Temperature coefficient of optical path of tellurite glasses doped with gold nanoparticles

This work reports on the thermo optical parameters of TeO₂–PbO–GeO₂ glasses as a function of the concentration of gold nanoparticles. Techniques such as thermal lens and heat capacity measurements allowed the determination of the thermal diffusivity D, thermal conductivity K and the temperature coefficient of optical path (ds/dT). It is shown an expressive decrease in ds/dT as a function of gold nanoparticles concentration, while the opposite effect was observed for the thermal diffusivity D.     

Modeling of an isolated liquid hydrogen droplet evaporation and combustion

In this model, diffusive governing equations of Liquid Hydrogen (LH) evaporation and combustion were solved. The simulation reveals that, there exists a critical radius (a_cri) where radiation heat is equal to conduction heat (Q_rad = Q_cond) and a_cri is a function of ambient temperature during LH droplet evaporation process. Under pure evaporation condition, for large liquid hydrogen droplets (a > a_cri) radiation heat is dominant at a given environment temperature, but as liquid droplet size decreases, radiation heat becomes insignificant and thermal conduction will be dominant for liquid evaporation. When LH droplet is burned in a cold environment (T_∞ = 300 K), there are two films above the LH surface, Film I is from LH surface to flame front within which a dense hydrogen gas cloud is formed; Film II is from flame front to the free stream where oxygen is diffused inward to react with hydrogen. The flame front is located about 95 times of the droplet radius (r_f = 95a) and the flame temperature could rise up to 2077 K. When an LH droplet is immersed in a hot environment (T_∞ = 2050 K), the flame front is located at a similar distance to the LH droplet (r_f/a = 114) and flame temperature could go up to 3769 K.     

A cryogenic fast response thermometer

Multipurpose carbon thin film resistance thermometers have been developed for cryogenic use. Carbon was electron beam evaporated onto polycrystalline alumina substrates with copper films as electrical contacts. The thermometers were coated with thin glass layers. The desired electrical resistance of the sensors was obtained with a final heat treatment.Their sensitivity at 4.2 K is approximately 300 Ω K^?1. Between 4.2 and 20 K the film resistance can be expressed as a function of temperature by a usual correlation of the form In R = A₀ + A₁T + A₂ (InT)². These sensors have been developed as separate devices to be soldered or glued onto experiments. Nevertheless, their response is fast; at 4.2 K their thermal relaxation time is better than 1 ms and their thermal delay time is of the order of 70 μs.     

Deposition temperature effect on the electric and dielectric properties of InSbSe3 thin films

Thin films of InSbSe₃ compound were obtained by thermal evaporation on to clean glass substrates maintained at various deposition temperatures from 423 to 593 K. At deposition temperature T_d?473 K, the films have an amorphous structure, while those prepared at T_d>473 K have a polycrystalline structure identified by X-ray diffraction analysis. The DC electrical conductivity of the films increases as T_d increases, whereas activation energy decreases with increasing T_d, which reflects a change in the degree of disorder. AC conductivity was studied as a function of frequency in the range (10²-10⁵ Hz) and as a function of deposition temperature. The dependence of T_d on the frequency exponent s in the conductivity-frequency relation confirmed that the mechanism of AC conductivity is correlated barrier hopping with a single polaron hopping mechanism. The discrepancy between DC and AC activation energies was studied as a function of deposition temperature. The maximum barrier height W_m and the density of defect states N were also determined. Finally, the dependence of dielectric constant and dielectric loss on T_d were studied. A Debye-like relaxation of dielectric behavior was observed for crystalline films and is found to be a thermally activated process. The position of maximum dielectric loss is shifted towards higher temperature with T_d treatment and there by reduces the relaxation time.     

