Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Using broad band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T(g) range. The alpha relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the relaxation. At high temperatures, beta remains nearly insensitive to temperature changes, whereas in the vicinity of T(g) a nearly linear increasing of beta with temperature is found. The temperature range where the change of the beta(T) behavior occurs is located for all the polymers investigated around 1.2T(g). Moreover, our results indicate a common value of beta approximately equal to 1/3 at the temperature where the relaxation time diverges. The beta(T) behavior near T(g) is discussed in terms of a "rugged landscape" phase space which allows us to rationalize both the beta(T) behavior observed as well as the similarities of our findings near T(g) with the results reported in simulations on Ising spin glasses and other model systems.     

Thermally tuned optical fiber for true time delay generation

A new technique for generating a continuous range of true time delay values is introduced. Heating optical fiber in order to change the effective index of the guided mode produces time delays. A 45-m section of single-mode silica fiber is demonstrated to produce a continuous range of time delay values from 0 to 211 ps over a temperature tuning range of 50°C (30–80°C). A thermal time delay factor is introduced and found to be 0.096 ps/m°C for Corning LEAF fiber. A 7.66-m section of multimode Lucina polymer fiber is demonstrated to produce a range of time delay values from 0 to 32 ps over a temperature tuning range of 30°C (30–60°C). The thermal time delay factor for this fiber is −0.1427 ps/m°C.     

High sensitivity long-period grating-based temperature monitoring using a wide wavelength range to 2.2 μm

Temperature effects on the various cladding modes of a long-period grating (LPG) fabricated in B-Ge co-doped fibre have been investigated to create a high sensitivity measurement device. The temperature sensitivities of the attenuation bands of the LPG over the wavelength region 1.2-2.2 μm, for a grating with a 330 μm period, were obtained by monitoring the wavelength shift of each attenuation band, with a temperature increment of 20 °C, over the range from 23 °C to 140 °C. The attenuation band appearing over the 1.8-2.0 μm wavelength range has shown a nearly five times higher temperature sensitivity than that of lower order modes, and thus it shows significant promise for fibre optic temperature sensor applications.     

Simultaneous strain and temperature measurement with long-period gratings

The differential modulation of the attenuation bands in a long-period grating is used for simultaneous sensing of axial strain and temperature. A grating fabricated in a conventional optical fiber is demonstrated for concurrent measurements of strain over a range of 2100 micro? and temperature over a range of 125 degrees C, with maximum errors of 58 micro? and 1 degrees C, respectively.     

Thermoluminescence characteristics of hydrogenated amorphous zirconia

This paper reports the experimental results concerning the thermoluminescent (TL) characteristics of hydrogenated amorphous zirconium oxide (a-Zr:H) powder prepared by the sol–gel method. The advantages of this method are the homogeneity and the purity of the gels associated with a relatively low sintering temperature. Hydrogenated amorphous powder was characterized by thermal analysis and X-ray diffraction. The main TL characteristics investigated were the TL response as a function of the absorbed dose, the reproducibility of the TL readings and the fading. The undoped a-Zr:H powder presents a TL glow curve with two peaks centered at 150 and 260 °C, respectively, after beta irradiation. The TL response a-Zr:H as a function of the absorbed dose showed a linear behavior over a wide range. The results presented open the possibility to use this material as a good TL dosimeter.     

Effect of Temperature on Cononsolvency of Polyvinylpyrrolidone in Water/Methanol Mixture

The behavior of polyvinylpyrrolidone in mixed water/methanol solvents was studied by rheoviscosimetry over a temperature range of 20°C–40°C. For the lower temperatures of this range, the intrinsic viscosity variation of the polymer vs. methanol molar fraction shows structural transitions (coil–globule–coil). This transition, which is usually attributed to the cononsolvency phenomenon, agrees with our previously published results obtained by dynamic light scattering. For higher temperatures, near 40°C, the intrinsic viscosity increase shows an expansion of the polymer over the alcohol molar fraction range 0.2 < X _A < 0.5. This last result can be attributed to the water/alcohol complex destruction under temperature increase. The “excess viscosity” of the polymer-mixed solvents vanishes with increasing temperature and becomes positive at 40°C. So, the polymer chain tends to transit from a globular to an ideal chain in the middle composition range of the mixed solvents.     

