Effect of cooling rates on dendrite spacings of directionally solidified DZ125 alloy under high thermal gradient

The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfme dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings 21 and the cooling rate Vc is λ1 = 0.013 Vc^-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2 = 0.00258 Vc^-0.31, which gives good agreement with the Feurer/Wunderlin model.

Triaxial compression strength for artificial frozen clay with thermal gradient

A series of triaxial compression tests for frozen clay were performed by KoDCGF （freezing with non-uniform temperature under loading after K0 consolidation） method and GFC （freezing with non-uniform temperature without experiencing Ko consolidation） method at various confining pressures and thermal gradients. The experimental results indicate that the triaxial compression strength for frozen clay in KoDCGF test increases with the increase of confining pressure, but it decreases as the confining pressure increases further in GFC test. In other words, the compression strength for frozen clay with identical confining pressure decreases with the increase in thermal gradient both in KoDCGF test and GFC test. The strength of frozen clay in KoDCGF test is dependent of pore ice strength, soil particle strength and interaction between soil skeleton and pore ice. The decrease of water content and distance between soil particles leads to the decrease of pore size and the increase of contact area between particles in KoDCGF test, which further results in a higher compression strength than that in GFC test. The compression strength for frozen clay with thermal gradient can be descried by strength for frozen clay with a uniform temperature identical to the temperature at the height of specimen where the maximum tensile stress appears.

A Methodology to Reduce Thermal Gradients Due to the Exothermic Reactions in Resin Transfer Molding Applications

Resin transfer molding(RTM)is among the most used manufacturing processes for composite parts.Initially,the resin cure is initiated by heat supply to the mold.The supplementary heat generated during the reaction can cause thermal gradients in the composite,potentially leading to undesired residual stresses which can cause shrinkage and warpage.In the present numerical study of these processes,a one-dimensional finite difference method is used to predict the temperature evolution and the degree of cure in the course of the resin polymerization;the effect of some parameters on the thermal gradient is then analyzed,namely:the fiber nature,the use of multiple layers of reinforcement with different thermal properties and also the temperature cycle variation.The validity of this numerical model is tested by comparison with experimental and numerical results in the existing literature.

What happens to the initial planar instability when the thermal gradient is increased during directional solidification?

The positive thermal gradient is one of the most important parameters during directional solidification. The increase of the thermal gradient usually stabilizes the planar interface in the steady state analysis. However, in the initial transient range of planar instability, the thermal gradient presents complicated effects. Time-dependent analysis shows that the increase of the thermal gradient can enhance both the stabilizing effects and the destabilizing effects on a planar interface. The incubation time first decreases and then increases with the increase of the thermal gradient. Moreover, the initial average wavelength always increases with the thermal gradient increasing, contrary to the effect of the thermal gradient on the steady cellular/dendritic spacing. This reveals the types of spacing adjustment after planar instability.

Geothermal Gradient and Heat Flow in the Nigeria Sector of the Chad Basin, Nigeria

Information on geothermal gradient and heat flow within the subsurface is critical in the quest for geothermal energy exploration. In a bid to ascertain the thermal potential of Nigeria sector of the Chad Basin for energy generation, subsurface temperature information from 19 oil wells, 24 water boreholes drilled to depths beyond 100 metres and atmospheric temperature from the Chad basin were utilized in calculating geothermal gradient of the area. Selected ditch cuttings from the wells were subjected to thermal conductivity test using Thermal Conductivity Scanner (TCS) at the Polish Geological Institute Laboratory in Warsaw. The terrestrial heat flow was calculated according to the Fourier’s law as a simple product of the geothermal gradient and the mean thermal conductivity. Results obtained indicated geothermal gradient range of 2.81 °C/100 m to 5.88 °C/100 m with an average of 3.71 °C/100 m. The thermal conductivity values from the different representative samples range from 0.58 W/m*K to 4.207 W/m*K with an average of 1.626 W/m*K. The work presented a heat flow value ranging from 45 mW/m2 to about 90 mW/m2 in the Nigerian sector of the Chad Basin.