Thermal explosion hazards on 18650 lithium ion batteries with a VSP2 adiabatic calorimeter

Thermal abuse behaviors relating to adiabatic runaway reactions in commercial 18650 lithium ion batteries (LiCoO₂) are being studied in an adiabatic calorimeter, vent sizing package 2 (VSP2). We select four worldwide battery producers, Sony, Sanyo, Samsung and LG, and tested their Li-ion batteries, which have LiCoO₂ cathodes, to determine their thermal instabilities and adiabatic runaway features. The charged (4.2 V) and uncharged (3.7 V) 18650 Li-ion batteries are tested using a VSP2 with a customized stainless steel test can to evaluate their thermal hazard characteristics, such as the initial exothermic temperature (T₀), the self-heating rate (dT/dt), the pressure rise rate (dP/dt), the pressure-temperature profiles and the maximum temperature (T_max) and pressure (P_max). The T_max and P_max of the charged Li-ion battery during the runaway reaction reach 903.0 °C and 1565.9 psig (pound-force per square inch gauge), respectively. This result leads to a thermal explosion, and the heat of reaction is 26.2 kJ. The thermokinetic parameters of the reaction of LiCoO₂ batteries are also determined using the Arrhenius model. The thermal reaction mechanism of the Li-ion battery (pack) proved to be an important safety concern for energy storage. Additionally, use of the VSP2 to classify the self-reactive ratings of the various Li-ion batteries demonstrates a new application of the adiabatic calorimetric methodology.     

Analysis of the lattice thermal conductivity of thin films by means of a modified Mayadas-Shatzkes model: The case of bismuth films

The lattice thermal conductivity λ_øx(d, T) of bismuth films in a direction of the film plane (x direction) with the thicknesses d ranging from 20 to 400 nm was determined in the temperature range 80 K ⩽ T ⩽ 400 K from the measured total thermal conductivity λ_x(d, T) and the calculated charge carrier contribution λ_ex(d, T). A modified Mayadas-Shatzkes model of phonon scattering on polycrystalline films is presented. Following this model the thickness and temperature dependence of λ_øx(d, T) can be interpreted.     

Compositional effects on the optical and thermal properties of potassium aluminophosphate glasses

The K₂O–Al₂O₃–P₂O₅ glass system has been examined and various compositions have been melted. Their optical and thermal properties have been measured to assess their potential for athermalisation. The addition of alumina (Al₂O₃) increases the refractive index (n) and glass transition temperature (T_g) and decreases the thermal expansion coefficient (α), consequently leading to positive thermo-optic coefficient (dn/dT). In addition to thermal expansion, polarisability of the glass also affects dn/dT. Generally, glasses must exhibit negative dn/dT to counter thermal expansion, in order to have potential application in athermalisation.     

Unsteady free convective boundary-layer flow near a stagnation point in a heat-generating porous medium

The free convection boundary-layer flow near a stagnation point in a porous medium is considered when there is local heat generation at a rate proportional to (T ? T _∞) ^p , (p ≥ 1), where T is the fluid temperature and T _∞ the ambient temperature. Two cases are treated, when the surface is thermally insulated and when heat is supplied at a constant (dimensionless) rate h _s from the boundary. If h _s = 0 the solution approaches a nontrivial steady state for time t large in which the local heating has a significant effect when p ≤ 2. For p > 2 the effects of the local heating become increasingly less important and the solution dies away, with the surface temperature being of O(t ^?1) for t large. When h _s > 0 and there is heat input from the surface, the solution for p ≤ 2 again approaches a nontrivial steady state for t large and all h _s . For p > 2 there is a critical value h _s,crit (dependent on the exponent p) of h _s such that the solution still approaches a nontrivial steady state if h _s < h _s,crit. For h _s > h _s,crit a singularity develops in the solution at a finite time, the nature of which is analysed.     

Thermomechanical properties of poly(vinyl alcohol) plasticized with varying ratios of sorbitol

The objective of this work is to study the thermal and mechanical properties of films based on blends of poly(vinyl alcohol) (PVA) with different weight percent of sorbitol. Solid-state PVA/sorbitol polymer membranes were prepared by a solution casting method. The characteristic properties of these polymer membranes were examined by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation methods and by Fourier Transform Infrared (FTIR) spectroscopy. It was found that the thermal properties (glass transition, T_g, melting point, T_m and decomposition temperature, T_d) for PVA blends showed a decrease proportional to the sorbitol concentrations. The hardness and elastic modulus obtained from nanoindentation test were also found to decrease with increase in plasticizer concentration. FTIR confirmed the reduction in hydrogen bonding between PVA chains in favour of formation new bonding between the plasticizer and the PVA chains.     

