K. D. Pae

Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2

A model to predict the densification and grain growth rates ofn-TiO2 during high pressure/low temperature sintering has been developed and validated by experiments. In this model, densification during intermediate stage sintering is based on a modified grain boundary creep process. For late stage sintering, a modified grain boundary diffusion model that governs atom migration from interparticle boundaries to pores is used. The rate equations for densification during both stages are affected by the increased driving force and decreased diffusivity induced by high pressure. During the sintering process, the concurrent grain growth is modeled through a pore-controlled pinning model, which indicates that grains grow little during the intermediate sintering stage, but experience rapid growth during the final stage.

Surface modification of polytetrafluoroethylene by Ar+ irradiation for improved adhesion to other materials

A surface of thin square polytetrafluoroethylene (PTFE) samples (1 × 1 × 0.2 cm3) was irradiated with Ar+ at 1 keV with varying ion dose from 5 × 1014 to 1 × 1017 ions/cm2 with and without an oxygen environment. The irradiated surface of the samples was examined by scanning electron microscopy (SEM) for surface textural changes and x-ray photoelectron spectrometry (XPS) for changes in chemical structure. A wettability test was conducted on the irradiated surface of PTFE samples by water droplets. A Scotch ™ tape adhesion test, after a thin film of Cu or Al was evaporated on the irradiated surface, and a tensile test after irradiated samples were glued to sample holders by an adhesive glue (Crystal Bond) was also run. The SEM micrographs showed increasing roughness with fiber forest-like texture with increasing ion dose. The Ar+ with an O2 environment produced finer and denser fiber forest-like texture than that without O2. The high-resolution XPS spectra showed decreased intensity of the F1s peak and formation of the O1s peak when irradiated with the O2 environment. The increase of the O1s peak may be attributed to the reaction of oxygen atoms and the free radicals created by Ar+ bombardment. The wettability of water droplets on the irradiated surfaces was found to be inversely proportional to the surface roughness. Adhesion tests were conducted on 2000 A thick Al or Cu film. Full detachment of the metal films was observed when PTFE samples were not modified. Partial detachment of the Al film occurred when PTFE was irradiated without the O2 environment, regardless of ion dose. No detachment of the film occurred when PTFE was irradiated with the O2 environment with the ion dose exceeding 1 × 1016 ions/cm2. Partial detachment of Cu film was observed with or without the O2 environment when the ion dose was 5 × 1014 ions/cm2. No detachment occurred with or without the O2 environment when the ion dose was 1 × 1015 ions/cm2 or greater. The tensile test showed that adhesion of an adhesive cement (Crystal Bond) to the irradiated PTFE samples increased significantly with increasing ion dose up to 1 × 1016 ions/cm2. Possible mechanisms for the improved adhesion are given.

High pressure and low temperature sintering of bulk nanocrystalline TiO2

Abstract Bulk n-TiO2 samples with a relative density as high as 95% and a grain size less than 50 nm were fabricated by hot-pressing at temperatures as low as 400 °C and at pressures up to 1.5 GPa. During hot-pressing, the anatase phase transformed to the rutile phase and the amount of transformation increased with sintering pressure. The grain size in both the anatase and the rutile phase increased with sintering pressure at a constant temperature but the grain size of the transformed phase is always smaller than that of the starting material. We believe that the smaller grain size of the rutile phase is related to multiple nucleation events in the anatase phase during sintering at very high pressure. The average grain size increased from 27 nm in the original powder to only 45 nm in the compact after hot-pressing. Analysis of the grain size and closed porosity by transmission electron microscopy suggested that closed pores at grain boundary triple junctions might also retard the grain boundary migration and thus prevent grain growth. A competing mechanism is also proposed in which the rate of grain growth is controlled by the pressure effect on the bulk diffusion rate and interface energy.