Study the Thermal Gradient Effect on Frequencies of a Trapezoidal Plate of Linearly Varying Thickness

In this paper, effect of thermal gradient on vibration of trapezoidal plate of varying thickness is studied. Thermal effect and thickness variation is taken as linearly in x-direction. Rayleigh Ritz technique is used to calculate the fundamental frequencies. The frequencies corresponding to the first two modes of vibrations are obtained for a trapezoidal plate for different values of taper constant, thermal gradient and aspect ratio. Results are presented in graphical form.

Study the Effect of Thermal Gradient on Transverse Vibration of Non-Homogeneous Orthotropic Trapezoidal Plate of Parabolically Varying Thickness

The present paper deals with the effect of linearly temperature on transverse vibration of non-homogeneous orthotropic trapezoidal plate of parabolically varying thickness. The deflection function is defined by the product of the equations of the prescribed continuous piecewise boundary shape. The non homogeneity of the plate is characterized by taking linear variation of the Young's modulus and parabolically variation of the density of the material. The non homogeneity is assumed to arise due to the variation in the density of the plate material and it is taken as parabolically. Rayleigh Ritz method is used to evaluate the fundamental frequencies. The equations of motion, governing the transverse vibrations of orthotropic trapezoidal plates, are derived with boundary condition clamped-simply supported-clamped-simply supported. Frequencies corresponding to first two modes of vibration are calculated for the trapezoidal plate for various combinations of the parameters of the non-homogeneity, thermal gradient, taper constant and for different values of the aspect ratios and shown by figures. All The results presented here are entirely new and are not found elsewhere. Comparison can only be made for homogeneous plates, and in that cases the results have been compared with those found in the existing literatures and are in excellent agreement.

Ionic Power Generation on a Scalable Cellulose@polypyrrole Membrane:The Role of Water and Thermal Gradients

The integration of ionic power generation with solar-driven water evaporation presents a promising solution to the critical global problems of freshwater scarcity and clean energy deficiency.In this work,a scalable normal temperature chemical vapor deposition(CVD)method is applied for the first time to the fabrication of a cellulose@polypyrrole(CC@PPy)membrane with efficient ionic power generation performance.The excellent ionic power generation is intimately related to the water and thermal gradients across the membrane,which not only induces fast water evaporation but also synergistically promotes the transport of counterions in charged nanochannels,and the corresponding mechanism is attributed to the streaming potential resulting from the ionic electrokinetic effect and the ionic thermoelectric potential originating from the Soret effect.Under one sun illumination,the CC@PPy film can produce a sustained voltage output of~0.7 V and a water evaporation rate up to 1.67 kg m^(−2)h^(−1)when an adequate water supply is available.This study provides new methods for the scalable fabrication of ionic power generation membranes and a design strategy for high-performance solar power generators.

Effects of Thermal Gradients and Rotational Flows on Grain Growth in 22 t Steel Ingots

Heavy ingots are widely used in many industrial fields. The coarse grains formed during the process of in- got solidification influence the properties and fracture behaviors of the final products. The coarse grain growth was simulated under different thermal gradients. A 30Cr2Ni4MoV steel ingot was melted in a cubic crucible with dimen-sions of 15 cm×10 cm×23 cm, and the cooling conditions on each side of the crucible were controlled by different thermal curves. The influences of thermal gradients and rotational flows on grain growth in heavy steel ingots were then investigated both numerically and experimentally. The results showed that when the amplitude of the rotation angle was 60°, the metal was solidified under a reciprocating horizontal rotational condition when the angular velocity was 10 （°）/s or 20 （°）/s. As the thermal gradient increased, the lengths of the primary columnar grains in- creased, and the diameters of equiaxed grains decreased. When the direction of flow rotation was perpendicular to the direction of grain growth, the columnar grain zone was nearly eliminated, and the average diameter of equiaxed grains was 0.5 mm.

Oxidation and Hot Corrosion of Gradient Thermal Barrier Coatings Prepared by EB-PVD

The performances of gradient thermal barrier coatings (GTBCs) produced by EB-PVD were evaluated by isothermal oxidation and cyclic hot corrosion (HTHC) tests. Compared with conventional two-layered TBCs, the GTBCs exhibite better resistance to not only oxidation but also hot-corrosion. A dense Al2O3 layer in the GTBCs effectively prohibites inward diffusion of O and S and outward diffusion of Al and Cr during the tests. On the other hand, an "inlaid" interface, resulting from oxidation of the Al along the columnar grains of the bond coat, enhances the adherence of AI2O3 layer. Failure of the GTBC finally occurred by cracking at the interface between the bond coat and AI2O3 layer, due to the combined effect of sulfidation of the bond coat and thermal cvcling.

