Thermal conductivity of Freon-12

The construction of a heated filament measurement device is described. The thermal conductivity of Freon-12 in the liquid phase is measured over the temperature range ?50–100°C at pressures up to 600·10⁵ N/m², and conductivity of the gaseous phase of Freon-12 is measured over the range 30–160°C at atmospheric pressure. The results obtained are approximated by equations and compared with the data of other authors.     

The effect of drying preconditioning on the South African durability index tests

This study investigated the effect of different drying regimes in the preconditioning stage on Durability Index (DI) test results. The moisture condition of specimens needs to be stable and uniform for the tests to be accurate and reliable. Three drying regimes were used: (a) standard oven drying method of 50 °C for 7 days, (b) oven drying at 50 °C to constant mass, and (c) drying using a solvent replacement method with isopropanol. Concrete mixes were designed using three w/c ratios (0.40, 0.50 and 0.65) and four binders. The isopropanol and oven drying to constant mass methods were found to remove different amounts of moisture compared with the standard drying method. Most (about 80%) of the moisture was removed within 7 days when oven dried. Statistical analysis suggested that, for certain mixes, the drying method had an effect on the DI results, with the chloride conductivity test being the most sensitive. For quality control purposes, it is impractical to wait until specimens are completely dry, which in some cases took up to 17 days, before performing the DI tests. The practical solution is for specimens to be tested at not less than 7 days and not more than 8 days of drying.     

Measurement and prediction of thermal properties of alkali-activated fly ash/slag binders at elevated temperatures

This paper presents a comprehensive experimental study of thermal properties of various alkali-activated binders at ambient and elevated temperatures. The binders were prepared using alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 90/10, 50/50 and 0/100 wt%. These binders can be considered as a composite of solid, water and air. Accordingly, a three-phase model is applied to predict thermal conductivity of the binders at ambient temperature. At elevated temperatures, the Hashin–Shtrikman model is used to estimate the bounds of thermal conductivity for alkali-activated binders containing of fly ash. To validate the above models, a transient plane source measurement technique was applied to measure the thermal conductivity and heat capacity at temperatures ranging from 23 to 600 °C. Data generated is then utilised to develop analytical expressions for estimating thermal properties as a function of temperature. The simplified relationships can be used for estimating the fire resistance of structural elements made from alkali-activated cementitious materials.     

Experimental study on thermal properties of frozen soils

A series of measurements of the effective thermal conductivity, specific heat and unfrozen-water content of various soils exposed to a cold environment have been carried out by a transient probe method and a calorimetric technique. The present experiments were performed at a very slow rate of cooling of the moist-soil and using four kinds of soil samples from fine to coarse grain-size. It was established that the amount of unfrozen-water in the frozen soil was dependent on the initial moisture content, the grain-size of the soil and the temperature (below 0°C). Measured values of the effective thermal conductivity and specific heat show a strong dependence on temperature, initial moisture content and grain-size of soil in the temperature range T = 0°C to ?10°C. The anomalous behavior of a decrease in the effective thermal conductivity of frozen soil for the finest grain-size with decreasing temperature (below 0°C) was observed for a large initial moisture content w = 30–40%.     

Determination of Moisture Diffusivity as a Function of Both Moisture and Temperature

The effect of moisture and temperature on liquid water transport in porous media was studied. Specimens of autoclaved aerated concrete were subjected to one-sided water penetration in isothermal conditions at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. After specified time intervals, moisture profiles were determined gravimetrically. The moisture diffusivity was calculated for a particular temperature as a function of moisture content, using an inverse analysis. The results demonstrate the dependence of the moisture diffusivity on the moisture content and the temperature of the samples. The moisture diffusivity for high moisture content can be as much as one order of magnitude greater than for the lowest moisture content studied. The moisture diffusivity was found to increase by as much as a factor of two when the temperature is increased from 20 °C to 80 °C.     

