Dimensional and ice content changes of hardened concrete at different freezing and thawing temperatures

Samples of concrete at different water-to-cement ratios and air contents subjected to freeze/thaw cycles with the lowest temperature at about ?80 °C are investigated. By adopting a novel technique, a scanning calorimeter is used to obtain data from which the ice contents at different freeze temperatures can be calculated. The length change caused by temperature and ice content changes during test is measured by a separate experiment using the same types of freeze–thaw cycles as in the calorimetric tests. In this way it was possible to compare the amount of formed ice at different temperatures and the corresponding measured length changes. The development of cracks in the material structure was indicated by an ultra-sonic technique by measuring on the samples before and after the freeze–thaw tests. Further the air void structure was investigated using a microscopic technique in which air ‘bubble’ size distributions and the so-called spacing factor, indicating the mean distance between air bubbles, were measured. By analyzing the experimental result, it is concluded that damages occur in the temperature range of about ?10 °C to ?55 °C, when the air content is lower than about 4% of the total volume. For a totally water-saturated concrete, damages always occur independently of the use of entrained air or low water-to-cement ratios. It is, further, concluded that the length changes of these samples correspond to the calculated ice contents at different temperatures in a linear fashion.     

Entropy generation theory for characterizing the freezing and thawing injury of biological materials

As an important approach for long-term preservation of biological material, cryopreservation has been widely studied and applied in clinics. However, one of the most critical issues involved–the injury of biology material induced during freezing and thawing process still remains to be incompletely answered due to complexity of the problem itself. In this paper, we proposed for the first time to interpret the freezing or thermal injury by using the irreversible thermodynamics theory which is generalized in concept and has wide applicability. Comprehensive entropy generation analysis was performed on several typical freezing/thawing processes of biological tissues subject to cryopreservation. Particularly, variation of entropy generation rate due to injury induced change of thermal properties of the biological materials was investigated. Several useful indexes were suggested to quantify the freezing injury or viability of the biological material, through a combination with certain specific measurements. This study may possibly open a new theoretical strategy for evaluating the final output of either cryobiology or hyperthermia practices.     

不同冻融介质作用下混凝土力学性能衰减模型

通过快速冻融试验,研究了三种不同冻融介质(水、3.5wt%NaCl、飞机除冰液)对混凝土质量损失、动弹模量以及力学性能的影响,比较了三种冻融介质对混凝土损伤程度的大小,分析了混凝土相对动弹性模量与相对剩余抗压强度和相对剩余抗折强度之间的关系,基于相对动弹性模量建立了相对剩余抗压强度和相对剩余抗折强度衰减方程。结果表明:3.5wt%NaCl溶液对混凝土的损伤度要远大于单纯水冻融循环对混凝土的损伤度,飞机除冰液对混凝土冻融损伤具有抑制作用;混凝土抗压、抗折强度以及相对动弹性模量随着冻融循环次数的增加而降低;三种冻融介质下混凝土抗压、抗折强度损失率大小关系为:3.5wt%NaCl水飞机除冰液;相对动弹性模量与相对剩余抗压强度、相对剩余抗折强度相关性好,可以通过测定混凝土相对动弹性模量来评估混凝土相对剩余强度。     

Chemical changes in powder reference materials examined by accelerated ageing methods

A method is proposed for planning and executing research involving accelerated ageing in studying the chemical stability of powder reference materials (RM) for spectral analysis. It is necessary to perform a series of experiments having various degrees of extremity in the instability factor in order to draw correct conclusions on the stability. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 64–66, November, 1999.     

On the propagation speed of heat in materials without memory

The propagation speed of thermal disturbances for a large class of heat-condicting materials without memory is studied. The results are compared with those recently obtained by F. L. Roetman.     

Damage evaluation in freezing and thawing test of concrete by elastic-wave methods

In the freezing and thawing test of concrete, elastic-wave methods are applied to estimate the frost damage nondestructively. Recently, two test methods are proposed in the RILEM recommendation. The alternative method A is based on the ultrasonic test, which measures the transmission time of the longitudinal wave (P wave), while the alternative method B measures the resonant frequencies of vibrations. With relation to these methods, evaluation of the frost damage by the elastic-wave methods is studied, focusing on the dynamic modulus of elasticity. The freezing and thawing tests were conducted by employing concrete samples after 1 year curing. As a result, the samples were not heavily damaged even after 300 cycles. Concerning the resonant frequencies of concrete samples, a three-dimensional analysis is performed by the boundary element method to identify actual vibration modes. It is clarified that an assumption of the one-dimensional resonant vibration is not applicable, resulting in a false discrepancy between the dynamic modulus and the static modulus of elasticity. It is found that relative modulus obtained from P-wave velocity is comparable to that from the tangential modulus of elasticity in the compression test, and is reasonably recommended to estimate the frost damage nondestructively.     