Variable-temperature micromagnetic study of epitaxially grown MnAs films on GaAs(001)

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs layers on GaAs(001). In MnAs, a structural and magnetic phase transition occurs at T_c40 °C from the hexagonal, ferromagnetic -phase below T_c to the orthorhombic, paramagnetic -phase above T_c. In the investigated MnAs-GaAs system, both phases coexist over a temperature range of 30 °C below T_c due to the involved strain. Using MFM we are able to distinguish between the ferromagnetic and the paramagnetic phases by measuring topographic and magnetic contrast of the same sample area. For VT-MFM studies, we have employed a temperature stage that allows heating and cooling in a controlled atmosphere with small thermal drift (for this system, the temperature was varied from below 0 °C to above 45 °C). The ratio of the ferromagnetic to the paramagnetic phases shows a temperature hysteresis, i.e. the ratio is dependent on whether the sample was heated or cooled to reach the measurement temperature. Detailed studies of the domains and their arrangements over the hysteretic temperature cycle are shown and compared. Mainly three different domain types are found that are dominant in their respective temperature ranges. PACS 68.37.Rt; 68.35.Rh; 75.70.-i     

Experimental investigation of thermo-optic effects in SiC and Si photonic crystal nanocavities

We experimentally investigate and compare the thermo-optic effects of silicon carbide (SiC) and silicon (Si) photonic crystal nanocavities on their resonant wavelengths over a temperature range of 25?°C to nearly 200?°C by using a laser source with a wavelength close to the telecommunication wavelength range of 1550?nm. The measured results clearly show that the shift in the resonant wavelength of the SiC cavity is significantly (by a factor of 3) less than that of the Si cavity for the same ambient temperature changes. In addition, the measured results provide direct estimates of the thermo-optic coefficients (dn/dT) for thin SiC and Si as 3.87×10(-5)/°C and 1.60×10(-4)/°C, respectively, for this temperature range.     

Electrical properties of CoO,NiO, CuO and ZnO doped beta″-alumina

The measurements of ionic conductivity of sintered beta″-alumina samples doped with CoO, NiO, CuO and ZnO were caried out. It was found that conductivities of these samples are lower than conductivity of Li₂O stabilized beta″-alumina. For CoO, NiO, CuO as well as Li₂O stabilized beta″-alumina the bending of Arrhenius plots was observed. For samples doped with ZnO the plots were linear in whole 20°C–450°C temperature range. The doping effect on bulk conductivity was stronger than on grain boundary conductivity.     

Specific Heat of Rhombohedral Polymeric C60 in Temperature Range 300--2K

Polymerization of C60 is realized under high temperature and high pressure. X-ray diffraction reveals a rhombohedral lattice structure in the product, and solid-state ^13C nuclear magnetic resonance spectroscopy confirms the formation of sp^3 bonds between C60 molecules. Specific heat is then measured over the temperature range of 300-2 K. It is found that its specific heat values are significantly less than those in fullerite within the region of 80-2K, and this huge reduction is attributed to the suppression of intermolecular librational modes in polymerized C60. An excellent fit to the experimental data over the entire temperature range is provided by a model, which needs to include only three-dimensional and two-dimensional translational modes in various contributions at different temperatures.     

Hyperthermia induces T1 relaxation and blood flow changes in tumors. A MRI thermometry study in vivo

Regional hyperthermia in combination with chemotherapy and/or radiotherapy has proven to be an effective treatment concept for locally advanced deep-seated tumors. Simultaneous MR-imaging could be a promising approach for therapy optimization. Purpose of this study was the in vivo investigation of temperature induced longitudinal relaxation time (T(1)) and blood flow changes in a tumor model. Using a 1.5 Tesla MR system, the T(1) sensitivity on temperature and the onset of tissue changes at temperatures >44 degrees C were investigated in muscle samples. Also, fourteen Syrian Golden Hamsters with amelanotic melanoma A-MEL-3 were examined during heating of the tumors. Temperature induced blood flow and T(1) changes were determined continuously during hyperthermia. Changes of T(1) correlated linearly with temperature over a wide range (27-44 degrees C) in the tissue sample. Tissue changes became notable above 44 degrees C. In the tumor model, relative changes of T(1) (unlike blood flow) showed linear correlation with temperature over the entire range of hyperthermia relevant temperatures (32-44 degrees C). For a low thermal dose, T(1) allows the assessment of temperature changes in tumors in vivo. At higher thermal doses, in addition to temperature changes, T(1) also shows tissue changes. Both temperature and tissue changes supply important information for hyperthermia.     

Current-controlled curvature of coated micromirrors

Precise control of micromirror curvature is critical in many optical microsystems. Micromirrors with current-controlled curvature are demonstrated. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2 mm over a current-induced temperature range from 22 degrees to 72 degrees C. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2/Y2O3 lambda/4 pairs) mirror was tuned from -0.68 to -0.64 mm over a current-induced temperature range from 22 to 84 degrees C. These results should be readily extendable to mirror flattening or real-time adaptive shape control.     

Monocrystalline ZnSe as an ionising radiation detector operated over a wide temperature range

This work presents an analysis of the main requirements for semiconductor detectors of ionising radiation that can be operated over a wide temperature range. The analysis shows that wide-gap semiconductors with a band gap greater than 2.0 eV are a better option for effective detection of ionising radiation at high temperatures. The results of an experimental investigation into the luminescent, electrical and spectrometric properties of the wide-gap semiconductor ZnSe are shown as an example. Undoped monocrystalline ZnSe has an extremely low leakage current over a wide range of temperatures up to 167 °C and can be used as a radiometric X-ray detector in pulse-counting mode over a wide temperature range up to at least 130 °C.     

