Decreasing Sample Thickness Research Articles

Slip traces caused by plastic deformation during recrystallization of thin metal sheets

During a study of mechanical properties of recrystallized B-free Ni3Al single crystals, regularly spaced parallel traces within individual grains were discovered on the surfaces of thin recrystallized sheets, see Fig. 1. They appeared to be slip traces, but since we could not find similar observations in the literature, a series of experiments was performed to identify them. We will refer to them “traces”, because they contain some, if not all, of the properties of slip traces. A variety of techniques, including the Electron Backscattering Pattern (EBSP) method, was used to ascertain the composition, geometry, and crystallography of these traces. The effect of sample thickness on their formation was also investigated.In summary, these traces on the surface of recrystallized Ni3Al have the following properties:1.The chemistry and crystallographic orientation of the traces are the same as the bulk. No oxides or other second phases were observed.2.The traces are not grooves caused by thermal etching at previous locations of grain boundaries.3.The traces form after recrystallization (because the starting Ni3Al is a single crystal).4.For thicknesses between 50 μm and 720 μm, the density of the traces increases as the sample thickness decreases. Only one set of “protrusion-like” traces is visible in a given grain on the thicker samples, but multiple sets of “cliff-like” traces are visible on the thinner ones (See Fig. 1 and Fig. 2).5.They are linear and parallel to the traces of {111} planes on the surface, see Fig. 3.6.Some of the traces terminate within the interior of the grains, and the rest of them either terminate at or are continuous across grain boundaries. The portion of latter increases with decreasing thickness.7.The grain size decreases with decreasing thickness, the decrease is more pronounced when the grain size is comparable with the thickness, Fig. 4.8.Traces also formed during the recrystallization of cold-rolled polycrystalline Cu thin sheets, Fig. 5.

Polymer degradation mechanisms: New approaches

In studying the thermal degradation of polymers, experimental problems such as that of taking the samples rapidly to chosen degradation temperatures can be solved by appropriate filament design and the use of sufficiently small samples. The pyrolysis-GC technique can then provide reliable quantitative data which can be used in the following ways to obtain mechanistic information about degradation processes. (I) Dependence of rate constants for volatiles evolution on the initial molecular weight of the sample, and on the pyrolysis temperature, can give specific information about the mechanisms of initiation and temination in chain degradation processes. (II) Direct comparisons of observed rate constants for volatiles evolution can indicate whether all observed products are primary products. (III) Anomalous dependence of product yields with decreasing sample thickness can give information about diffusion control of degradation rates. (IV) Anomalous dependence of product yields and ratios which increases with increasing sample thickness can distinguish primary, secondary, and even tertiary products, and can moreover lead to the interpretation of the overall pyrolysis mechanisms in terms of a reaction scheme involving parallel and consecutive reactions. All of the above are illustrated with examples from studies by the authors on a number of polymeric systems.

Separating the surface and volume plasmon energy loss intensity distributions in MgO and MgAl2O4

Broadening on the low energy side of the MgAl2O4 (spinel) volume plasmon, which increases with decreasing sample thickness (Fig. 1a), affected multiple least squares (MLS) fitting when spectra such as these were used in quantitative analysis. The inverse dependency of broadening on thickness is indicative of a surface effect. Potential artifacts from sample preparation (argon milling) were investigated. Spinel stock was ground in methanol on a grinding disk of 15μm-diameter diamond, and the tailings were placed onto a holey-carbon film. Electron-transparent regions of spinel that extended beyond holes in the film were used to obtain the energy loss single scattering distributions (SSD) shown in Fig. 1b. Parallel-detection electron energy loss spectroscopy (PEELS) was performed at 100 kV with a Philips EM400T/FEG analytical electron microscope. These SSD were obtained using the procedures described earlier, and show, relative to those in Fig. 1a, identical broadening on the left side of the plasmon with decreasing specimen thickness. This similarity indicates the broadening effect is not due to milling but to surface plasmon excitation.

Electrical breakdown and space-charge effect in polyphenylene sulfide films

Electrical breakdown of polyphenylene sulfide (PPS) films, one of the high-temperature engineering plastics, has been investigated. The results show that the dc breakdown strength is lower than the impulse one, and that the difference in electric strength between dc and impulse becomes wider as the sample thickness decreases. The result suggests space-charge accumulation when dc voltage is applied. Further, the impulse breakdown strength in the presence of a dc prestressing voltage is measured in order to elucidate the space-charge effect. The data on the contribution of the prestress voltage to electrical breakdown are used to estimate the amount of space charge accumulated in the specimen during the dc prestressing.