Thresholds for nonlinear effects in high- intensity focused ultrasound propagation and tissue heating

For a variety of reasons, including their simplicity and ability to capitalize upon superposition, linear acoustic propagation models are preferable to nonlinear ones in modeling the propagation of high-intensity focused ultrasound (HIFU) beams. However, under certain conditions, nonlinear models are necessary to accurately model the beam propagation and heating. In analyzing the performance of a HIFU system, it is advantageous to know before the analysis whether a linear model suffices. This paper examines the problem of determining the thresholds at which nonlinear effects become important. It is demonstrated that nonlinear interaction has different effects on different physical and derived quantities, such as compressional pressure, rarefactional pressure, intensity, heat rate, temperature rise, and thermal lesion volume. Thresholds are determined as a function of the dimensionless gain, nonlinearity, and absorption parameters. The relative difference between linear and nonlinear predictions is plotted as a series of contours, enabling practitioners to locate their system in parameter space and determine whether nonlinearity significantly affects the quantities of interest.     

Prediction of freezing and thawing times for multi-dimensional shapes by numerical methods

Testing the accuracy of freezing and thawing time prediction methods requires accurate experimental data. To complement existing data, 175 experimental measurements, 68 for thawing of rectangular bricks and 107 for both freezing and thawing of 12 different multi-dimensional irregular shapes, were made using Tylose, a food analogue, over a wide range of conditions. Twelve additional experiments were conducted using an actual food material, minched lean beef. Details of all the experimental conditions are reported.     

Prediction of rates of freezing, thawing or cooling in solids or arbitrary shape using the finite element method

The finite element method was formulated for the solution of three dimensional heat transfer problems in solids of arbitrary geometry with variable thermal properties. These variable properties can include latent heat effects. The accuracy of the method was tested against available analytical solutions and against experimental data for freezing of regular shapes. The method gave comparable predictions to finite difference methods for similar problems and precision was therefore more limited by the data uncertainties than by the numerical approximations inherent in the method. A simplified formulation investigated gave only slightly reduced accuracy yet much shorter computation times than the general formulation. The computer codes for both formulations are available from the authors.The flexibility of the method allows application to a wide variety of situations. For example, rates of chilling, freezing or thawing food or other solids of any shape under any type of external heat transfer environment can be predicted, heterogeneous composition can be taken into account in these predictions, and simultaneous heat and mass transfer can be considered.     

Pore pressure and consolidation of saturated silty clay induced by progressively heating/cooling

The thermal pore pressure and consolidation of a saturated silty clay are experimentally studied. The specimen was heated progressively from the room temperature of 25 °C to 85 °C (T = 25 °C → 35 °C → 45 °C … → 85 °C), and then cooled from 85 °C to 25 °C (T = 85 °C → 75 °C → 65 °C … → 25 °C), with (case 1) or without (case 2) drainage between temperature stages at four confining pressures (50, 100, 150 and 200 kPa, respectively). It shows that during the undrained heating/cooling, the pore pressure reaches a peak at the time when the specimen is heated to the required temperature and then decreases slightly, and it declines to a minimum at the time when the specimen is cooled to the required temperature and then increases slightly. Such phenomenon can be attributed to the adjustment of soil structure induced by the interaction of solid grains with pore water. On the other hand, during the drained heating/cooling, the maximum pore pressure increases with the increase of temperature stages, and the normalized negative pore pressures show a decreasing trend with the decrease of temperature stages. This appears to be especially obvious as the applied confining pressures decrease. It is also observed that consolidation volumetric strains during the isothermal drainage increase significantly with the increase of temperature stage, and the reduction in the volumetric strains due to absorption after cooling shows a decreasing trend with the decrease of temperature stage.     

Properties of hydrogels synthesized by freezing and thawing aqueous polyvinyl alcohol solutions and their applications

The hydrogel synthesized by freezing a polyvinyl alcohol (PVA) aqueous solution and thawing it slowly has high water content, excellent mechanical properties of high tensile strength, elongation and good shape recovery by elasticity. The PVA used had a degree of polymerization of 2500 and a degree of saponification of 99.5 mol%. The solution was obtained by dissolving 7.5 g of PVA in 80 g of water, this was frozen at –50°C for 3 h and then warmed up to room temperature over 10h. This freezing–thawing process was repeated once again and a hydrogel was synthesized. The hydrogel had a water content of about 90 wt%. Its tensile strength was 0.6 MPa and the elongation at break was 130%. The shape of the hydrogel which was deformed by an external force recovered in a short time when it was released from the force. This recovery had good persistence and repeatability. Applying these properties a strain sensor and a gas pressure sensor were tested. Furthermore, a PVA hydrogel rod containing polyacrylic acid was used as a bending actuator. This hydrogel had the ability to deform when direct current was applied.     