Demonstration of high extinction ratio modal interference in a two-mode fiber and its applications for all-fiber comb filter and high-temperature sensor

We show that high extinction ratio (>20 dB) modal interference in a two-mode dispersion compensating fiber can be utilized to build a compact, easy-to-fabricate tunable all-fiber optical comb filter. Wavelength tunability over the full free spectral range of the comb filter is demonstrated with an electrical power of 115mW using an on-fiber thin film micro-heater deposited directly on the fiber. In another configuration, the comb filter is used as a temperature sensor with dynamic range of >300 °C and sensitivity of <0.1 °C. The temperature sensor is capable of measuring a temperature as high as 500 °C.     

Temperature dependence of the 1H chemical shift of tetramethylsilane in chloroform, methanol, and dimethylsulfoxide

The chemical shift of tetramethylsilane (TMS) is usually taken to be zero. However, it does vary slightly with temperature, having obvious implications for studies of temperature effects on chemical shifts. In this work, we measure the variation in the chemical shift of TMS with temperature in three solvents, CDCl3, CD3OD, and DMSO-d6, relative to the resonant frequency of 3He gas, which can be reasonably assumed to be temperature independent. In all three solvents, the average temperature coefficient over a wide temperature range is about -6 x 10(-4) ppm/degrees C, a factor of five smaller than that previously reported in the literature. Data are included for 3He resonance frequencies over a temperature range of -110 to +180 degrees C, along with new measurements of volume magnetic susceptibilities of the three solvents and estimates of their temperature dependence. A novel method is used to provide temperature measurement via 2H resonances of methanol and ethylene glycol samples, which can concurrently be used for field/frequency locking.     

Temperature dependence of luminescence time-resolved spectra from quartz

Pulsed optical stimulation of luminescence has been used to study the thermal dependence of luminescence lifetimes in quartz over the temperature range 20–200°C. Time-resolved spectra for lifetime analysis were recorded from samples of quartz over a dynamic range of 64 μs following stimulation of luminescence by pulsed 525 nm green light emitting diodes (LEDs) using an 11 μs pulse and 12% duty cycle. It has been demonstrated that an increase in measurement temperature generally leads to a decrease in lifetimes from about 30 μs at 20°C to about 7 μs at 200°C. The form of the decrease is influenced by the initial optical or thermal pre-treatment of samples.     

Temperature measurements using fiber optic polarization interferometer

A novel measurement method of temperature based on the phenomena that the phase difference between principle polarization states in the optical retarder is function of temperature is described. The polarization state of optical beam is changed as it passes through the optical retarder, which depends on the temperature. The temperature of optical retarder is determined by comparison of the power difference between principal polarization states. We demonstrate successfully the temperature measurement by using a polarization maintaining fiber as the optical retarder. With a 100 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.236 rad/°C and the measurement error was ±0.038°C over the temperature range of −2.6 – +3.4°C. With a 11.5 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.021 rad/°C and the measurement error was ±0.79°C over the temperature range of −8.5 – +86.5°C.     

DISTORTION PROPERTIES OF GaN SWITCHES AT HIGH-TEMPERATURES

The origins of HEMT distortion in passive control applications as SPST switch are presented in this paper. Also, this paper describes the change of the AlGaN/GaN HEMT switch distortion properties (second-and third distortion intercept points) over a wide range of temperature. The results indicate that the change in second-and third-order distortion intercept points is smaller (about 2dBm) over a wide range of temperature from −50 to +300°C. A comparison of the GaN-based HEMT switch with InP-and GaAs-HEMT switches shows that the GaN technology generates lower distortion than its InP and GaAs technologies counterpart.     

Beta processes in a high-temperature field and nuclear multibeta decays

Sources of the temperature dependence of rates of nuclear beta processes in matter of massive stars are systematized. Electron and positron beta decays and electron capture (K capture and the capture of unbound electrons) fromexcited nuclear states (thermal decays) are considered along with the photobeta decays from ground and excited nuclear states. The possible quantum degeneracy of an electron gas in matter and the degree of ionization of an atomic K shell in a high-temperature field are taken into account. For a number of multidecay odd-nuclei, the temperature dependences of the ratios of the total rates of their β ^? decays to the sum of the total rates over all of decay modes for the same nuclei are calculated in the range of nuclear temperature from 2 to 3 × 10⁹ K. It is shown that the deviation of this ratio from the experimental value obtained at “normal” temperature may be quite sizable. This circumstance should be taken into account in models that consider the problem of synthesis of nuclei in matter of massive stars.     

Logarithmic Fermi-liquid breakdown in NbFe2

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.