Conduction mechanism and dielectric properties of BiFeO3–BaTiO3 solid solutions

The first measurements are reported for the frequency-dependent conductivity of (1?x)BiFeO₃–xBaTiO₃ (x = 0.10, 0.15 and 0.30) solid solutions in the frequency range of 10⁰–10⁶ Hz and in the temperature range of 50–300 °C. Powder X-ray diffraction confirms the formation of solid solutions. The dielectric properties were seen to improve with increasing BaTiO₃ (BT) content. The conductivity (AC and DC) measurements reveal an inverse variation of the frequency exponent ‘s’ with temperature, high density of states and thermally activated process. The calculated density of states was found to be N(E_f) = 80.2 × 10³² eV^?1 cm^?1 at 1 kHz and 50 °C for BiFeO₃–10 % BaTiO₃ (BFO–10 % BT) solid solution. The impedance spectroscopy analysis confirms the presence of grain and grain boundary affecting the conductivity. Our results provide the first unambiguous evidence of conduction in crystallite BFO–BT solid solutions through correlated-barrier-hopping model.     

Transient plane source measurements of the thermal properties of hydrating cement pastes

A transient plane source measurement technique is applied to assessing the heat capacity and thermal conductivity of hydrating cement pastes in their fresh state and during the course of 28 d of hydration at 20°C. Variables investigated include water-to-cement mass ratio (w/c – 0.3 or 0.4) and curing conditions (sealed or saturated curing). The heat capacity data for the fresh cement pastes are compared to a simple law of mixtures, and analytical expressions are developed to estimate the heat capacity as a function of degree of hydration for the two curing conditions. The measured thermal conductivities of the fresh pastes along with the known thermal conductivity of water are used to estimate the thermal conductivity of the original cement powder via application of the Hashin-Shtrikman (H-S) bounds. Hydration is seen to have only a minor influence on the measured thermal conductivity. Extension of the law of mixtures for heat capacity and the H-S bounds for thermal conductivity to predicting the corresponding properties of concretes are discussed.     

Effects of Sintering Process on Microstructure and Properties of Flake Graphite-Diamond/Copper Composites

Flake graphite-diamond/Cu–Cr–Zr composites with good two-dimensional thermophysical properties were prepared by vacuum hot-pressing technology. The influence and working mechanism of the hot-pressing temperature on the relative density and thermal conductivity of the composites were studied to obtain the optimum sintering process. The results showed that with a pressing pressure of 10 ～ 20 MPa, the relative density and thermal conductivity of the composite materials increased as the sintering temperature increased from 950 to 1010°C. When the temperature rose to 1010 ～ 1040°C, a near fully dense composite material was obtained and thermal conductivity reached maxima of 410 and 119 W/m K parallel and perpendicular to the graphite planes, respectively, both of which are close to the theoretical value. However, relative density and thermal conductivity drastically decreased as the temperature continued to increase beyond 1070°C. This is attributed to the combined effect of sintering temperature and wettability between the matrix and the reinforcements.     

Stability,viscosity, thermal conductivity,and electrical conductivity enhancement of multi-walled carbon nanotube nanofluid using gum arabic

This experimental investigation discussed on the stability and rheological behavior of multi-wall carbon nanotubes (MWCNTs) nanofluids with and without gum arabic (GA). The stability of MWCNT in the presence of GA dispersant in solar glycol is systematically investigated by taking into account the combined effect of different parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of MWCNT and GA have been varied from 0.2 to 0.6% volume concentration and from 0.25 to 1.25 wt%, respectively, and the sonication time has been varied in between 30 and 120 min. The effect of sonication time on viscosity was discussed. It was perceived that the shear thinning behavior is exhibited by all the nanofluid samples. The stability of nanofluid is measured in terms of MWCNT concentration as a function of sediment time using UV-Vis spectrophotometer. Rheological behavior of MWCNT nanofluids is measured using Bohlin CVO Rheometer in the temperature range of 30–50°C, with step sizes of 5°C. Optimal GA concentration is obtained for the entire range of MWCNT concentration and 0.25–1.25 wt% of GA is found to be sufficient to steady all MWCNT range in solar glycol. Rapid sedimentation of MWCNTs is observed at higher GA concentration and sonication time. The presence of MWCNT and GA enhanced the thermal conductivity of the nanofluids by 30.59% at 0.6 vol.% particle concentration and 1.25 GA wt% at 50°C. The electrical conductivity is enhanced in a linear manner with respect to the loading of MWCNT and GA. Nevertheless, the electrical conductivity is increased linearly with increasing the temperature of the nanofluid. At particle concentration of 0.6 vol.% of MWCNT and 1.25 wt% of GA, the electrical conductivity of the nanofluid is improved by 190.57% at a temperature of 50°C.     

Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)₂ which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.     

A glycerol–water-based nanofluid containing graphene oxide nanosheets

Nanofluids are simply the dispersion of nanometer-sized particles in different fluids. Graphene oxide nanosheets (GONs) were prepared by exfoliating the graphite oxide. The GONs were investigated using Fourier transform-infrared spectroscopy, Raman spectroscopy, XRD analysis, high-resolution emission electron microscopy, transmission electron microscopy, and UV–visible spectroscopy. GONs/glycerol–water-based nanofluid was prepared by the two-step method. The stability of the nanofluid was investigated with respect to time. Thermal and electrical conductivity of the prepared nanofluid was examined with different temperatures (25–45 °C) and weight fractions (0.02–0.1 wt%). The nanofluid is found to be stable for more than 5 months. The results showed an enhancement in thermal conductivity of about 4.5 % at 25 °C with a weight fraction of 0.02 %. The improvement was up to 11.7 % with a weight fraction of 0.1 wt% at 45 °C. The electrical conductivity was increased with increasing the weight fraction and temperature. The improvement in electrical conductivity was about 5890 % at 25 °C and 0.1 wt%.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Influence of curing regimes on compressive strength of ultra high performance concrete

The present paper is aimed to identify an efficient curing regime for ultra high performance concrete (UHPC), to achieve a target compressive strength more than 150 MPa, using indigenous materials. The thermal regime plays a vital role due to the limited fineness of ingredients and low water/binder ratio. By activation of the reaction kinetics, the effectiveness of the binder is enhanced which leads to improvements in mechanical as well as durability properties. The curing cycle employed are ambient air curing, water curing and hot air curing. The specimens were exposed to thermal regime at (90°C/150°C/200°C) for duration of 24, 48 or 72 hours at the age of 3rd and 7th day followed with air curing or water curing till 28 days. The results showed a marked difference in compressive strength ranging from 217 to 142 MPa with change in curing regimes. The samples when thermally cured at the age of 3rd and 7th day produced an average ultimate strength of 217–152 MPa and 196–150 MPa, respectively.     

Experimental investigation of the effect of moisture on thermal conductivity and specific heat of porous ceramic materials

Thermal conductivity and specific heat of porous ceramic materials display a unique behaviour when moisture is present in their structure. When compared with the corresponding dried materials, these properties are drastically altered, and the magnitude of these changes depends on the moisture content. In this work it was experimentally investigated the effect of adsorbed water on the thermal conductivity and specific heat of the most commonly structural material: castables and concrete of Portland cement. The experimental technique employed was the hot wire parallel technique, and measurements were carried out from room temperature up to 300°C during the heating and cooling cycle. The thermal conductivity and the specific heat were simultaneously determined from the same experimental thermal transient. Experimental results show a drastic influence of the adsorbed water on the thermal conductivity and specific heat of green castables. It was also observed that the addition of glassy phase on sample composition decreases the thermal conductivity and promotes the inversion of the slope of the curve thermal conductivity versus temperature for the dried material.     

Effects of particle size and carbon coating on electrochemical properties of LiFePO4/C prepared by hydrothermal method

In this study, well-crystallized phase pure LiFePO₄/C (LFP/C) powders were synthesized using the hydrothermal reaction method. To improve the electronic conductivity of the LFP/C powder after ball-milling, the LFP/C powders were double-coated with carbon. Scanning electron microscopy and transmission electron microscopy were employed to observe the micromorphology of the samples and the carbon coating, which was analyzed using Raman spectroscopy. Furthermore, the electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectra, and the charge–discharge cycling test. The ball-mill and the process for double-coating carbon decrease the particle size and increase the conductivity of the LFP/C, thereby reducing the Li-ion diffusion length and improving the reversibility of the Li-ion intercalation/de-intercalation in the LFP/C crystallites. The capacity of the small-particle LFP/C with the double-layer carbon coating was 133 mAh/g at 0.1 °C, and remained at 83 mAh/g as the charge–discharge rate increased to 10 °C. In addition, good cycle stability was observed, with a retention rate of 98 % after 50 cycles at 1 °C.     

