Simultaneous photoacoustic measurements of specific heat and thermal conductivity critical behaviour at a smectic A–nematic phase transition

Abstract

Simultaneous measurements of the thermal conductivity and specific heat at the smectic A-nematic phase transition in 4-n-octyl-4′-cyanobiphenyl (8CB) have been carried out with the photoacoustic technique. A critical increase in the thermal conductivity is reported. A critical decrease in the thermal diffusivity data confirms that the transition in 8CB is fluctuation dominated as also shown by the critical behaviour of the specific heat.     

Static and dynamic thermal quantities near the consolute point of the binary liquid mixture aniline-cyclohexane studied with a photopyroelectric technique and adiabatic scanning calorimetry

We studied the thermal conductivity, thermal effusivity, and specific heat capacity at constant pressure of the critical binary liquid mixture aniline-cyclohexane near the consolute point, using a photopyroelectric (PPE) technique and adiabatic scanning calorimetry (ASC). According to recent theoretical predictions based on renormalization group theory calculations, a substantial (but not diverging) enhancement in the thermal conductivity in the homogeneous phase near the critical temperature was expected for this binary system near the consolute point. However, within an experimental precision of 0.05%, we found no deviation from linear behavior in the range of 5 K above Tc down to Tc. The specific heat capacity calculated from the results for the thermal conductivity and effusivity is in good agreement with that measured by ASC. For the ASC results, the theoretical power law expression with the Ising critical exponent was fitted to the specific heat capacity both above and below the transition temperature. Good agreement with theory was found both for the amplitude ratio and the two-scale universality.     

Photopyroelectric method

High resolution measurements have been performed with a photopyroelectric (PPE) technique to study the static and dynamic thermal parameters behaviour in the critical region of different liquid and solid samples. In particular the specific heat (c), the thermal diffusivity (D) and the thermal conductivity (k) have been simultaneously determined in the critical region around the antiferromagnetic/paramagnetic (AP) phase transition occurring in FeF₂ and around the Smectic-A/Nematic (AN) phase transition occurring in the 8S5 liquid crystal.The high resolution of the measuring technique has allowed the determination of thec andD critical exponents and amplitude ratio of their critical terms.     

Thermal conductivity, thermal effusivity, and specific heat capacity near the lower critical point of the binary liquid mixture n-butoxyethanol-water

Experimental investigations on binary liquid mixtures near the critical mixing point are presently leading to a controversy about the anomaly in the thermal conductivity. A photopyroelectric technique is used to determine the thermal conductivity and the effusivity of the binary liquid mixture n-butoxyethanol-water at its critical concentration near the critical mixing point. It is proven that, contrary to previous reports, there is no critical enhancement in the thermal conductivity. The specific heat capacity is calculated from these results and compared with the results from measurements performed by adiabatic scanning calorimetry.     

酞侧基聚芳醚酮的热学性能

酞侧基聚芳醚酮的热学性能谢红卫，李滨耀（中国科学院长春应用化学研究所长春１３００２２）关键词酞侧基聚芳醚酮，ｐｖＴ行为，导热系数，定压比热容酞侧基聚芳醚酮（ＰＥＫ－Ｃ）具有较高的玻璃化转变温度，其流变加工性能和力学性能已被广泛研究［１～３］，被证明是...     

超临界水临界区域判定方法研究

对超临界水在临界区域进行合理的判定和区域划分,对于深入理解超临界水在临界过渡区域的流动和换热相关特征具有重要的作用。本文分析了超临界水从拟液态区向拟汽态区过渡的过程中,其导热系数、动力粘度、定压比热和膨胀系数等相关参数的变化规律特征,并归纳了已有超临界水在临界区域的划分判定模型。分析结果表明,在临界过渡区域,超临界水的流动特征参数和换热特征参数均会发生一系列连续剧烈的变化;只有同时考虑超临界水的膨胀特性和最大比热特性,才能更加合理地对临界区域进行划分。在此分析基础上,本工作完善了超临界水的三区分析判定模型,得到了新的超临界水在临界区域的判定划分数据,并由此拟合得到了新的超临界水分区边界计算关系式。新的计算关系式的误差范围在±0. 3℃之内,满足计算分析的要求。     

Applicability of the du pont 900 DTA apparatus in quantitative differential thermal analysis

Some problems of quantitative differential thermal analysis have been studied by means of a Du Pont 900 DTA instrument. Applying the equation of heat transfer the features of a thermogram (base line and peak surface) may be interpreted in terms of heat of transition, specific heat and thermal conductivity. Satisfactory results are obtained for substances with thermal conductivities lower than 10^-3 cal/sec deg.     

A combined photoacoustic DSC for simultaneous temperature modulated measurements: does it really work?

