Stress-induced Shifts Research Articles

Piezoreflectance spectra of 1s Z 12 exciton in CuBr

We have investigated the piezoreflectance spectra (Δ R/ R) of the 1s Z 12 exciton in single crystals of copper bromide CuBr at the temperature of 95 K with linearly polarized light. The spectra were successively studied with the pressure p along the [001] and [111] axis and the wave vector k of the incident light parallel to the [1 1 0] direction. The shear deformation potentials b and d of the 1s Z 12 exciton were deduced from the stress-induced shift and splitting. The results are compared with those obtained in other works.

Effect of surface treatment upon the pull-out behaviour of aramid fibres from epoxy resins

A detailed study has been undertaken of the pull-out behaviour of aramid fibres with different surface characteristics from blocks of an epoxy resin matrix. The fibres employed had either no surface treatment (HM), a standard surface finish (HMF) or had been treated with a special epoxy-based adhesion-activating finish (HMA). The point-to-point variation of axial fibre strain along the fibres both inside and outside of the resin matrix has been determined from stress-induced Raman band shifts. This has enabled the distribution of interfacial shear stress along the fibre/matrix interface to be determined and, in combination with scanning electron microscope analysis of the specimens following pull-out testing, the failure mechanisms to be elucidated. It is found that pull out of the HM fibre takes place by a debond propagating along the fibre/matrix interface at a low level of interfacial shear stress. The HMF fibre showed better adhesion to the epoxy matrix with pull out occurring in a complex manner through both separation of the fibre skin and failure at the fibre/finish interface. No evidence of debonding was found for the HMA fibre and failure occurred by fracture of the fibre at the point where it entered the resin block.

Analysis of SiC fibres and composites using raman microscopy

The application of Raman spectroscopy in the analysis of the microstructure of SCS-6 silicon carbide fibres using a Renishaw Raman microscope is described. It is demonstrated that the technique allows a detailed study to be made of the point-to-point variation in microstructure across a fibre section. It has been possible to monitor the variation of the concentration of SiC and carbon in the fibre microstructure and to detect differences in the forms of carbon present. It is also shown that Raman spectroscopy can be used to follow the micromechanics of both the deformation of silicon carbide fibres and of the fibres within a model composite. Well-defined Raman spectra have been obtained from a variety of Nicalon and Tyranno fibres and the positions of the Raman bands shown to shift on the application of stress or strain. From such stress-induced Raman band shifts, the point-to-point variation of axial fibre stress or strain along an individual fibre in an epoxy matrix can be determined. An example is given of the use of the technique to map the distributions of axial fibre strain in a Nicalon/epoxy fragmentation test specimen and to model the failure processes at the fibre/matrix interface.

Development and comparison of methods to estimate the catabolic versatility of metal-affected soil microbial communities

Three methods designed to estimate the catabolic versatility of metal-affected soil microbial communities were compared with each other and with a method described previously. All are based on the differential catabolism of 21 aromatic acids. In contrast to the original USIB-method which aims at analysing the utilisation of these substrates by isolated bacteria, the CFU-method allows to determine substrate-specific colony-forming units. The simpler SSC- and SOIL-methods were designed to measure substrate-specific degradation times in soil suspension cultures or in soil, respectively. While bacteria were exclusively multiplying in the neutral soil suspension cultures, oxidation in soil of the nonneutralised aromatic acids was predominantly due to fungal activities. Assuming a short degradation time is correlated with a high biomass possessing the corresponding catabolic potential and vice versa, relative inverse substrate-specific degradation times were calculated from the data obtained with the SSC- and SOIL-methods. They could be directly compared with the relative frequencies of the substrate-specific capabilities determined with the CFU- and USIB-methods. Although all methods were applied to the same soils the metal-induced shifts in substrate utilisation potentials were not very consistent. This insufficient agreement between method-specific data is explained by the fact that, with the USIB-method, only positive or negative reactions are recorded whereas with the CFU, SSC and SOIL methods, enzyme synthesis as well as growth rate and selection of organisms play unknown roles and mask the actual distribution of catabolic capabilities. A correct assessment of the distribution of the catabolic capabilities is, therefore, only possible with approaches similar to the USIB-method. In spite of these inconsistencies, the CFU-, SSC- and SOIL-methods are capable, as the original USIB-method, of indicating stress-induced shifts in the catabolic potentials of bacterial communities by eco-physiological indices, such as ‘catabolic versatility’. Though this index depends on the method applied it decreases with increasing metal contamination of soils. When comparing the control and the most contaminated soils, decreases in the catabolic versatility of about 40, 37, 30 and 24% were found for the USIB-, SSC-, CFU- and SOIL-methods, respectively.