One-dimensional wave propagation in materials with different moduli in tension and compression

One-dimensional wave propagation in compressible and incompressible elastic materials behaving differently in tension and compression is investigated. The constitutive equations of these materials are nonlinear even though small deformations are considered. The characteristic wavespeeds are derived, the hyperbolicity condition is investigated, and analytical simple wave solutions are obtained in a compressible and incompressible semi-infinite half-space. The presented solutions exhibit interesting phenomena of wave propagation, like that of a coupled shear-normal plane wave propagating steadily with a constant velocity in a compressible medium with different moduli in tension and compression.     

共沉淀-微波法合成LiFePO4/C正极材料

以有机表面活性剂聚乙二醇(PEG)为碳源,采用共沉淀-微波法合成了锂离子电池正极材料LiFePO4/C,探讨了微波烧结时间对样品结构和性能的影响,并用XRD、TEM、激光粒度分析和恒电流充放电测试对LiFePO4/C样品的结构、形貌和电化学性能进行了表征.结果表明:微波烧结9 min的样品为单一的橄榄石晶体结构和较好的电化学性能,在室温下,以0.1C、0.2C和1C进行充放电,首次放电比容量分别达到154.3mAh/g、139.7mAh/g和123.9mAh/g,循环20次后仍保持在152.3mAh/g、134.3mAh/g和118.5mAh/g.     

Molecular dynamics investigations of structural changes accompanying with freezing a molten Cu135 cluster on cooling

We use molecular dynamics (MD) simulations within the framework of the embedded-atom method (EAM) to investigate structural changes during freezing a molten Cu cluster containing 135 atoms. The simulations show how the structural changes can strongly cause internal energy to change accordingly, and reveal that continuous interchange positions of atoms are the key in the formation of the icosahedral (Ih)-like Cu₁₃₅ cluster. By using visual inspection on atomic packing according to atomic density profiles, we analyze crystallization processes during freezing the molten cluster. At the initial stage of the freezing, one atom moves into the center of this cluster. Then the packing in interior atoms is changed into an ordered Ih structure, while outer atoms are becoming locally ordered with a fivefold Ih symmetry. Subsequently, nanocrystallization at lower temperatures propagates outward from the interior Ih structure, leading to the Ih-like cluster.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae part 2: irregular shapes

Calculated and experimental data for multi-dimensional irregular shapes wer used to assess various methodologies to include the effect of shape in empirical freezing and thawing time prediction methods. The principles underlying two existing geometric factors, EHTD and MCP, were found to be valid; so there seems to be no need for other approaches. Used in conjunction with accurate slab freezing and thawing time prediction methods, the proposed empirical formulae for EHTD and MCP gave accurate predictions for all of the two-dimensional shapes and most of the three-dimensional shapes tested, except those with oval cross-sections in the third dimension. This was attributed to the lack of data for this group of shapes.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae Part 1: regular shapes

Numerical prediction methods were used to generate data to assess and develop geometric factors taking account of the effect of product geometry on freezing and thawing time. Improved empirical formulae for two existing geometric factors were developed; these depend only on the Biot number and parameters that describe object shape. The new formulae are accurate for both freezing and thawing of an extended range of regular multi-dimensional shapes and for a wider range of conditions than the original formulae. Used in conjunction with accurate slab freezing and thawing time prediction formulae, the improved geometric factors accurately predicted a large set of experimental freezing and thawing times for various shapes. As the improved geometric factors are both accurate and generally applicable there is no need for shape-specific freezing and thawing time prediction formulae to be developed.     

地源水-水热泵冬夏暖冷联供实验研究

介绍了地源水-水热泵全年冬夏暖冷联供的实验结果,分析了系统流量对热泵性能参数的影响,获得了最佳运行流量为1000(L/h)。并实测得到单位管深换热率及埋管的有关性能指标。对比分析了冬季吸热曲线和夏季放热曲线。     

Peculiarities of the nonequilibrium distribution of moisture in the freezing and thawing of disperse soils

The article presents the results of numerical simulation of the process of moisture migration in disperse soils with different regimes of freezing and with a view to the nonequilibrium effects accompanying the phase transformations of water. The results of simulation are compared with data of experimental investigations from the literature.