Thermal conductivity of Tetryl by modulated differential scanning calorimetry

An investigation of the use of modulated differential scanning calorimeter (MDSC) to measure thermal conductivity (κ) of the explosive Tetryl using isothermal and non-isothermal methods. Issues surrounding the use of silicone oil as a heat transfer aid are discussed. Using these methods the calculated isothermal and non-isothermal properties of specific heat capacity were observed to be 0.844 and 0.863 J/(g K) and the calculated thermal conductivity values were found to be 0.165 and 0.186 W/K. Calibration experiments using polystyrene indicate that the non-isothermal method is more reproducible but has a larger offset (35%) from the true value. Our corrected values for Tetryl fall in the middle of the considerable range of values reported in the literature.     

Thermal conductivity behaviour of polymers around glass transition and crystalline melting temperatures

The thermal conductivity of five semi-crystalline and four amorphous polymers was determined within a wide range of temperature, starting at room temperature and going up to temperatures above the polymer melting point (Tm) for semi-crystalline polymers or above the glass transition temperature (Tg) for amorphous polymers. Two transient techniques were employed in the experimental investigation: the hot wire technique for the group of amorphous polymers, and the laser flash technique for the semicrystalline polymers. As expected, the experimental results show that Tg exerts a measureable influence on the thermal conductivity of amorphous polymers. In the case of semi-crystalline polymers, a singular behaviour of the thermal conductivity is observed within the Tm range. In order to explain the anomalous behaviour, the influence of these transition temperatures on the thermal conductivity behaviour with temperature has been analysed in terms of a phonon conduction process and temperature variations of specific heat and modulus of elasticity of the analyzed polymers.     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

The effect of inclusion on the heat capacities of some simple molecules in adducts of Dianin's compound

The heat capacities of unsolvated (empty) Dianin's compound (4p-hydroxypheny]-2,2, 4-trimethylchroman) and the ethanol and carbon tetrachloride adducts of Dianin's compound have been measured in the 30–300K temperature range. The molar heat capacities of the guest species were calculated from the experimental results and compared with molar heat capacities of bulk CCl₄ and bulk C₂H₅OH The stoichiometry of the CCl₄ adduct was determined to be six host molecules per guest CCl₄, in contrast to an earlier report.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Thermal conductivity measurements using the flash method

Thermal diffusivity is the speed with which heat propagates through a material. It has a multitude of direct applications, such as determining heat transfer through brake pads at the moment of contact, etc., but more often it is used to derive thermal conductivity from the fundamental relationship tying it with specific heat capacity and density. Using a new multi-sample configuration system, and testing a reference sample adjacent to the unknown, specific heat capacity can be obtained parallel with thermal diffusivity. Thus, a single test yields thermal diffusivity and thermal conductivity with prior knowledge of density. The method is fast and produces results with high accuracy and very good repeatability. The sample size, 12 to 30 mm diameter and 2 to 5 mm thickness, is easy to handle and is well suited for a broad range of materials, even for composites, often a problem for other methods. Typical data on two polymers, Pyrex glass and Pyroceram 9606 are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Advanced two-channel ac calorimeter for simultaneous measurements of complex heat capacity and complex thermal conductivity

The advanced construction of a two-channel ac calorimeter for simultaneous measurements of frequency-dependent complex heat capacity C(ω) and complex thermal conductivity λ(ω) is presented. In the new calorimeter, the number of interfaces with thermal-wave reflections was reduced. Thus, the new construction can be easily calibrated with higher precision and is simpler in handling than the previous one. The new construction allows to measure thermal conductivity in steady-state mode, as well as frequency-dependent complex thermal properties in ac mode, in the same measuring cell. The capabilities of this technique were demonstrated, being applied for simultaneous measurements of complex effusivity, diffusivity, heat capacity, and thermal conductivity of glycerol in the glass transition region. The so-called ac and dc thermal conductivities of glycerol were measured as a function of temperature. It was shown that the double-channel ac calorimetry is a technique, which can be used for reliable distinguishing of relaxation processes related to relaxing thermal conductivity or relaxing heat capacity.In the region apart from phase transitions, the calorimeter provides the unique possibility of simultaneous measurements of the thermal contact properties together with the sample’s thermal parameters. The improvement of the accuracy gave us the possibility to observe the thermal contact resistance, leading to a step of 1 and 5% in the temperature-modulation amplitude at the cell/sample interface in the case of liquid samples such as Apiezon™-H grease and glycerol, respectively. A step of 25% was observed in the case of a dry thermal contact between the cell and an ethylene-1-octene copolymer sample. Thus, the thermal contact resistance must be taken into account in the temperature-modulated calorimetry, especially in the case of a dry cell/sample contact.     