Molecular deformation processes in aromatic high modulus polymer fibres

Structure–property relationships in a number of different aromatic high-modulus fibres have been examined in detail. Fibres studied include Kevlar, Twaron, Technora and PBO and it has been shown using Raman spectroscopy that these fibres exhibit similar molecular deformation processes. The stress-induced shift of the 1610cm−1 Raman band, assigned to a stretching mode of the p-phenylene ring, is found to be approximately −4.0cm−1/GPa for all of the fibres, independent of their chemical structures and morphologies. A similar rate of shift is found for the same band in poly(ethylene terephthalate) fibres. This implies that the molecular deformation processes in these different fibres are similar even though the fibres have different chemical structures and morphologies, and have been processed differently. It is shown that the mechanical behaviour of the fibres is consistent with deformation following a uniform stress series aggregate model.

Time-resolved surface differential diffraction study of Ag electrodeposition on Au(111) and the role of surface stress

An insitu X-ray diffraction investigation using surface differential diffraction (SDD) was carried out to study the mechanism and kinetics of Ag electrodeposition on Au(111). The effect of the surface stress on SDD measurements employing a thin film electrode has been investigated, which reveals that stress-induced peak shifts can account for some inconsistency in previous SDD results. Potential dynamic and potential step modes of deposition were used and the adlayer spacing of Ag on Au(111) was determined as a function of potential and time, respectively. A structural change in adlayer spacing during Ag electrodeposition on Au(111) was observed in both modes of deposition, which is consistent with a transition from initial adsorption in a mixture of bridge and atop sites to three-fold hollow sites.

Greatly suppressed stress-induced shift of GIDL in N 2O-based n-MOSFET's

Considerably suppressed gate-induced drain leakage (GIDL) shifts of N 2O-based n-MOSFET's after hot-carrier stress with different gate voltages are observed. The mechanisms involved are studied by dividing gate-oxide traps into sub-interface and bulk-oxide traps, and by means of computer simulations on electric-field distribution and carrier filling of these traps. It is demonstrated that sub-interface and bulk-oxide hole detrapping during stressing are mainly responsible for the respective GIDL shifts under two different stress conditions of V G=0.5 V D and V G= V D, with the effect of the former larger than the latter. In view of this, it is proposed that MOSFET's with N 2O-nitrided or especially N 2O-annealed NH 3-nitrided gate oxide have not only fewer pre-existing sub-interface and bulk-oxide hole traps, but also greatly suppressed generation of hole and neutral electron traps due to the formation of a nitrogen-rich layer near the SiO 2/Si interface through N 2O treatment.

Formation and Properties of Model Crystalline Blends Comprising Diacetylene-Containing Polyester and Polyolefin. Tensile Behavior and Deformation Micromechanics

Model blends comprising a cross-polymerized diacetylene-containing polyester (PE) and poly-[ethylene-co-(vinyl acetate)] (EVA) were prepared and their mechanical properties evaluated. The dependence of the Young's modulus of the blends upon composition has been modeled by using a variety of theories for the deformation of multiphase polymers. Deformation of the cross-polymerized polyester phase in the blends was monitored by using Raman spectroscopy. The polyester phases within the blends produce well-defined Raman spectra in which the C?C stretching bands undergo significant stress-induced band shifts during deformation of the blends. The results show that the Raman technique allows direct measurement of the stress in the polyester phase in a blend, independent of the overall deformation of the blend. Simultaneous deformation and Raman spectroscopy studies, therefore, give a unique insight into the deformation micromechanics of polymer blends.