Combination of two techniques, photoacoustic (PA) and differential scanning calorimetry (DSC), is a combination of quasi stationary thermodynamic DSC method and nonstationary dynamic PA method. Especially favorable and easy to realize is the combination with power compensated type of DSC. It has several advantages over the use of two techniques separately and allow to perform measurements simultaneously. The most obvious is simultaneous determination of thermodynamic properties such as specific heat, heat of transition and dynamic properties such as effusivity, thermal conductivity at the different phase transitions and complex specific heat at the glass transition. Unlike other temperature modulated techniques PA–DSC is especially suitable for studying polymer materials since their low thermal conductivity is an additional advantage. Conditions for simultaneous measurements are examined. It is proved that the combination of two techniques and necessary changes in construction do not essentially change adequate work of the instrument. A little disbalance of DSC operation due to the construction change can be corrected simply by recalibration. The procedures for testing and calibration for the proper operation of the combined PA–DSC are given together with some details of experimental methodology. Several measurements could serve as examples of widespread applicability of PA–DSC to study different types of phase transitions as well as time dependent processes such as glass transition.     

Composite materials based on zeolite 4A for adsorption heat pumps

Some additives and binders were chosen for the preparation of 4A-zeolite-based composites with high equivalent thermal conductivity for heat pumps application. Additives (SiC, Si₃N₄, graphite) and binders (PTFE, Al(OH)₃) were tested for their effectiveness in terms of equivalent thermal conductivity and maximum water adsorption capacity of the composites. The influence of the equivalent thermal conductivity of the composite adsorbents on the specific power of the heat pump was also calculated. Results show a significant improvement in the equivalent thermal conductivity of the composite samples which are prepared using aluminum hydroxide as binder, over that of zeolite pellet beds. Such composite materials could be used to build adsorption heat pumps with higher specific power and, consequently, with lower investment cost.     

Measuring the thermal conductivity of heat transfer fluids via the modified transient plane source (MTPS)

Heat transfer fluids are often a critical performance component in industrial processes and system design. Fluids are used in heat dissipation to maintain stable operating temperatures in a variety of applications, such as diesel engines, chemical production, asphalt storage, and high-power electric transformers. A wide range of fluids specific to various applications are available, thus a reliable and accurate thermal conductivity characterization is extremely important. Thermal conductivity analysis of heat transfer fluids with traditional methods is time-consuming and error-prone due to the impact of convection. Convection often distorts effective thermal conductivity measurement as an additional source of heat transfer. The modified transient plane source method implemented in the C-Therm Technologies TCi Analyzer provides an easy way to accurately measure the thermal conductivity and distinguish this form of heat transfer in negating the impact of convection by (a) employing the shortest test time in commercially available sensors (0.8 s), (b) offering a minimal sample volume requirement (1.25 mL), and (c) employing a low-energy power flux to the specimen under test (approximately 2,600 W m^?2). This work presents thermal conductivity results generated on three types of heat transfer fluids over a wide temperature range and discusses the significance of the data in relevance to the application.     

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.     

Critical behavior of the disorder-induced Raman modes close to the lambda-phase transition in NH4Cl

This study gives our analysis of the disorder-induced Raman modes of v7 (93 cm(-1)) and v5 (144 cm(-1)) close to the lambda-phase transition (P = 0) in the NH4Cl crystal. We describe the critical behavior of the frequency shifts for those modes by a power-law formula and using our Raman data we obtain the value of 0.1 for the critical exponent which can also describe the critical behavior of the thermal expansivity and the specific heat of NH4Cl close to its lambda-phase transition (Tlambda = 242.9 K).     

石蜡/膨胀石墨/石墨烯复合相变储热材料的制备及性能

以石蜡(PA)作为相变储热材料、 膨胀石墨(EG)作为主导热材料和支撑材料, 石墨烯气凝胶(GA)作为导热增强材料和辅支撑材料制备了PA/EG/GA复合相变材料, 研究了GA添加量对复合相变材料相变温度、 相变潜热、 导热性能以及循环稳定性的影响. 结果表明, 所制备的80%PA-17%EG-3%GA复合相变材料导热性能良好, 循环稳定性出色. 与80%PA-20%EG复合材料相比, 该材料的相变温度、 相变潜热以及循环稳定性无明显变化, 但导热系数由4.089 W/(m·K)提升到了5.336 W/(m·K), 显示出良好的应用前景.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.     

A non-isothermal activated kinetics of K-SmA transition of the aligned octylcyanobiphenyl

This study explores a non-isothermal activated kinetics of the Crystalline to Smectic A (K-SmA) transition of the aligned octylcyanobiphenyl (8CB) liquid crystal. High resolution calorimetric technique was used to study the molecular motion and rearrangement of the 8CB molecules near the K-SmA transition as a function of temperature, rate, and time. In the presence of magnetic field, the transition peak was found to be shifted towards lower temperature by 0.5?K when compared with results of un-magnetized 8CB. The K-SmA transition showed a rate dependent kinetics following Arrhenius behavior where the increased shifting rate showing an increased thermal kinetics for the transition. The 8CB molecules get more aligned and more ordered that pushes the temperature of the transition towards lower temperature in the presence of magnetic field. Hence they show a temperature decrease in the peak of the transition temperature with a decrease in the enthalpy and hence needs more activation energy. This study may be useful to understand the liquid crystal behavior to upgrade liquid crystal devices (LCDs).     