FEM ANALYSIS OF THERMAL STRESSES IN GRADIENT THERMAL BARRIER COATINGS PRODUCED BY EB-PVD

Gradient thermal barrier coatings (GTBCs) produced by co-deposition of mixtures ofAl-Al2 O3-YSZ onto metallic bond coat exhibited longer lifetimes than the two-layeredTBCs. The finite element method (FEM) numerical models were used to investigatestress and strain states in the GTBCs and traditional two-layered TBCs as they cooledto 750℃ from a stress-free state at 850℃.

Finite Element Analysis of Thermal Stresses in Ceramic/Metal Gradient Thermal Barrier Coatings

This paper studied the thermal stresses of ceramicl metal gradient thermal barrier coating which combines the conceptions of ceramic thermal barrier coating （TBC） and functionally gradient material （FGM）. Thermal stresses and residual thermal stresses were calculated by an ANSYS finite element analysis software. Negative thermal expansion coefficient method was proposed and element birth and death method was applied to analyze the residual thermal stresses which have non-uniform initial temperature field. The numerical results show a good agreement with the analytical results and the experimental results.

Electrical, Thermal, and Mechanical Properties of Cu/Ti3AlC2 Functional Gradient Materials Prepared by Low-temperature Spark Plasma Sintering

Cu/Ti3AlC2 composite and functional-gradient materials with excellent electrical conductivity and thermal conductivity as well as good flexural properties were prepared by low-temperature spark plasma sintering of Cu and Ti3AlC2 powder mixtures. The phase compositions of the materials were analyzed by X-ray diffraction, and their microstructure was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. Further, the electrical conductivity, thermal conductivity, and flexural properties of the materials were tested. Results show that, for the composite materials, the resistivity rises from 0.75 × 10^-7 Ω·m only to 1.32 × 10^-7 Ω·m and the thermal diffusivity reduces from 82.5 mm^2/s simply to 39.8 mm^2/s, while the flexural strength improves from 412.9 MPa to 471.3 MPa, as the content of Ti3AlC2 is increased from 5 wt%to 25 wt%. Additionally, the functional-gradient materials sintered without interface between the layers exhibit good designability, and their overall electrical conductivity, thermal conductivity, and flexural strength are all higher than those of the corresponding uniform composite material.

Genetic Algorithm for the Thermal Stresses Optimum Design ofFunctionally Gradient Material Plate

Based on the thermal stress distribution for functionally gradient material (FGM) plates, a Genetic Algorithm (GA) method for the thermal stresses optimum design of FGM plate with computer technologies is given. The minimum thermal stresses combination distribution for FGM is obtained.

Coherent gradient sensing method for measuring thermal stress field of thermal barrier coating structures

Coherent gradient sensing （CGS） method can be used to measure the slope of a reflective surface, and has the merits of full-field, non-contact, and real-time measurement. In this study, the thermal stress field of thermal barrier coating （TBC） structures is measured by CGS method. Two kinds of powders were sprayed onto Ni-based alloy using a plasma spraying method to obtain two groups of film-substrate specimens. The specimens were then heated with an oxy-acetylene flame. The resulting thermal mismatch between the film and substrate led to out-of-plane deformation of the specimen. The deformation was measured by the reflective CGS method and the thermal stress field of the structure was obtained through calibration with the help of finite element analysis. Both the experiment and numerical results showed that the thermal stress field of TBC structures can be successfully measured by CGS method.

Thermal Shock Property of Al/Ni-ZrO_2 Gradient Thennal Barrier Coatings

Al/Ni-ZrO2 gradient thennal barrier coatings are made on aluminum substrate using plasma spraying method and one direction thermal shock properties of the coatings are studied in this paper. The results show that pores in coatings link to form cracks vertical to coating surface. They go through the whole ZrO2 coating once vertical cracks form. When thermal shock cycles increase, horizontal cracks that result in coatings failure forms in the coatings and interface. And vertical cracks delay appearance of horizontal cracks and enhance thermal shock property of coatings. Failure mechanisms of coating thermal shock are discussed using experiments and finite element method.