An improved procedure for obtaining and maintaining well characterized partial water saturation states on concrete samples to be used for mass transport tests

A conditioning procedure is proposed allowing to install into the concrete specimens any selected value of water saturation degree with homogeneous moisture distribution. This is achieved within the least time and the minimum alteration of the concrete specimens. The protocol has the following steps: obtaining basic drying data at 50 °C (water absorption capacity and drying curves); unidirectional drying of the specimens at 50 °C until reaching the target saturation degree values; redistribution phase in closed containers at 50 °C (with measurement of the quasi-equilibrium relative humidities); storage into controlled environment chambers until and during mass transport tests, if necessary. A water transport model is used to derive transport parameters of the tested materials from the drying data, i.e., relative permeabilities and apparent water diffusion coefficients. The model also allows calculating moisture profiles during isothermal drying and redistribution phases, thus allowing optimization of the redistribution times for obtaining homogeneous moisture distributions.     

Experimental investigation of thermal conductivity of ordinary and heavy water at temperatures of 25–350°C and pressures of 0.1–245.3 mN/m2

The results of experimental investigations of the thermal conductivity of ordinary and heavy water by the plane-layer method at temperatures of 25–350°C and pressures of 0.1–245.3 mN/m² are given. Equations are given for the thermal conductivity of ordinary and heavy water at pressures of 0.1–245.3 mN/m² and temperatures of 25–350°C.     

Physical, electrical, transport and magnetic properties of Nd(Ba,Nd)2.1Cu3O7−δ system

In this study, the superconducting Nd(Ba,Nd)_2.1Cu₃O_7?δ system has been prepared using conventional solid-state reaction technique. Transport properties including structural/microstructural evolution, electrical, magnetic and critical current density properties were investigated. After high temperature heat treatments at over 1,000 °C, large and strongly connected grains were obtained but weakly connected and small in size granular formation were obtained for the low temperature heat treated samples at around 900 °C. The best T _c and T ₀ values were obtained as 93 and 89 K respectively for the sample prepared at 1,020 °C for 24 h, which is very close to peritectic temperature of YBCO material. Magnetization of the sample heat treated at 1,020 °C was investigated in detail. The magnetization hysteresis loops are expounded to be the product of Nd-123 grains and unscreened Nd³⁺ ions within intergranular boundaries and vortex cores. The peak effect on the magnetization curves was described by the extended critical state model. Temperature dependencies of the irreversibility field, the peak field and the full penetration field correlate and there is scaling behavior of the pinning force as well. Thermoelectric power data was analyzed by “Modified two band model with linear T-term for superconductors”. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below the T _c value. Thermal conductivity of samples prepared was calculated by using “The Modified Callaway Model and Wiedermann–Franz law” and results obtained discussed.     

Cost-effective liquid-phase exfoliation of molybdenum disulfide by prefreezing and thermal-shock

In this work, we present a facile method without hazardous material for improving the liquid-phase exfoliation of MoS₂ nanosheets by use of pre-freezing and thermal shock. The MoS₂ bulk is easily exfoliated and functionalized by prefreezing and thermal shock of MoS₂ powder in the ethanol solvent. Atomic force microscopy confirms that the approach can exfoliate MoS₂ powder to nanosheets. UV–visible spectroscopy of the prepared samples shows that fingerprint excitonic peaks appear in the spectrums and they become sharper by repeating the process. Optical band gap from Tauc plot of UV–visible spectrum shows an increase in the band gap of exfoliated MoS₂ up to 1.85 eV and the surface energy of the exfoliated MoS₂ is measured as 29.8 mJ/m². Annealing the prepared samples at temperatures up to 400 °C decreases the contact angle of water droplet from 130° down to 2°. X-ray diffraction patterns and Fourier transform infrared spectroscopy confirm that exfoliated MoS₂ is functionalized during the exfoliation process and molybdite is formed on the surface by crumpling and agglomerating nanosheets due to heating, which is mainly responsible for increasing the surface energy as well as superhydrophilicity of the samples at 400 °C.