ポリフェニレンサルファイド(PPS)フィルムの絶縁破壊と空間電荷効果

Electrical breakdown of polyphenylene sulfide (PPS) films, one of high temperature engineering plastics, has been investigated. The results show that the DC breakdown strength is lower than the impulse one, and that the difference in electric strength between DC and impulse becomes wider as the sample thickness decreases. The result suggests space charge accumulation when DC voltage is applied. For further information, the impulse breakdown strength with DC prestressing voltage is measured in order to discuss the space charge effect. On the basis of the results of the prestress effect on the breakbown, an attempt is made to estimate the amount of space charge accumulated in the spacimen during the DC prestressing.

Finite-size effects and anisotropic melting of the vortex solid in high-temperature superconductors.

A mean-field model of anisotropic melting of the vortex solid in high-temperature superconductors is proposed. For a slab sample with dimensions ${\mathit{l}}_{\mathit{a}\mathit{b}}$\ensuremath{\gg}${\mathit{l}}_{\mathit{c}}$, where 2${\mathit{l}}_{\mathit{a}\mathit{b}}$ and 2${\mathit{l}}_{\mathit{c}}$ are the average diameter of the ab plane and the c axis thickness, respectively, large thermal fluctuations and finite-size effects may result in anisotropic two-dimensional melting at crossover temperatures ${\mathit{T}}_{\mathit{X}}$(H) below the three-dimensional-melting transition ${\mathit{T}}_{\mathit{M}}$(H). Thus a quasi-two-dimensionally ordered vortex-liquid phase may exist in ${\mathit{T}}_{\mathit{X}}$(H) T${\mathit{T}}_{\mathit{M}}$(H). Generally, ${\mathit{T}}_{\mathit{X}}$(H) decreases with the decreasing sample thickness, increasing magnetic field, and larger Ginzburg-Landau parameter \ensuremath{\kappa}(==\ensuremath{\lambda}/\ensuremath{\xi}). In the limit of 1/2${\mathit{H}}_{\mathit{c}2}$\ensuremath{\ll}H${\mathit{H}}_{\mathit{c}2}$, the geometric anisotropy plays a more important role in determining ${\mathit{T}}_{\mathit{X}}$(H) than the electronic-mass anisotropy.

Size effects and charge-density-wave pinning in NbSe3.

We report measurements of the impurity-concentration and crystal-size dependences of the threshold field ${E}_{T}$ for charge-density-wave (CDW) depinning in ${\mathrm{NbSe}}_{3}$. These measurements establish that CDW's in Ta-doped ${\mathrm{NbSe}}_{3}$ are weakly pinned. Size dependence of ${E}_{T}$ occurs when the transverse crystal dimensions become comparable to the transverse CDW phase-phase correlation length, resulting in a crossover from 3D to 2D weak pinning. A divergence of the T=4.2-K Ohmic resistance with decreasing sample thickness is attributed to diffuse scattering by crystal surfaces.

Continuous Two-Dimensional Melting of Vortex-Solid in High Temperature Superconductors

AbstractA model of continuous two-dimensional melting in the mixed state of high temperature superconductors is proposed. Two-dimensional melting sets in at a cross-over temperature Tx(H) below the three-dimensinal phase transition Tx(H) due to finite size effects, and Tx(H) is a function of the sample thickness (lc), applied magnetic field (H), and k(= λ/ξ) For a given zero-field transition temperature Tc0 and material properties, (such as defect density), the onset temperature of 2D-melting (Tx(H)) decreases with decreasing sample thickness and increasing magnetic field. In transport studies, thermally induced melting is further complicated by the depinning effect of high current densities.

Ionized‐Impurity‐Mediated Free‐Carrier Absorption in n‐GaAs Slabs

AbstractThe theory of impurity‐mediated free‐carrier absorption in n‐GaAs bulk material is extended to quasi‐two‐dimensional structures. The surfaces are treated within the classical infinite‐barrier model and the electron—electron interaction within the random‐phase approximation. Screening due to phonons is also considered. Using temperature‐dependent Green's functions and starting from the Kubo formula a general expression is derived for the free‐carrier absorption coefficient including both bulk and surface contributions. In thick n‐GaAs slabs coupled phonon—plasmon bulk excitations give rise to distinct peaks in the absorption spectrum. These structures are increasingly washed out with decreasing thickness. For comparison impurity‐mediated free‐carrier absorption spectra of non‐interacting electrons have also been calculated. At and below the bulk plasmon frequency the absorption is reduced due to screening. The absorption increases on the other hand with decreasing sample thickness.

