of H for a butt joint subject to remote shear; the obtained solution complements existing solution for remote tension and uniform change in temperature. The theoretical solution of the singular shear stress is shown to be in good agreement with the corresponding finite element solution. The effect of combined remote tension, remote shear and uniform change in temperature on the failure loads and failure mechanisms is experimentally determined for brass/araldite/brass butt joint. It is shown that the failure envelope in tensile stress – shear stress space is elliptical and the failure loads decrease with increasing cure temperature due to thermal residual stress associated with the curing process. The application of the results to the assessment of onset of failure in composite patch repair is discussed.     

Constrained Sintering of Silver Circuit Paste

Densification kinetics and stress development during constrained sintering of a silver film on a rigid silicon substrate have been studied. Compared with free sintering, the sintering of constrained silver film exhibits a much lower densification and slower densification kinetics. The densification-controlled mechanism changes from fast grain-boundary diffusion kinetics for free sintering to slow lattice diffusion kinetics for constrained sintering. The in-plane tensile stress developed during constrained sintering of silver film, measured using a noncontact laser-scanning optical system, increases rapidly to a maximum level of 1.0–1.5 MPa initially, gradually decreases, and then becomes constant at 0.8–1.0 MPa. The maximum stress observed increases with increasing sintering temperature as a result of the faster densification rate. It is believed that the retardation of densification kinetics of constrained silver film is caused by a change in densification mechanism and the existence of in-plane tensile stress.     

Field assisted sintering of ceramic constituted by alumina and yttria stabilized zirconia

We show that a two-phase 50 vol% 3YSZ-alumina ceramic flash-sinters at a furnace temperature of 1060 °C under an electrical field of 150 V cm⁻¹. In contrast undoped, single-phase alumina remains immune to field assisted sintering at fields up to 1000 V cm⁻¹, although single-phase 3YSZ flash sinters at 750 °C (furnace temperature). The mechanisms of field assisted sintering are divided into two regimes. At low fields the sintering rate increases gradually (FAST), while at high fields sintering occurs abruptly (FLASH). Interestingly, alumina/zirconia composites show a hybrid behavior such that early sintering occurs in FAST mode, which is then followed by flash-sintering. The specimens held in the flashed state, after they had sintered to near full density, show much higher rate of grain growth than in conventional experiments. These results are in contrast to earlier work where the rate of grain growth had been shown to be slower under weak electrical fields.     

The sintering kinetics of ultrafine tungsten carbide powders

The sintering kinetics of nano grained tungsten carbide (n-WC) powders has been analyzed by non isothermal and isothermal sintering. Non isothermal sintering experiments reveal a multi staged sintering process in which at least three major sub-stages can be distinguished. The isothermal shrinkage strain also exhibits an asymptotic behavior with time indicating an end point density phenomenon in most of the temperature ranges. Combined microstructural and kinetic data analyses suggest that differences in the sinterability of inter and intra agglomerate pore phases introduce sub-stages in the sintering process which manifest as stagnant density regions in both the isothermal and non isothermal experiments. Kinetic analysis of the data reveals very low activation energies for sintering suggesting that particle rearrangement and agglomeration at low temperatures may be brought about by surface diffusion leading to neck growth and grain rotation. At higher temperatures rapid grain boundary diffusion by overheating along inter particle boundaries induced by sparking may be a dominant sintering mechanism. Although grain growth and densification in conventional WC powders generally obey an inverse relation to each other, in n-WC powders both can act synergistically to increase the net densification rate. In fact, complete densification cannot be achieved in n-WC powders without grain growth as one abets the other.     

Extrudate swell during the melt spinning of fibers—influence of rheological properties and take-up force

A theoretical and experimental study has been carried out on extrudate swell B, especially the influence of rheological properties and applied take-up force on the emerging melt. The problem is analyzed in terms of (1) dimensional analysis, (2) force–momentum balances, (3) partially constrained elastic recovery. Analyses in terms of force–momentum balances are only able to give extrudate swell B in the asymptote of high Reynolds numbers. For low Reynolds numbers, they simply relate the take-up force to the pressure field in the spinneret. Increasing the take-up force predicts a decrease in the exit pressure. The partially constrained elastic recovery theory yields an expression for B as a decreasing function of applied take-up force. Specifically, this is where B(0) is the extrudate swell in the absence of applied forces, λ_eff is the effective relaxation time, μ is viscosity (both evaluated at the capillary wall), and D is the spinneret capillary diameter. An experimental study of extrudate swell of several rheologically characterized melts (high density polyethylene, low-density polyethylene, polypropylene, polystyrene) has been carried out at 180°C by four different methods (frozen, annealed in hot silicone oil, photographed emerging into air, photographed emerging through 180°C silicone oil) in the absence of applied take-up forces. Extrudate swell for a melt emerging from dies with differing diameters correlates with capillary-wall shear rate. A comparison of extrudate swell with normal stress–shear stress ratio shows the best agreement for frozen extrudates and photographs of melts emerging into air. The data is compared to the Tanner theory of extrudate swell. B has been determined during melt spinning and shown to be a function of take-up force for both a high-density polyethylene and polypropylene melt. B decreases rapidly with applied take-up stresses. The results are compared to the predictions of the partially constrained elastic recovery theory.     

Polyoxymethylene orientation by equal‐channel multiple angular extrusion

The effect of conditions and routes of deformation in the course of equal‐channel multiple‐angular extrusion (ECMAE) on physical and mechanical properties of polyoxymethylene (POM) have been studied. As deformation routes, Route C (shear planes are parallel, and the simple shear direction of every deformation zone is changed through 180°) and Route E (shear planes are turned through ±45° around the extrusion axis and the normal to the axis, and simple shear direction is changed through 180° or ±90° with respect to the deformation zone) were selected. It has been shown that ECMAE provides the increase of modulus of elasticity E more than twice, tensile strength σ_T increases in four times. At the same time, strain at break ε_b is reduced by 1.5%. The value of the achieved effects depends on the accumulated deformation and the selected deformation route. The best set of physical and mechanical characteristics was observed in the case of Route E. According to SEM data, Route C results in partial pore healing and E provides total pore healing both in longitudinal and transversal direction. The observed effects are related to orientation order formation, increase of cristallinity degree and reduction of structure imperfection of extrudates. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012