Deformation processes in poly(ethylene terephthalate) fibers

The structure/property relationships in a series of melt-spun poly(ethylene terephthalate) (PET) fibers processed under different conditions have been investigated. The structure of the fibers was characterized using a variety of techniques, including gel permeation chromatography (GPC), differential scanning calorimetry (DSC), optical birefringence, and wide-angle x-ray scattering (WAXS). The mechanical properties of the PET fibers were also investigated, and the fibers showed different stress-strain and modulus- strain curves as a result of the different processing conditions employed. Raman spectroscopy was used to study molecular deformation processes in the PET fibers. It was found that most of the PET Raman bands shifted to a lower wavenumber when the fibers were subjected to axial tension, and the shift of the 1616 cm−1 band was examined in detail. It was found that there was a linear relationship between the position of the center of gravity of the band and stress and that the stress-induced band shift factor of −4].0 cm−1/GPa was identical for all PET fibers studied. Band broadening was also found under stress, and the Raman bandwidth (full width at half maximum, FWHM) provided information about the spread of molecular stresses resulting from the application of macroscopic tensile stress to the fibers. The molecular deformation processes in the PET fibers have been interpreted in terms of a modified series aggregate model in which the molecules are subjected to a constant average level of stress.

Crack bridging and fibre pull-out in polyethylene fibre reinforced epoxy resins

An investigation has been undertaken of the stress distributions in high-performance polyethylene fibres bridging cracks in model epoxy composites. The axial fibre stress has been determined from stress-induced Raman band shifts and the effect of fibre surface treatment has been followed using untreated and plasma-treated polyethylene fibres. It is found that when the specimen is cracked, the fibres do not break and stress is transmitted from the matrix to the fibre across the fibre/matrix interface. A debond propagates along the fibre/matrix interface accompanied by friction along the debonded interface. The axial stress distributions in the fibres can be analysed using a partial-debonding model based upon shear-lag theory and it is found that the maximum interfacial shear stress at the bond/debond transition is a function of the debond length. The debonding process has been modelled successfully in terms of the interfacial fracture energy-based criterion developed by Hsueh for the propagation of a debond along a fibre/matrix interface accompanied by constant friction along the interface.

Zeeman study of the orthorhombic FeIn pair center in silicon

The excitation spectrum of the neutral orthorhombic $({\mathrm{C}}_{2v})$ FeIn center in silicon has been studied by Fourier transmission infrared spectroscopy. The Zeeman results show that the electronic structure of the ground and excited state is similar. Both comprise two nearby Kramers doublets derived from a ${}^{4}{B}_{1}$ term split by spin-orbit interaction with a ${}^{4}{B}_{2}$ term. The crystal-field and spin-orbit parameters for the ground state are close to those previously determined for the orthorhombic FeIn center by electron-paramagnetic-resonance, which shows that the same center is studied with both techniques. Previous electron-paramagnetic-resonance results on the ground state have been well accounted for by considering the electronic structure of the ${\mathrm{Fe}}_{i}$ ion perturbed by the nearby inert In ion. Almost identical sets of electronic parameters were determined for the ground and excited state. The excitation lines do not split under uniaxial stress, which is most easily explained by equal stress-induced energy shifts for the initial and the final states. The Zeeman and stress results thus strongly indicate that the excitation lines are due to internal ``$d$'' transitions at the interstitial Fe ion.

Interleukin-1 and defensins in thermoregulation, stress, and immunity.

Our previous studies suggested that IL-1 and defensins, which play a critical role in mechanisms of host resistance, participated in the realization of stress reaction. The present report describes functional relations between IL-1 and defensins during stress reactions and the influence of defensins administration on thermoregulation and the IL-1 level in blood. It is shown that stress-induced shifts in the humoral immune response are accompanied by diverse changes in the IL-1 alpha level in blood and LAF production by peritoneal macrophages, which are possibly associated with the intensity and strength of applied stressors. Functional interrelations between actions of defensins and IL-1 during stress reaction and in the conditions of an LPS-induced rise in body temperature was demonstrated. These results suggest that antibacterial peptides defensins may be involved in thermoregulation in case of increased body temperature probably influencing the blood level of IL-1.