Phase transitions in the hydroquinone hydrogen sulfide clathrate compound

Heat capacities of [C₆H₄(OH)₂]₃·(H₂S) _x were measured between 1 and 15 K. Heat capacity peaks were found at (7.56±0.09) K, (7.61±0.05) K, and (7.65±0.07) K for the compounds withx=0.92, 0.95, and 0.96. A weak anomaly was observed around 6.75 K for the compound withx=0.85. The temperatures of these anomalies are unusually low among the phase transitions of molecular crystals. The decrease of the transition temperature from that of crystalline H₂S (=103.52 K) is a clear indication of the effect of enclathration on the molecular interaction. A comparison of the rotational heat capacity of the trapped hydrogen sulfide molecules with that of crystalline hydrogen sulfide shows that the trapped hydrogen sulfide molecules have a large rotational freedom at low temperatures (13 K). This agrees with the results from far infrared spectroscopic data. The dielectric constant of the clathrate compound obeyed the Curie-Weiss law above 30 K and no significant dielectric loss was found over the whole temperature range. These results showed that the trapped hydrogen sulfide molecules execute free rotation or are orientationally disordered above 20 K.Dedicated to Professor H. M. Powell.     

Low Temperature Specific Heat Capacity of 3d Transition Metal Chlorine Boracites (T3B7O13Cl; T=Cr,Mn, Fe,Co, Ni,Cu, Zn or Mg)

The heat capacities of eight chlorine boracites T₃B₇O₁₃Cl (T=Cr, Mn, Fe, Co, Ni, Cu, Zn or Mg) have been measured in the temperature range 2 to 100 K. Magnetic phase transitions occur below 20 K in the compounds studied except in the two non-magnetic substances Zn₃B₇O₁₃Cl and Mg₃B₇O₁₃Cl. The magnetic specific heat capacities give information on magnetic ground state of the transition metals and the entropy related to the phase transitions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal conductivity anisotropy of expanded graphite/ chlorate salt composites for thermal energy storage

ABSTRACT

Expanded graphite (EG)/LiCl-NaCl phase change composites are prepared by aqueous solution method with different EG amount and forming pressure to enhance heat conduction for high-temperature latent heat thermal energy storage application. Their microstructure and thermal conductivity are characterized. Results indicate that the composites are uniform and the LiCl-NaCl eutectic is well dispersed in the graphite flakes. Thermal conductivity of the LiCl-NaCl can increase to as much as 40.51 W/(m·K), which is 46 times higher than that of pure eutectic salt. With forming pressure, the thermal conductivities of the samples show anisotropy because of a flattened irregular honeycomb network of graphite. Within certain limits, the greater the forming pressure is, the more pronounced the anisotropy performs. In addition, the formulas to calculate the thermal conductivity in the axial direction and the radial direction are given based on the average rotation angle φ of EG basal plane, and experimental data show that the formula in the radial direction is especially useful for calculating the thermal conductivity.     

Heat capacity measurement and DSC study of hafnium hydrides

Heat capacities, electrical conductivities and phase transition temperature of hafnium hydrides, HfH_x (0.99≤x≤1.83), were studied using a direct heating pulse calorimeter and a differential scanning calorimeter from room temperature to above 500 K. The heat capacity of HfH_1.83 was larger than that of pure hafnium and showed no anomaly of heat capacity. In contrast, there were λ-type peaks for the heat capacity and DSC curves for HfH_x (1.1≤x≤1.6) near 385 and 356 K. The anomalies of heat capacity and electrical conductivity of HfH_x (1.1≤x≤1.6) were considered the result of phase transition and order-disorder phase transition for hydrogen in the hafnium hydride lattice for HfH_x (1.1≤x≤1.3).     