Control of substrate surface temperature in millisecond annealing technique using thermal plasma jet

Temporal variations of substrate surface temperature in scanning Ar thermal plasma jet has been investigated based on an analysis of transient changes in optical reflectivity. The accuracy of the temperature measurement has been evaluated to be 30 K at temperature around 1760 K. The maximum surface temperature (T_max) is controlled in the range from ∼ 960 to ∼ 1780 K with keeping the annealing duration (t_a) around ∼ 3 ms by changing the Ar gas flow rate (f) and distance between the plasma jet and the substrate (d) under a constant scanning speed (ν) of 500 mm/s.     

‘Heat from Above’ Heat Capacity Measurements in Liquid 4He

We have made heat capacity measurements of superfluid ⁴He at temperatures very close to the lambda point, T _λ, in a constant heat flux, Q, when the helium sample is heated from above. In this configuration the helium enters a self-organized (SOC) heat transport state at a temperature T _soc(Q), which for Q≥100 nW/cm² lies below T _λ. At low Q we observe little or no deviation from the Q=0 heat capacity up to T _SOC(Q); beyond this temperature the heat capacity appears to be sharply depressed, deviating dramatically from its bulk behaviour. This marks the formation and propagation of a SOC/superfluid two phase state, which we confirm with a simple model. The excellent agreement between data and model serves as an independent confirmation, of the existence of the SOC state. As Q is increased (up to 6 µW/cm²) we observe a Q dependent depression in the heat capacity that occurs just below T _SOC(Q), when the entire sample is still superfluid, This is due to the emergence of a large thermal resistance in the sample, which we have measured and used to model the observed heat capacity depression. Our measurements of the superfluid thermal resistivity are a factor of ten larger than previous measurements by Baddar et al.     

Determination of the steady temperature field in a toothed ring

An examination is made of a method of calculating the temperature field in the region illustrated in Fig. 1. The temperature T₀ of the shaded heat regions is found, under the assumption that the shaded regions possess high thermal conductivity, and that the temperature is the same at all points of these regions. The method is applied to calculation of the temperature field in a phase shifter rotor. The dependence of T₀ on the thickness of thermal insulation of the rotor conductors is given.     

Electrical resistivity studies of thermally evaporated manganese-rhenium thin films

Electrical resistivity studies have been carried out on thermally evaporated Mn_100−xRe_x thin films (with X=0.1-0.5 and 1 at.% Re) over the temperature range from 300 to 1.4 K using the van der Pauw four probe technique. A resistivity minimum a notable characteristic of α-Mn was found in all the specimens with a shift of T_min corresponding to the resistivity minimum to upper values as the concentration of Re increases. The results show a tendency towards saturation of the resistivity as the temperature approaches zero implying a Kondo scattering mechanism in the samples. The shift of T_min and the characteristic Kondo temperature T_K to upper values may be explained in terms of the Kondo scattering.     

Control of the deposition temperature by the use of a magnetic field in r.f. sputtering

The effect of a magnetic field on the deposition temperature T_d of a thin film prepared by r.f. sputtering is reported. The variation in T_d with the configuration of the magnetic field is illustrated. For the sputtering parameters used, T_d rises to 200°C when no field is applied, but it remains close to room temperature when a d.c. magnetic field is applied at the level of the substrate, parallel to the substrate surface, perpendicular to the motion of the sputtered particles and essentially localized outside the plasma. The results are explained by considering the various contributions to the heating. The application of the present experimental process to the preparation of amorphous thin films is pointed out.     