Stress-induced parametric shift in plastic packaged devices

In order to determine the shift in device characteristics due to stresses the die experiences during assembly, long channel NMOS and PMOS transistors (6.25/spl times/6.25 /spl mu/m) were encapsulated in plastic quad flat packages (PQFPs) and the shifts in device parameters were monitored. Shifts as much as -5.3% in drain current (I/sub dsat/) were recorded for the NMOS devices, and +1.64% for PMOS devices. These shifts indicate a biaxial compressive stress level of some 60-87 MPa after plastic packaging. Die attach alone (prior to plastic encapsulation) results in a biaxial tensile stress in the range 35-55 MPa. Different mold compounds can exert different stress levels and accompanying device parameter shifts. Prolonged high temperature storage, leading to resin shrinkage, also affects device characteristics. Short channel submicron devices, typical of what is used in products, are expected to show little shift. Monitoring device parameter shift is a simple technique and is well suited for quantifying the stresses the die experiences in a plastic package.

Impact of Social Confrontation on Rat CD4 T Cells Bearing Different CD45R Isoforms

The impact of social defeat on lymphocyte subpopulations and T helper subsets was investigated in Long Evans rats. CD4 T helper cell subsets with distinct functional properties and different cytokine profiles can be distinguished by using the mAbs OX-22 (anti-CD45RC) and OX-7 (anti-CD90, Thy1.1). Male intruders were exposed for 2, 6, or 48 h to aggressive resident pairs. All intruders were attacked upon introduction and were defeated as indicated by frequent display of full submissive postures. After 2 and 48 h of confrontation, drastic but differential effects on blood leukocyte numbers, CD4 and CD8a cells, and CD4 subsets were evident. However, after 6 h of confrontation most lymphocyte subset numbers corresponded to baseline levels. Focusing on CD4 subsets after 2 h of confrontation, we demonstrated that only the number of the CD45RC−CD90−subset declines, whereas neither the number of the CD45RC+CD90−subset nor the number of the CD45RC−CD90+subset (recent thymic emigrants) was influenced. Con A stimulation of sorted subsets identified the CD45RC−CD90−as a poor producer of IFN-γ. The data clearly demonstrate that social factors might differentially influence not only T cell subsets but also T helper cell subsets with distinct cytokine profiles in a possibly time-dependent manner. Such a stress-induced shift toward a CD45RC+CD90−-dominated milieu may have important consequences in interpreting results obtained from mitogenic stimulation of blood lymphocytes and cytokine production profiles measured after such a stimulation. In addition, a shift toward a CD45RC+CD90−dominance may modify the type and magnitude of immune response, at least temporarily.

Distinct effects of heat shock and ATP depletion on distribution and isoform patterns of human Hsp27 in endothelial cells

To study the cytoprotective capacity of Hsp27 under various cellular stresses, we compared the effects of heating and energy deprivation on its distribution and isoform composition. Cultured endothelial cells from human aorta or umbilical vein were subjected to heat shock (45°C) and ATP-depleting metabolic stress (CCCP or rotenone in a glucose-free medium). Both exposures led to the translocation of Hsp27 into the Triton X-100-insoluble cellular fraction, whereas the immunofluorescent Hsp27 pattern was characteristic for each stress employed. Heating (5–30 min) caused unexpected association of Hsp27 with thick bundles of actin microfilaments (stress fibers). ATP depletion within 30–120 min resulted in the appearance of Hsp27-containing compact granules in the nucleus. The insolubilization and relocalization of Hsp27 were reversible in both cases. The stress-induced shifts in the Hsp27 isoform spectrum indicate an increase in phosphorylation of Hsp27 in heat-shocked cells and its dephosphorylation in ATP-depleted cells. We suggest that these stresses diversely affect the phosphorylation status of endothelial Hsp27, thus altering its localization, supramolecular organization and functional activity toward actin.

