多孔结构分形分析及其在材料性能预报中的应用

采用分形理论表征孔隙的结构特征,通过对烧结不锈钢纤维多孔材料的扫描电镜(SEM)照片进行图像处理和分形分析,详细研究图像分辨率、获取图像时的放大倍数、灰度图转化为二值图时阈值的大小等因素对孔结构分形分析结果的影响,并研究它们与分形维数的定性关系.结果表明,采用的烧结纤维多孔材料具有明显的分形特征,图像分辨率、放大倍数、阈值等因素对分形维数计算结果影响显著:与SEM图像的原始分辨率相比,分辨率变化越大则分形维数越大;分形维数随着放大倍数的增大而减小;灰度图转化为二值图时的阈值存在最佳值,且随着阈值的增大,分形维数先增大后减小.通过理论推导,建立了多孔材料孔隙度和渗透性能与孔结构分形维数之间的数学关系.可通过孔结构分形维数、面孔隙度、孔径等参数合理预测多孔材料的体孔隙度、渗透性能,预测结果与实测结果吻合良好.     

热喷涂热障涂层孔隙与涂层性能关系研究进展

孔隙在热障涂层中较为常见,孔隙对热障涂层的性能有利有弊。对热障涂层陶瓷层中孔隙的形成机理进行了综合分析,总结了热障涂层孔隙结构的调控方法,讨论了孔隙结构特征对热障涂层隔热性能和力学性能的影响。孔隙结构的引入将引起力学性能的下降,同时降低热障涂层的热导率,提高隔热效果。孔隙结构特征参数包括孔隙形状、孔隙间距、孔隙倾斜角、孔隙高宽比等,其中孔隙的倾斜角和高宽比对涂层导热性能的影响尤为重要,是孔隙结构的关键特征参数。通过原始粉末孔隙结构的保留、造孔剂(有机造孔剂、无机造孔剂)的搭配造孔、制备工艺(临界等离子喷涂参数、粒子扁平率等)的调节以及后续的孔隙处理,可实现热障涂层孔隙结构的调控。在实际应用过程中应同时兼顾力学性能和隔热性能,最重要的是保证热障涂层的有效寿命,需要综合考虑力学性能与导热性能的匹配。通过热障涂层孔隙结构特征的设计及分布控制,可实现孔隙结构高性能、高可靠性热障涂层的制备。     

关于连铸凝固传热数值模拟中钢液有效导热系数的探讨

在建立板坯连铸一维非稳态凝固传热数学模型的基础上,考虑到液相区的流动和传热状态随拉坯方向的变化,研究了有效导热系数与固相导热系数的比值m（λeff/λs）的处理方法对计算结果的影响。结果表明,在相同的二冷条件下,m取不同的常数对模型计算结果影响很大。在相同的二冷条件下,将m取为常数和取为随拉坯方向变化的变量都可以得到相同的液相穴深度,但二者的凝固壳厚度随拉坯方向的变化有一定的差别,并且出结晶器坯壳厚度差别较大。改变二冷条件,上述二者液相穴深度不再相等。因此,将m取为常数的处理方法是不合理的。     

Effect of Pore Structure on Performance of Porous Metal Fiber Materials

孔结构是影响金属纤维多孔材料各项性能的关键因素之一,为此研究了孔结构对其性能的影响规律。采用气流铺毡法和烧结技术制备了FeCrAl纤维多孔材料,利用SEM观察其微观组织,同时测试了其拉伸强度、透气性和吸声系数（声强为 90~140 dB,频率为1000~3000 Hz）。利用自主研发的分形软件计算了孔结构的分形维数。另外,研究了孔结构对多孔材料拉伸强度、吸声系数和透气性的影响规律,建立了拉伸强度、透气性与分形维数之间的本构关系。研究表明,随着分形维数的增加,抗拉伸强度呈线性下降,而透气性显著增大;在相同的声强和频率下,吸声系数随着分形维数的增加而逐渐降低     

工艺参数对藕状多孔Mg合金结构影响的模拟仿真

在金属/气体共晶定向凝固工艺(Gasar)中,气体压力、过热度以及凝固速率是影响藕状多孔金属结构的最重要参数。建立了一个描述Gasar工艺过程的传热、传质、气孔形核、生长、合并及气孔与固相的协同生长的三维非稳态理论模型。并且采用有限差分的方式,实现了定向凝固铸造法Gasar工艺过程中藕状多孔结构形成及演变的计算机仿真,揭示了气体压力和凝固速率对藕状多孔金属最终结构参数的影响。基于Mg-H系的模拟与相应试验结果吻合较好,验证了所建仿真模型的合理性。     