Thermal conductivity of ammonia borane complex and its composites with aluminum powder

The thermal conductivity of ammonia borane (AB) complex, in the temperature range of 300-420 K, was measured experimentally using ASTM method E 1225. At 300 K, the thermal conductivity of pure AB was found to be approximately 15 W/m-K. A composite pellet prepared by mixing 10 wt% aluminum powder with AB had a thermal conductivity that was a factor of 4 higher than that of pure AB complex. The extent of the pyrolytic weight loss for AB/Al composite and pure AB complex was 25.4% and 33.9%, respectively—indicating comparatively lower levels of volatile species evolved as impurities (e.g. monomeric aminoborane, borazine, diborane, etc.) in the product hydrogen.     

Calorimetric study of phase transitions in the thiourea 1,1,2,2-tetrachloroethane clathrate compound

Heat capacities of the thiourea clathrate compound of 1,1,2,2-tetrachloroethane, {(NH₂)₂CS}₃(CHCl₂)₂, were measured at temperatures between 13 and 330 K. Two phase transitions were found. The enthalpy and entropy changes of the transition are 5940 J·mol^–1 and 28.1 JK^–1· mol^–1 for the one occurring at 224 K and 2756 J·mol^–1 and 11.3 JK^–1·mol^–1 for the other at 248 K. It is concluded from the transition entropy values that the guest molecules are orientionally disordered nearly to the same extent as in the neat liquid.Contribution No. 56 from the Microcalorimetry Research Center     

Anomalous change of viscosity and conductivity in blood plasma lipoproteins in the physiological temperature range

Study on temperature dependencies of viscosity (η) and conductivity (σ) of blood lipoproteins [high‐density lipoprotein (HDL), low‐density lipoprotein (LDL), and very low density lipoprotein (VLDL)] and A‐I apolipoprotein (human and rats) has revealed the presence of the anomalous region at temperature 35–38±0.5°C (T_c). Transition width is 2°C. Viscous flow enthalpy, activation energy (ΔH), and transition enthalpy (ΔH_trans.) as well as thermal coefficients Δη/ΔT and Δσ/ΔT on either side of T_c have been calculated. Transition heat is very low in human HDL, VLDL and apoA‐I, and in LDL it is higher by a factor of 4–5. Some mechanisms of the cortisol interaction with HDL and apoA‐I have been studied by infrared (IR) spectroscopy and conductometry. The hormone has been found to strengthen the tangle → α‐helixes and tangle → β‐structures transitions and increase the ordering of lipids. Therewith, ΔH_trans. rises markedly (13 and more times), and at the same time the anomalous region is shifted by 1–2°C in apoA‐I. The anomalous change of viscosity and conductivity in the physiological temperature range for all lipoprotein fractions and apoA‐I seems to be due to the structural phase transition in both proteins and lipids. In view of the heat capacity jump and a low value of ΔH_trans. in human HDL, one may assume the phospholipids of these particles to exhibit the orientation transition of smectic A↔C type, which is assigned to the second type of phase transition. The structural transition into apoA‐I is likely to contain the elements of phase transition of the first and second types. In human and rat VLDL, the smectic → cholesteric phase transition seems to occur. Enthalpy of viscous flow and structural transition in VLDL is higher for rats than for human. The pH shift of the medium to the neutral region (pH 6.1) results in shifting the anomalous region by 1–2°C. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 348–369, 2001     

Thermal conductivity enhancement of MWNTs on the PANI/tetradecanol form-stable PCM

We prepared PANI/tetradecanol/MWNTs composites via in-situ polymerization. DSC results indicated that the composites are good form-stable phase change materials (PCMs) with large phase change enthalpy of 115 J g⁻¹. The MWNTs were randomly dispersed in the composites and significantly enhanced the thermal conductivity of the PCMs from 0.33 to 0.43 W m⁻¹ K⁻¹. The form-stable PCMs won’t liquefy even it is heated at 80°C, so that the MWNTs were fixed in the composite and the phase separation of the MWNTs from PANI/tetradecanol/MWNTs composites won’t occur.     