Speed and effectiveness of windowless GaAs étalons as optical logic gates

The effectiveness of surface recombination in speeding up the relaxation of a GaAs étalon is reported. Various thickness (1.5, 0.5, 0.3, 0.13 μm) bulk GaAs windowless samples were fabricated and tested as optical logic gates. Times for complete recovery of transmission lie between 400 and 30 ps. The response shows a roughly linear increase in speed as the sample thickness decreases, consistent with surface recombination being the dominant relaxation mechanism. A very fast cycle time of 70 ps is demonstrated using a 0.3-μm-thick windowless GaAs étalon as an all-optical logic device. Proton-bombarded samples show slower recovery and poorer contrast, and they require more gating energy.

Conduction of Sodium and Nonbridging Oxygen Ions in Sodium Silicate Glass

From thermally-stimulated polarization/depolarization current studies of bulk glasses of Na2O–2SiO2 and Na2O–3SiO2, the following conclusions were made. First, the great reduction of dc conductivity down to a few hundredths with decreasing sample thickness is explained in terms of the presence of inherent microheterogeneity in the glass structure. Second, the peak which previously was assigned generally as due to localized motion of nonbridging oxygen ions in the alkali-depleted region near the anode is better assigned as due to conduction polarization of those ions. Then the activation energy for conduction of nonbridging oxygen ions is estimated to be roughly 6±2 kcal/mole more than that for alkali conduction. And third, because of the high conductivity of bulk samples, degradation of alkali conduction by space charges accumulating near the electrodes during polarization generates the peak at around 10-8–10-7 Ω-1 cm-1.

Influence of the surface anisotropy on the r.f. collapse effect

The influence of magnetic surface anisotropy on the fast relaxation of the hyperfine field forced by an r.f. field in invar (r.f. collapse effect) has been studied using the Mössbauer technique. The Mössbauer measurements were performed as a function of sample thickness (2.5–12 μm) and intensity (1–9 Oe) of the 50 MHz r.f. field applied. Due to the very high sensitivity of the r.f. collapse effect to the anisotropy field it was possible to detect the influence of “spin pinning” on the r.f. collapse effect. It is shown that a decrease of the sample thickness causes a decrease of the r.f. collapse effect at a given r.f. field frequency and intensity which is connected with the increase of the anisotropy field due to surface anisotropy. The dependence of the r.f. sidebands effect, which accompanies the r.f. collapse effect, on the sample thickness is discussed. The r.f. sidebands effect increases with decreasing sample thickness, which is in good agreement with the magnetostriction model of sidebands formation.

Sample thickness dependence of the exciton polariton absorption coefficient in Cu2O

The sample thickness dependence of the absorption coefficient of the yellow exciton 1s line of Cu2O was investigated experimentally using present state of the art crystals. The integrated absorption coefficient increases as the sample thickness decreases and also has an oscillatory component with period 0.053 cm. The experimental results are discussed in terms of the exciton-polariton formalism described by Pekar (1958) and Hopfield (1963).

Ionic conductivity of Li 5AlO 4 and Li 5GaO 4 in moist air environments: Potential humidity sensors

In moist air environments, the ionic conductivity of polycrystalline Li 5AlO 4 and Li 5GaO 4 at 500°C is found to increase 2–3 orders of magnitude in nearly an exponential manner with increasing moisture content (measured for vapor pressures up to ∼ 24 torr). The conductivity rate of response with changing humidity increases with decreasing sample thickness and appears to depend upon the surface-to-volume ratio. Results suggest that these materials may be uniquely suited as humidity sensors for high temperature environments.

Excitonic insulator transition in thin films

A gap Δ 2 in the elementary excitation spectrum of a thin semimetal film is calculated. The gap associated with electron-hole pairing is shown to be much larger in a two-dimensional system than in a three-dimensional one. It is noted that the gap decreases significantly if the film is placed on a metallic substrate. The possibility of anisotropic semimetal-insulator (or, at the triplet pairing, antiferromagnetic) transition when the sample thickness decreases is considered.

The effect of sample size on the steady state creep characteristics of Ni-6 pct W

The effects of sample size and grain morphology on the steady state creep properties of Ni-6 pct W in the temperature range of 0.55T m to 0.74T m have been studied. It is shown that a decrease in sample thickness results in a corresponding decrease in the number of grains per thickness and gives rise to a decrease in the measured values of bothQ creep andn when compared to existing data on thick samples with many grains per thickness. The observed effects of sample configuration on the creep properties are explained with a model for creep deformation which is based on the interaction of free surfaces with grain boundary sliding and grain deformation. Using this model, an expression for the stress and temperature dependence of the total steady state strain rate is obtained as a function of the grain matrix strain rate and the grain boundary sliding strain rate. The results of this model are shown to correlate well with the observed deformation characteristics of the thin samples and to explain the variations ofQ{creep} andn with sample morphology.