High pressure melting and crystallization of nylon-11

Differential thermal analysis (DTA), high pressure differential thermal analysis (HP-DTA), and high temperature X-ray studies are combined to elucidate the origin of the two melting peaks in Nylon-11. The results of the studies suggest that two species of crystals are involved in the melting of Nylon-11 for samples crystallized at atmospheric pressure or when the environmental pressure is below 4 kbar. At atmospheric pressure, the high melting species is predominant. However, under hydrostatic pressures, the high melting species undergoes phase transition to the low melting species before melting. The amount of the material involved in the transition depends on the pressure. At pressures of 4 kbar or greater, the entire high melting species transforms to the low melting species. The melting behaviour, at atmospheric pressure, of samples crystallized at high pressures also shows two melting peaks if the crystallization pressure is below 4 kbar. The amount of the low melting species increases with increasing pressure and, at 4 kbar or higher, only melting of the low melting species is observed. The X-ray photographs taken at room temperature suggest that samples crystallized between atmospheric pressure and 3 kbar contain both theα-form and theδ′-form crystals but the samples crystallized at 4 kbar and higher contain only theα-form crystal. However, it appears from X-ray scans taken at high temperatures near melting that the low melting species is of theδ-form and the high melting species of the δ′-form crystals for samples crystallized below 4 kbar. Theδ-form crystals result from theα-δ transition that occurs at 95° C. Moreover, the melting at high pressures (<4 kbar) of samples crystallized at atmospheric pressure also appears to involve aδ′-δ transition. These results suggest that both the crystal forms,δ andδ′, are stable at high temperatures, if the environmental pressure is below 4 kbar, and that only theδ-form crystals are stable up to melting at pressures greater than 4 kbar.

Influence of hydrostatic pressure on the mechanical behavior and properties of unidirectional, laminated, graphite-fiber/epoxy-matrix thick composites

Abstract This paper reviews and gives new insight into earlier work by the author and his co-workers on the experimental investigation of the influence of superimposed hydrostatic pressure on the mechanical behavior and properties of the epoxy used for the matrix and unidirectionally laminated, graphite-fiber/ epoxy-matrix thick composites. The direction of the fibers was, respectively, 0°, 45° and 90° for the compressive test samples and 0°, 45° -45° and 90° for the shear samples. Hydrostatic pressure induces very significant, often dramatic changes in the compressive and shear stress/ strain behavior of composites, and consequently in the elastic, yielding, deformation and fracture properties. The range of pressures covered for the compressive experiments was 1 bar to 4 kbar, and for the shear tests 1 bar to 6 kbar. The shear modulus (G) of the epoxy increased bilinearly with pressure, with the break, or the discontinuity point, occurring at ∼2 kbar. The compressive elastic modulus (E) and the shear modulus (G) of the composites increase in the same manner as for the epoxy. The break, which is located at ∼2 kbar, represents a pressure at which physical changes in the molecular motion of the matrix epoxy occur. That is, segmental motion of molecules between the cross-links is frozen in by 2 kbar pressure. This pressure is known as the secondary glass transition pressure of the epoxy at room temperature. Alternatively, the sub-zero secondary glass transition temperature of the epoxy is shifted to ambient temperature by 2 kbar pressure. The increase in the moduli may also be given a mechanical interpretation. The elastic or shear modulus of an isotropic, elastic material due to small compressive or shear deformations, respectively, superimposed on a finite volume deformation, which is caused by hydrostatic pressure, increases with pressure. Such an increase in E or G has been predicted using finite deformation theory of elasticity. The normally brittle epoxy develops yielding when the superimposed hydrostatic pressure exceeds 2 kbar. The shear yield stress (1% off-set) of the epoxy increases linearly with pressure above 2 kbar. This kind of yielding behavior can be predicted by a pressure-dependent yield criterion. The compressive yield strength of the 45° and 90° composites increases bilinearly with pressure, and the shear yield strength of the 0°, 45° and 90° composites also increases bilinearly with pressure. This bilinear behavior is also due to the secondary glass transition pressure of the matrix epoxy, being located at 2 kbar. The fracture strength of the composites also increases with pressure linearly and the greatest increase occurs in the 45° composite in compression and in the −45° composite in shear. The fracture modes of the composites undergo changes with increasing hydrostatic pressure. For instance, the 0° composite undergoes a brittle-ductile transition under shear stress, while no such transition appears to set in under compressive stress. The fracture mode of the 45° composite changes from matrix failure at lower pressures to fiber failure at high pressures under shear stress.