Thermal conductivity ofn-Tetracosan around the liquid-liquid transition temperatureT u

Forn-Tetracosan the temperature dependence of the thermal conductivity has been determined between 52 °C and 150 °C by the transient hot strip method. A decreasing thermal conductivity is followed by a constant one forT100 °C.The change in slope can be well estimated by using data of Brillouin scattering and specific volume measurements thus confirming the existence of theT _u -phenomenon.     

Thermal conductivity measurements on ferrofluids

The thermal conductivity of a number of ferrofluids consisting of colloidally dispersed Fe₃O₄ particles in diester, hydrocarbon, water and fluorcarbon carriers have been measured at 38°C. The variation in thermal conductivity with particle concentration is well described by Tareef's equation (1940). This has enabled the ratio of the physical to magnetic size to be determined and compared with estimates of the ratio obtained from electron micrographs and magnetic measurements.The fit between theory and experiment is particularly good for hydrocarbon carrier fluids giving the ratio of solid to magnetic radiusR _i/R _m=1.24±0.03 compared with the value obtained from magnetic data and electron micrographs of 1.19±0.07. The corresponding value from the fluids with a diester carrier ranges between 1.1<R _d/R _m<1.3 which is again consistent with microscopy and magnetic data.The application of a magnetic field of 0.1 T had no noticeable effect on the thermal conductivities of ferrofluids.     

Electrical conductivity and phase transition studies of some selected Congo red-metal complexes

The electrical properties of Congo Red dye (L) and its metal (M) complexes ofML molar ratios of 11 and 21, whereM=Cr(III), Fe(lII), Cu(II), Zn(II), Zr(IV), Th(IV) and UO₂(II), indicated semiconducting behaviour. The activation energies and electrical conductivities depend on the ionic charge of the metal chelates. The activation energies of 11 metal complexes are greater than those of 21 metal complexes. The conductivity data obtained are interpreted on the basis of the molecular structure of the Congo Red complexes. One phase transition was obtained during DTA study of Congo Red dye at 212°C. This phase transition may be due to the rotation of the two naphthalene rings and the N=N groups around the diphenyl group centre.     

Thermal conductivity of gel-spun polyethylene fibers

The thermal conductivities of unidirectional gel-spun polyethylene fiber-reinforced composites have been measured parallel (K∥?) and perpendicular (K⊥) to the fiber axis from 15 to 300K. The axial thermal conductivity K∥? varies linearly with volume fraction v_f of fiber, while the transverse thermal conductivity K⊥ follows the Halpin-Tsai equation. Extrapolation to v_f = 1 gives the thermal conductivity of gel-spun polyethylene fiber which, at 300K, has values of 380 and 3.3 mW cm^?1K^?1 along and perpendicular to the fiber axis, respectively. The axial thermal conductivity is exceptionally high for polymers, and is more than twice the thermal conductivity of stainless steel. This high value arises from the presence of a large fraction of long (> 50 nm) extended chain crystals in the fiber. Further improvement of up to a factor of 10 is possible if the length and volume fraction of the extended chain crystals can be increased. © 1993 John Wiley & Sons, Inc.     

Contributions on the Properties of Titanates with Ilmenite Structure. IV. The effect of an order-disorder transition on the high temperature electrical conductivity of NiTiO3

NiTiO₃ shows an order-disorder transition from an ordered ilmenite structure to a corundum structure at high temperatures. The transition is followed by a strong increase of the specific electrical conductivity. The conductivity was investigated as a function of temperature and oxygen activity. An order parameter according to common phase transition theories can be used to describe the behaviour of the conductivity in the transition region and vice versa. A model for the defect structure of NiTiO₃ is presented.