Field Effects in Extremely Thin Crystals of Bi8Te7S5

Field effect conductivity measurements have been performed on extremely thin crystals of Bi8Te7S5. Only the thinnest (5 atoms thick) crystals show an unusual behavior, which can be readily explained in terms of the Fermi sphere, which does not intersect quantized Fermi momentum levels at zero bias. It is shown that both the volume density of carriers and the spacing of quantized kF levels increase at the same rate with decreasing sample thickness; thus the limitation in the observability of quantized kF levels is in the areal carrier density σ that the dielectric material between the film and the gate will allow.

Influence of sample surface on thermomagnetic effects in Fe40-Ni60

It is shown that the effect of an induced anisotropy energy due to directional order in Fe40-Ni60 is strongly dependent on the sample-to-volume ratio. Torodial samples of various thickness (6,3,1,0.5 mm) were machined from the same ingot, chemically etched, and annealed at 1300°C in H 2 . All further annealings were performed at 450°C in N 2 in one of the following magnetic conditions: a) demagnetized state; b) under magnetic field; c) remanent state. After these annealings loops show a constricted shape (a), a closely square shape with small coercive field H c (b), or a square shape with large H c (c). However, the width of the constricted region after annealing a), and the magnitude of H c after b) or c), are found to increase for decreasing sample thickness; in general, changes of loop shape induced by a given annealing are markedly stronger on thinner specimens. These differences are attributed to some influence of sample surface on the thermomagnetic treatment. This conclusion was further checked by electrochemically etching off a sample layer (0.01-0.1 mm); as a consequence the shapes of the loops obtained by the previous annealing suffered changes which were strongly different depending on the original thickness of the sample. It is qualitatively observed that the induction of a directional anisotropy energy K u occurs according to the domain pattern present during annealing, and that surface patterns are certainly characterized by smaller spacings of 180° domains and larger densities of 90° closure domains than are bulk patterns. Therefore the effectiveness on the loop shape of a K u distribution in a surface layer is stronger than the one of a K u distribution induced in a bulk layer. By etching off a sample layer, the new surface domain pattern bears little correlation to the old K u distribution, and there is a memory loss of previous annealing results, which is more pronounced on samples with larger surface-to-volume ratios.

EFFECTS OF PROCESS VARIABLES ON RETENTION OF VOLATILES IN FREEZE‐DRYING

SUMMARY— Retention of organic volatiles in freeze‐dried solutions of mono‐, di‐ end polysaccharides was studied as a function of different process conditions. Under the experimental conditions used, slow freezing and decreasing sample thickness promoted retention. The effects of platen temperature and of concentration of solution components were more complex. Effects of process variables were explained on the basis of a mechanism which postulates formation of microregions due to association of carbohydrate molecules. These microregions become impermeable to organic compounds when their water content decrease below a critical level.

Mooney Viscosity Changes in Freshly Prepared Raw Natural Rubber

Abstract In the course of investigations carried out in connection with the technical classification of natural rubber, large permanent increases in Mooney viscosity of the order of 30 to 40 units have been observed when freshly prepared raw or masticated rubber is stored under conditions of low relative humidity. The rate of change and the extent of the increase are both affected by the relative humidity of storage, the effects being most marked in the region below about 30 per cent relative humidity. Rate of change in dry air increases with decreasing sample thickness and with increasing temperature, although the effects are masked somewhat by oxidative breakdown on prolonged storage at temperatures higher than about 30° C. Mooney values continue to increase long after the rubber has reached constant weight on drying; the removal of acetone soluble and nitrogenous non-rubber components and of oxygen does not prevent the changes taking place. The effects are only partially reversed by subsequent storage at high relative humidity, and there is evidence to suggest that the hardening is associated with the development of a cross-linked benzene-insoluble gel structure throughout the rubber. The rate of mill-breakdown of dry stored rubber is greater than for fresh rubber, but in comparisons using a fixed breakdown procedure, the hardened rubber still has a higher Mooney value after mastication. Rubbers hardened by dry air storage exhibit slightly improved vulcanized properties in a pure-gum mix even after the additional breakdown required to reach normal stock viscosity levels. Fluctuation of temperature and humidity in commercial smokehouses and drying sheds may well account for part of the variability of natural rubber as measured by Mooney viscosity determinations on the freshly prepared product, and different conditions of storage will add a further contribution to the total hardness variation experienced by the consumer.