The effect of high pressure on phase transformation of nanocrystalline TiO2 during hot-pressing

Bulk n-TiO2 samples with a relative density as high as 95% have been fabricated by means of hot-pressing at temperatures as low as 400 °C and at pressures up to 1.5 GPa. During hot-pressing, the anatase phase transformed to the rutile phase and the amount of the transformation increased with sintering pressure. We believe that the smaller grain size of the rutile phase compared with that of the pre-transformed anatase phase is related to multiple nucleation events in the anatase phase during sintering at very high pressure.

Ferroelectric hysteresis effects in poly(vinylidene fluoride) films

Investigations of the piezoelectric response of biaxially oriented poly(vinylidene fluoride) films subjected to a series of static positive and negative poling fields produces a hysteresis curve for d31 and e31 showing the ferroelectric character of phase IV. Examination of changes in the crystalline regions, using differential scanning calorimetry and x‐ray methods, of films cycled (statically) through large positive and negative poling fields provides insight into the switching mechanisms of both phase I and phase IV. The experimental results show a cyclic broadening and narrowing of the endothermic melting peaks as the film polarization is switched. Cyclic changes in the intensities of the phase I (110) (200) and phase II (or phase IV) (100), (020), and (110) reflections are also observed.

The pressure dependence of the pyroelectric response of poly (vinylidene fluoride) films

The pressure dependence of the pyroelectric coefficient Py was determined from atmospheric pressure to 7 kbar over a temperature range from −80 to +48 °C for poled uniaxially oriented poly (vinylidene fluoride) film. The effect of the glass transition on Py as a function of pressure is compared to the pressure dependence of the glass‐transition temperature obtained from dielectric studies. The ratio of dp, the hydrostatic pressure piezoelectric coefficient, to Py is presented as a function of both temperature and pressure. The details of a high‐pressure system design for these measurements are also presented.

Viscoplastic Behavior of a Glass at High Pressures.

The glass transition pressure Pg for a polyurethane elastomer (Solithane 113, 50/50 resin‐catalist ratio, manufactured by Thiokol Chemical Co.) is located at 2.5 kbar at room temperature and the glass transition temperature Tg is at −20 °C. Mechanical behavior of the elastomer, namely the tensile and the compressive stress‐strain behavior, in the glassy state as well as in the rubbery state has been determined. The Young’s modulus increases from ∼107 dyn/cm2 in the rubbery state to ∼1010 dyn/cm2 in the glassy state. The tensile fracture strain increases rapidly from 60% at atmospheric pressure to greater than ∼200% at 1 kbar and higher. In the glassy state, the samples exhibit yielding, yield drop, and cold drawing. The yield drop is not accompanied by necking. Rather the samples undergo uniform drawing throughout the entire gage length. A series of sequential loading, unloading, and reloading tests in the plastic range was also conducted in the glassy state. It was observed that the plastic strain recove...

Pressure‐volume‐temperature studies of a polyurethane elastomer

Experimental pressure‐volume‐temperature data for a polyurethane elastomer above Tg are fitted to equation of state theories. The cell and hole theories, as applied to polymer liquids, both represent the data fairly well for appropriate choices of P*, V*, and T*; the characteristic pressure, volume, and temperature scaling parameters, respectively. P*, V*, and T* are found to obey a semiempirical equation which holds for a large number of polymers, and, by fitting the data to the hole theory it is found that the hole fraction, 1−y, can be used as an ordering parameter to describe the pressure dependence of Tg.