Temperature-induced Degradation Research Articles

Chemorheological response of elastomers at elevated temperatures: Experiments and simulations

An experimental study and a method for simulating the constitutive response of elastomers at temperatures in the chemorheological range (90–150 °C for natural rubber) are presented. A comprehensive set of uniaxial experiments for a variety of prescribed temperature histories is performed on natural rubber specimens that exhibit finite elasticity, entropic stiffening with temperature, viscoelasticity, scission, and oxygen diffusion/reaction effects. The simulation approach is based on a multi-network framework for finite elasticity, isothermal incompressibility, thermal expansion, and temperature-induced degradation. The model extends previous work to account for kinetics of scission for arbitrary time-varying temperature histories and incorporates the effects of viscoelastic relaxation and diffusion-limited oxidative scission. The model is calibrated to experiments performed on a commercially-available filled natural rubber material, and numerical simulations are compared favorably to experiments for a variety of temperature histories.

P-73: The Development of Natural Graphite-Based Heat Spreaders for Reducing Temperature and Increasing Temperature Uniformity in Flat Panel Displays

Screen temperature reduction and improved screen temperature uniformity are ongoing objectives for flat panel display manufacturers as they seek to improve image quality, increase brightness and luminous efficiency, and extend screen life. This paper describes the development of new natural graphite-based heat spreaders for both plasma and liquid crystal displays. Reductions in peak hot-spot temperatures of up to 9 °C have been demonstrated in commercial flat panel displays, significantly reducing the potential for temperature-induced performance degradation mechanisms in all types of display.

Photo-CVD SiO2 Layers on AlGaN/GaN/AlGaN MOS-HFETs

High-performance AlGaN/GaN/AlGaN double heterojunction metal oxide semiconductor (MOS)-heterojunction field effect transistors (HFETs) with high-quality layers deposited by photochemical vapor deposition (photo-CVD) technology were fabricated and reported. With a 1 μm gate, the maximum saturated maximum and gate voltage swing of the photo-CVD fabricated MOS-HFETs were 755 mA/mm, 95 mS/mm, and 8 V, respectively. Even at a high temperature of 300°C, the maximum saturated and maximum of the fabricated MOS-HFETs were still kept at 323 mA/mm and 41 mS/mm, respectively. Moreover, the simulation results clearly suggest that the temperature-induced degradation of saturation velocity is predominantly responsible for the degradation mechanisms of and © 2004 The Electrochemical Society. All rights reserved.

Injection locking of VCSELs

Injection locking has been actively researched for its possibility to improve laser performance for both digital and analog applications. When a modulated follower laser (also termed slave laser) is locked to the master laser, its nonlinear distortion and frequency chirp may be reduced. As well, the resonance frequency can increase to several times higher than its free running case. In this paper, we show that the frequency response (S21) of an injection-locked laser is similar to a parasitic-limited laser with a high resonance frequency. The S21 was studied experimentally and the condition to achieve a flat, enhanced frequency response was identified. For analog applications, a record 112 dB-Hz/sup 2/3/, single-tone third harmonic spur-free dynamic range of a 1.55-/spl mu/m vertical cavity surface emitting lasers (VCSEL) was demonstrated. An improvement was attained for a wide-injection parameter space. In a 50-km 2.5-Gb/s digital link, a 2-dB power penalty reduction at 10/sup -9/ bit error rate was also demonstrated. As a novel application, an injection-locked uncooled tunable VCSEL was shown to have a reasonable modulation performance in a wide abient temperature range. The VCSEL was locked to a designated wavelength and the injection compensated the temperature-induced performance degradation. This concept can be extremely attractive for low-cost dense wavelength division muliplexed transmitters.

On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*β subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investigations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*β phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*α, SmC*β, SmC*γ and SmC*a. We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA* -SmC*a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*β and SmC*γ have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved.

Poly(acrylonitrile-methyl methacrylate) as a non-fluorinated binder for the graphite anode of Li-ion batteries

Poly(vinylidene fluoride) (PVDF) has been a choice of the binder for both cathode and graphite anode in stateof-the-art Li-ion batteries [1, 2]. However, there is currently a demand to replace PVDF with non-fluorinated binder since at elevated temperatures the fluorinated polymers react with lithiated graphite (LixC6) and metal lithium to form more stable LiF and >C@CF– double bonds. In particular, the reaction of PVDF and metal lithium produces an enthalpy as high as 7180 J (g PVDF) [1]. It has been reported that in the presence of electrolytes, PVDF and lithiated graphite undergo a series of exothermic reactions, including (i) temperature-induced degradation of the solid electrolyte interface (SEI) at 120–140 C, (ii) reactions of lithiated graphite and electrolyte at 210–230 C, and (iii) dehydrofluorination of PVDF initiated by LixC6 at >260 C [1, 2]. Among these reactions, the last one is known to be very exothermic and is believed to be a potential source for the thermal runaway of Li-ion batteries under abuse conditions. Therefore, safety concerns with Li-ion batteries may arise from the use of PVDF in the graphite anode. To replace the rather reactive PVDF, we attempt to evaluate poly(acrylonitrile-methyl methacrylate) (AMMA) as a non-fluorinated binder for the graphite anode of Li-ion batteries. Differential scanning calorimetry (DSC) study has shown that the heat of reaction of the lithiated graphite and electrolyte can be reduced significantly by using AMMA instead of PVDF [3]. That is, total enthalpies for the reactions (as indicated by two exothermic peaks in DSC curves) of the fully lithiated graphite (MAG-10, Hitachi Chemical) and electrolyte (1.2 M LiPF6 3:7 EC/EMC) in the temperature range 280–340 C can be reduced to 1211 J (g AMMA) from 2699 J (g PVDF). In this paper, we will electrochemically evaluate AMMA as a binder of the graphite anode of Li-ion batteries. 2. Experimental details