THERMAL STRESSES ANALYSIS OF CERAMIC/METAL FUNCTIONALLY GRADIENT MATERIAL CYLINDER

In this paper, the composite cylinder system is made of three layers: metal, functionally gradient material (FGM) and ceramics is studied. The formulas of the steady_state temperature distribution and the associate thermal stress distribution in the cylinder are obtained. For ZrO 2/Ti_6Al_4V system, the distribution of steady_state temperature and thermal stress are calculated and discussed.

Relationship Between Thermal Shock Behavior and Cutting Performance of a Functionally Gradient Ceramic Tool

Based on the deep understanding of the requirements of cutting conditions on ceramic tools, a design model for functionally gradient ceramic tool materials with symmetrical composition distribution was presented in this paper, according to which an Al 2O 3-TiC functionally gradient ceramic tool material FG-1 was synthesized by powder-laminating and uniaxially hot-pressing technique. The thermal shock resistance of the Al 2O 3-TiC functionally gradient ceramics FG-1 was evaluated by water quenching and subsequent three-point bending tests of flexural strength diminution. Comparisons were made with results from parallel experiments conducted using a homogeneous Al 2O 3-TiC ceramics. Functionally gradient ceramics exhibited higher retained strength under all thermal shock temperature differences compared to homogeneous ceramics, indicating the higher thermal shock resistance. The experimental results were supported by the calculation of transient thermal stress field. The cutting performance of the Al 2O 3-TiC functionally gradient ceramic tool FG-1 was also investigated in rough turning the cylindrical surface of exhaust valve of diesel engine in comparison with that of a common Al 2O 3-TiC ceramic tool LT55. The results indicated that the tool life of FG-1 increased by 50 percent over that of LT55. Tool life of LT55 was mainly controlled by thermal shock cracking which was accompanied by mechanical shock. While tool life of FG-1 was mainly controlled by mechanical fatigue crack extension rather than thermal shock cracking, revealing the less thermal shock susceptibility of functionally gradient ceramics than that of common ceramics.

基于高热梯度目标的多通道FDM喷头的结构优化

熔融沉积式3D打印技术通常使用单喷嘴打印,这就限制了打印效率。提出一种线阵式多通道喷头设计以提升效率,但面临散热难而导致材料热梯度过低并引发软化和堵塞的难题,因此热梯度成为影响打印过程的关键。为解决此问题,首先进行热传导和散热特性分析,采用有限元数值仿真方法深入研究了喷头内部材料的不良温度分布及低热梯度现象。仿真研究表明,由于进丝通道布置在体积庞大的打印头内很难散热,从丝材的下端到进料喉管的上端的温度差即温度梯度太小,在进料的位置丝材的温度超过了软化温度。为此,设计了开放式扇形散热结构并集成风扇散热。仿真结果表明,通过优化喷头结构和实施强制散热措施,可以显著改善材料的温度梯度。最后,通过热成像仪采集喷头表面的温度数据生成温度分布图,验证了所采用的热管理机制有效提升了材料的热梯度,成功防止了材料提前熔融和喉管堵塞问题,确保了3D打印的质量与效率。

INJECTION VOLUME CONTROL BY THERMAL WAY IN TRANSGENIC DNA MICRO-INJECTION SYSTEM

Nowdays there are several manual or half-automatic methods developed to drivethe DNA micro-fluid of transgenic micro-injection and they often fail to control precisely theinjection volume at picolitres level. Micro-size of the injector tip and viscosity of the DNA liquidalso lead to dead area of volume control. An adequate way is presented utilizing temperaturegradients to direct liquid flow in the pipette from the wanner to the cooler. Compared with theprevious ones, this way is helpful in decreasing the dead area of controlling through decreasing theviscous rate of DNA liquid, which changes as the temperature varies. The DNA liquid is pushed by asheer Stress at the liquid-pipette interface, which emerges when viscous rate of the liquid changes.Preliminary experimenting results show the efficiency and convenience of this way in improving thesystem's characteristics.