Charge trapping and device behavior in ferroelectric memories

The electric field emanating from the surface of a poled ferroelectric (FE) can control the conduction properties of an overlying semiconducting (SC) film; this combination of materials can thus serve as a nondestructive readout (NDRO), nonvolatile memory device. We have characterized prototypes of these devices which utilize semiconducting In2O3 deposited on thin film lead zirconate titanate and bulk BaTiO3 FEs. The remanent state SC resistance in thin film FE NDRO devices is often opposite to that predicted from the known direction of FE polarization. In these cases charge injected from the SC film into the FE and trapped near the interface appears to control the electric field at the SC/FE interface. By contrast, the response of SC films on bulk FEs is largely controlled by just the FE remanent polarization. The measured SC resistance values in the ‘‘up’’ and ‘‘down’’ polarization states can be fairly accurately predicted by calculating the accumulation and depletion charge densities from the measured carrier concentrations, mobilities, and FE hysteresis behavior. We also observe a correlation between charge trapping in bulk and thin film of NDRO memory devices and the presence or absence of temperature-bias–stress-induced voltage shifts (imprint) of the FE hysteresis curves. We suggest that the presence of near-interfacial traps in the FE controls both imprint and NDRO memory response.

Dynamic Step-Scan Two-Dimensional Fourier Transform Infrared Studies of Uniaxially Drawn Poly(Ethylene Terephthalate) Film

Dynamic infrared spectra of uniaxially drawn poly(ethylene terephthalate) (PET) under a sinusoidal strain were examined. A very intense dynamic band at 973 cm−1 assigned to the trans C-O stretching mode indicated stress-induced high mobility around the C-O bond in the ethylene glycol units. It was supposed that derivative-like skeletal bands observed in the dynamic spectra originated from the stress-induced frequency shift. Two-dimensional correlation analyses of the dynamic spectra were also carried out and revealed that the phenyl ring 18a band at 1018 cm−1 and the phenyl ring 19b band at 1410 cm−1 were composed of three and two independent components, respectively. The correlation peaks between the phenyl ring and CH2 vibrational modes showed that orientation of the methylene group in the ethylene glycol unit, induced by mechanical stretching, is faster than that of the phenyl ring in the terephthalate unit.

Imprint in Ferroelectric Capacitors

We show that voltage offsets in the polarization-voltage characteristics of Pb(Zr, Ti)O3 capacitors can lead to imprint in ferroelectric memory devices. The thermal-induced voltage shifts (internal bias field) are in part attributed to the role of oxygen vacancy-related defect dipoles throughout the film. In support of this, it is found that donor doping at the Ti(Zr) sites reduces the thermally-induced voltage shifts. The stress-induced voltage shifts are found to be dependent on the Zr/Ti cation ratio. This compositional dependence is explained by considering the role of deep bulk Ti3+ centers and/or a compositional dependent oxygen vacancy density.

Stress Effect on the Reliability of pMOS TFTs for 16 Mb SRAM: DC Stress at Room and Elevated Temperatures

We present a systematic experimental study of the electrical stress effects in p-channel metal-oxide-semiconductor thin film transistors (pMOS TFTs) used as active loads in high density static random access memory (SRAM) circuits. Specifically, the effects of dc stresses occurring during standby were investigated in devices both with and without offset and at room and elevated substrate temperatures. The stresses associated with the OFF regime biases give rise to a drastic reduction of leakage current and at the same time, a significant increase in the ON current. These effects are particularly pronounced in devices without offset and for stress biases applied at room temperature or below. Although at elevated substrate temperature the leakage current itself is enhanced by a few orders of magnitude, the stress-induced parameter shifts are shown to be smaller than those at lower temperatures. Additionally, the stress induced reliability aspects of pMOS TFTs used for 4 Mb and 16 Mb SRAM are compared and discussed.

Deformation micromechanics in high-volume-fraction aramid/epoxy composites

It is shown that Raman spectroscopy is an excellent technique for analysis of the relationship between structure and deformation processes in both high-performance fibres and fibre-reinforced composites. It is demonstrated that Raman spectroscopy can be used to follow molecular deformation processes in Kevlar 49 aramid fibres through stress-induced band shifts. The shift of Raman bands in a high-volume-fraction unidirectional composite is analysed and it is found that there are significant variations in local fibre strain due to imperfections in the composite microstructure. Furthermore, it is shown that it is possible to map the local fibre strain distributions around a hole in the composite. The distributions of axial fibre strain have also been mapped in both a plain weave and a four-harness satin weave Kevlar 49/epoxy composite. Complex distributions of local fibre strain that follow the pattern of the repeat units of the woven structures are found. The power of the Raman technique to follow composite micromechanics is therefore demonstrated.