L-band DFB laser diodes with output power of 106 mW at 1625 nm

For the first time, distributed-feedback lasers emitting at 1625 nm with high output power and low operation current are fabricated. Temperature-induced performance degradation is minimised by optimising carrier and optical confinement. Consequently, these L-band lasers show no additional degradation in threshold current with rising temperature compared to C-band lasers.

Anharmonicity and fragility in semi-interpenetrating polymer networks

Two series of semi-interpenetrating polymer networks (semi-IPN) based on the same linear polyurethane and two different heterocyclic polymer networks were characterized in terms of the complex dynamic modulus and the mechanical loss tangent, measured between 150 and 500 K at a frequency varying in the range between 0.3 and 30 Hz. The fragility and the deviations from exponentiality of the dynamics of the segmental motion of semi-IPNs in the glass transition interval have been obtained by modelling the mechanical a-relaxation. It was observed that the incorporation of linear polyurethane softens the heterocyclic polymer networks, lowers their resistance to the temperature-induced structural degradation, and increases the complexity of the long-range segmental dynamics. There has also been found a correlation between the fragility and the anharmonicity of the system, the most fragile polymer being characterized by the largest degree of anharmonicity. By extending the analysis to linear polymers, it was established that a growing anharmonicity is predictive of an increasing fragility.

Moisture and Temperature Induced Degradation in Tensile Properties of Kevlar-Graphite/Epoxy Hybrid Composites

This paper presents results of a comparative study of the effects of mois ture and temperature on tensile properties of Kevlar 49/epoxy, graphite/epoxy, single fiber and Kevlar 49-graphite/epoxy hybrid composites. The work reported here shows varia tions in tensile strength, elastic modulus and Poisson's ratio with changes in the hygrother mal conditions. A temperature range of 23°C to 150°C was considered for both dry and wet specimens exposed to moisture for 200 days. Moisture was introduced into the speci men by immersion in distilled water. Effects of the kind of outer fiber of the same hybrid system was also studied for better understanding of the behavior of hybrid composites. Variation in the tensile modulus determined by various test methods is also reported. The results indicate that moisture and temperature degrade the tensile strength of hybrid and single fiber composites. The tensile modulus of Kevlar 49-graphite/epoxy is compara tively higher than that of Kevlar 49/epoxy. The degradation of the tensile modulus is mostly temperature dependent. The presence of moisture either increases or decreases the tensile modulus depending upon the laminate and exposure conditions. Poisson's ratio in general appears to increase with temperature. In the presence of moisture the Poisson's ratio further increases between the temperatures of 45°C to 150°C.

Temperature dependence of light-induced degradation in a-Si solar cells

Stability tests were conducted on amorphous silicon solar cells with a glass/indium-tin-oxide/(p-i-n)a-Si:H/metal structure and on hydrogenated amorphous silicon (a-Si:H) film under various combinations of light and ambient temperature. The degradation of characteristics was dependent on temperature and was governed by the extent of the temperature rise. Results of the tests showed that light-induced degradation and recovery from temperature-induced degradation progress together as time passes. It had been expected that no light-induced degradation would occur if cells were kept at 130–140 °C for long periods of time, and, in order to investigate these phenomena, the changes in diode characteristics and the photo-current excited by low energy light in solar cells were measured.

Inhibition of poliovirus RNA synthesis by supraoptimal temperatures.

Summary In HeLa cells infected with poliovirus, shifts from optimal to supraoptimal temperatures in the middle of the infectious cycle resulted in extensive degradation of newly made virus RNA. This temperature-induced degradation was not observed until about 3 hr after infection. It did not require simultaneous synthesis of proteins. During the first 3 hr of infection at 39·5°, RNA of temperature-resistant poliovirus strains replicated much faster than at 36°. Subsequently, degradation became predominant. Thus, temperature-resistant strains partially escaped the effects of degradation by faster replication. During the first 150 min. approximately, the RNA of temperature-sensitive strains replicated as well at supraoptimal as at optimal temperatures. Subsequently, newly made virus RNA was degraded. The inhibitory effect of supraoptimal temperatures may be due to release, or activation, of a ribonuclease.