In situ probe nanophase transition in nanocomposite using thermal AFM

Nanocomposites made from inorganic nanoparticles and polymers have many applications in optics, electronics and biomaterials. However, the glass transition temperature (T_g) of a nanocomposite is very difficult to measure accurately by conventional thermal analysis such as DSC or TMA when the concentration of the nanoparticle reaches a threshold of the percolation network. At this threshold stage, the phase transition in the nano domains of the matrix is too small to be detected by macroscale thermal analysis. We have developed a methodology basis on thermal atomic force microscope (AFM) to monitor the nanophase transition of the nanocomposite in situ upon heating. This method has demonstrated the capability in determining the T_g of a nanocomposite made by spherical SiO₂ nanoparticles dispersed in polyacrylate. The threshold of the percolation network of this nanocomposite is at 40 wt% of SiO₂ nanoparticles according to the results of refractive index, AFM, nanoindentation, DSC, TMA and TGA.     

Effect of CO2 fixation reaction on the thermal and dynamic mechanical properties of tetrafunctional epoxy resin

A tetrafunctional epoxy resin was modified using CO₂ fixation process in the presence of tetra-n-butyl ammonium bromide as catalyst. The unmodified tetrafunctional epoxy resin (UMTE) and CO₂ fixated modified tetrafunctional epoxy resin (CFMTE) were cured by diethylenetriamine. A bifunctional glycidyl ether compound was used as a reactive diluent to control the viscosity of CFMTE. The activation energy of curing reaction was computed using the advanced integral isoconversional method. The activation energy, which depends on the conversion, was considerably changed due to the CO₂ fixation process. The thermal stability parameters including the initial degradation temperature, the temperature at the maximum rate of weight loss (T _max), and the decomposition activation energy (E _d) were determined by thermal gravimetry. Dynamic mechanical thermal analysis measurements showed that the CO₂ fixation decreases the T _g of the epoxy resin. The surface morphology of UMTE and CFMTE were determined by scanning electron microscope. It is concluded that CO₂ fixation reaction improves the properties of tetrafunctional epoxy resin.     

Size and temperature dependences of the resistivity of thin alkali metal wires at liquid nitrogen,hydrogen and helium temperatures

The temperature variation of the resistivity ?, the size effect on the resistivity and the residual resistivity were investigated for thin wires of the alkali metals Na, K, Rb and Cs (which were of high purity and had a high crystalline order) from room to liquid He temperatures.The results obtained allowed us to determine the fundamental quantity ?_∞l and also the resistivity ?(T)_∞ of the bulk metal and the mean free path l(T) of the conduction electrons.The values of ?_∞l obtained near 0 K for these thin alkali metal wires of high crystalline order were found to correspond, within experimental error, to those obtained previously using highly disordered thin films of the same alkali metals at liquid nitrogen temperatures. Both sets of ?_∞l valuescorrespond to the values expected theoretically according to the fundamental equation $\text{1}\text{?}{}_{\mathrm{\infty }}\text{l}\text{=}\text{const.}\text{× N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ (where N is the density of effective conduction electrons), thus proving that ?∞l is independent of temperature and of degree of crystalline order as expected.The results on the temperature variation of the resistivity ?(T)_pho due only to electron-phonon scattering reveal a T⁵ law for Na in the temperature interval 15?T?20 K and for Rb and Cs in the range 1.6?T?3 K, and a T³ law for K in the range 14?T?20 K. Moreover, at temperatures below 4 K the resistivities of the thinnest wires of Cs and, to a lesser extent, of Rb deviate from Matthiessen's rule, thus revealing a temperature-dependent size effect.     

A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal–Insulator Transition

Thermal radiation from a black body increases with the fourth power of absolute temperature (T⁴), an effect known as the Stefan–Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (ε_int), is insensitive to temperature (|dε_int/dT| ≈ 10⁻⁴ °C^–1). The resultant radiance bound by the T⁴ law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which ε_int can be engineered to decrease in an arbitrary manner near room temperature (|dε_int/dT| ≈ 8 × 10⁻³ °C^–1), enabling unprecedented manipulation of infrared radiation. As an example, ε_int is programmed to vary with temperature as the inverse of T⁴, precisely counteracting the T⁴ dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.