Ionic liquid-based stable nanofluids containing gold nanoparticles

A one-phase and/or two-phase method were used to prepare the stable ionic liquid-based nanofluids containing same volume fraction but different sizes or surface states of gold nanoparticles (Au NPs) and their thermal conductivities were investigated in more detail. Five significant experiment parameters, i.e. temperature, dispersion condition, particle size and surface state, and viscosity of base liquid, were evaluated to supply experimental explanations for heat transport mechanisms. The conspicuously temperature-dependent and greatly enhanced thermal conductivity under high temperatures verify that Brownian motion should be one key effect factor in the heat transport processes of ionic liquid-based gold nanofluids. While the positive influences of proper aggregation and the optimized particle size on their thermal conductivity enhancements under some specific conditions demonstrate that clustering may be another critical effect factor in heat transport processes. Moreover, the remarkable difference of the thermal conductivity enhancements of the nanofluids containing Au NPs with different surface states could be attributed to the surface state which has a strong correlation with not only Brownian motion but also clustering. Whilst the close relationship between their thermal conductivity enhancements and the viscosity of base liquid further indicate Brownian motion must occupy the leading position among various influencing factors. Finally, a promisingly synergistic effect of Brownian motion and clustering based on experimental clues and theoretical analyses was first proposed, justifying different mechanisms are sure related. The results may shed lights on comprehensive understanding of heat transport mechanisms in nanofluids.     

Effect of carbon black type and concentration on the performance of semiconductive shielding material of high-voltage cable

Semiconductive shielding layer as an important part of high-voltage cable, its performance directly affects the safe operation and the service life of the cable. Carbon black (CB) is the main conductive filler of shielding materials, and its type and concentration directly affect the performance of the shielding layer. In this paper, CB-A with higher structure and CB-B with lower structure were used as conductive fillers and EBA was chosen as the matrix resin to prepare the shielding materials. The CB concentrations of the shielding materials were 35, 45, and 55 phr. The influences of CB type and concentration on the physicochemical, electrical, thermal and mechanical properties of the shielding materials were investigated. The research shows that when the CB types are the same, the higher the concentration of CB, the more intensive the CB network in the shielding material, and the more serious the CB agglomeration phenomenon. With increasing CB concentration, shielding materials show a decreasing trend of volume resistivity, an increasing trend of thermal conductivity, and a decreasing trend of mechanical properties. When the CB concentration is the equal, the CB-A has better dispersion in the matrix resin, CB-A/EBA shielding material has lower volume resistivity and weaker PTC effect, CB-A/EBA shielding material has higher thermal conductivity at low temperature and CB-B/EBA shielding material has higher thermal conductivity at high temperature, CB-A/EBA shielding material has better mechanical properties. A comprehensive comparison shows that CB-A/EBA shielding material with a concentration of 45 phr has excellent overall performance, with volume resistivity of 15.3 and 68 Ω·cm at 25°C and 90°C, respectively. The thermal conductivity is 0.434 W/(m K) at room temperature and 0.536 W/(m K) at 90°C. The stress is 31.08 MPa and the strain is 570.2%. This work has important reference for the selection of conductive fillers and performance improvement of semiconductive shielding materials.     

Development of a thermal conductivity cell with nanolayer coating for thermal conductivity measurement of fluids

Various techniques and methodologies of thermal conductivity measurement have been based on the determination of the rate of directional heat flow through a material having a unit temperature differential between its opposing faces. The constancy of the rate depends on the material density, its thermal resistance and the heat flow path itself. The last of these variables contributes most significantly to the true value of steady-state axial and radial heat dissipation depending on the magnitude of transient thermal diffusivity along these directions. The transient hot-wire technique is broadly used for absolute measurements of the thermal conductivity of fluids. Refinement of this method has resulted in a capability for accurate and simultaneous measurement of both thermal conductivity and thermal diffusivity together with the determination of the specific heat. However, these measurements, especially those for the thermal diffusivity, may be significantly influenced by fluid radiation. Recently developed corrections have been used to examine this assumption and rectify the influence of even weak fluid radiation. A thermal conductivity cell for measurement of the thermal properties of electrically conducting fluids has been developed and discussed.     

Existence of time lag in crystalline to smectic A (K–SmA) phase transition of 4-decyl-4-biphenylcarbonitrile (10CB) liquid crystal

This study explores the presence of time lag in crystalline to smectic A (K–SmA) phase transition of 4-decyl-4-biphenylcarbonitrile (10CB) liquid crystal. A non-isothermal heating and cooling study were performed for 10CB liquid crystal using calorimetric technique where heating scan was performed from 250 to 350 K, and cooling scan was performed from 350 to 250 K. A clear difference in K–SmA phase transition was observed between heating and cooling scans. An attracting inclination effect in K–SmA transition was observed on cooling which is completely absent on heating. The inclination of the K–SmA transition peak increases and shows an existence of time lag during cooling, whereas other family member shows no effect i.e., 8CB. K–SmA peak shows a lower enthalpy with higher activation when compared with 8CB. The presence of time lag and increase in activation can be explained in terms of the density and nature of the material.