FRACTAL PORE DIFFUSION MODEL OF FLUIDS IN POROUS MEDIA

1.IntroductionTherateprocessesofporediffusionandreactonoffluidsinporousmediaincludingcat-alyticandnoncatalyticreactionshavebeenbeingadifficultproblemduetoverycomplexityandirregularityofgeometricalstructureofporousmedia,whichmakesitimpossibletogetasimpleanddistinctmathematicalsolutionofthemassbalancesorcontinuityequationswithvarioussurfacemorphologiesandporeshapes[1-11].However,withthehelpofMan-delbrot'sfractalgeometryl12],afractalsurfacereactionmodeloffluidsinporousmediawasderived[13].lnthepre…     

连铸凝固传热过程的数值模拟

研究和开发了连铸凝固传热过程数值模拟程序,并以生产厂的铸坯为研究对象,计算了铸坯断面温度分布和凝固壳厚度,该模型考虑了结晶器表面散热的不均匀性,处理了凝固时相变所产生的结晶潜热,将计算出的断面温度、坯壳厚度等数据与生产实验测得的数据相比较,吻合性很好.他可用来优化连铸工艺参数,是进一步开发在线控制模型的基础.     

Thermal conductivity of zirconia coatings with zig-zag pore microstructures

Highly porous zirconia based thermal barrier coatings have recently been synthesised with zig-zag morphology pores which appear to impede heat flow through the thickness of the coating. A combined analytical/numerical study of heat conduction across these microstructures is presented and compared with thermal conductivity measurements. The effects of pore volume fraction, pore type, pore orientation and pore spacing, together with the wave length and the amplitude of zig-zag pore microstructures on overall thermal performance are quantified. The results indicate that even a few volume percent of zig-zag inter-column pores oriented normal to the substrate surface reduce the overall thermal conductivity of the coatings by more than 50%.     

Fractal analysis of granular ore media based on computed tomography image processing

The cross-sectional images of nine groups of ore samples were obtained by X-ray computed tomography(CT)scanner. Based on CT image analysis,the fractal dimensions of solid matrix,pore space and matrix/pore interface of each sample were measured by using box counting method.The correlation of the three fractal dimensions with particle size,porosity,and seepage coefficient was investigated.The results show that for all images of these samples,the matrix phase has the highest dimension, followed by the pore ...     

An analytical model for simulation of heat flow in plasma-sprayed thermal barrier coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.     

厚板坯连铸二次冷却过程数值模拟及实验研究

基于凝固一传热学基本理论,建立板坯连铸三维凝固传热数学模型.该模型考虑了组元成分、固相分数、温度等因素对热物性参数的影响,根据糊状区溶质传输和反扩散理论,采用钢中多组元成分的百分含量计算液、固相线温度.以鞍钢第一炼钢厂厚板坯连铸机为对象,根据二冷区铸坯表面实际水流密度分布和辊子接触传热建立边界条件,计算整个铸坯任意点、线、面的温度及固相分数分布.多钢种多拉速条件下铸坯表面温度实测值和射钉实验结果与计算值吻合良好.     

绝热材料的显微结构及绝热性能

着重分析了材料的显微结构对材料导热系数的影响:高孔隙率减弱固相传热作用从而使材料的导热系数减小,但是,高孔隙的也增大辐射传热作用.适当降低孔隙可减少辐射传热作用,这在高温时特别明显.尺寸<1 mm的孔隙中对流传热被制止,不再对传热起作用.在保持孔隙总量不变的前提下减小孔径可使孔隙数量增大,从而减小导热系数.小于100 nm的孔隙可以消除气体本身的热传导,从而极大地减小材料的导热系数.展示了几种不同显微结构的材料及其绝热性能.     

Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.     

Low thermal conductivity vapor deposited zirconia microstructures

Low thermal conductivity yttria stabilized zirconia (YSZ) coatings have been grown using a low-vacuum (0.20 Torr) electron beam directed vapor deposition process. In this approach, a transsonic helium jet was used to entrain and transport an evaporated YSZ flux to a substrate. The interaction of the helium jet with the coating surface resulted in many of the evaporated species making oblique angles of contact with the substrate. This resulted in the formation of a highly porous, columnar microstructure without substrate rotation. When the substrate was positioned perpendicular to the axis of the jet, coatings with intercolumnar pores normal to the substrate surface were formed. The ambient temperature thermal conductivity of a coating grown in this arrangement was 1.9 Wm/K, comparable to that of conventional, high-vacuum electron beam coatings. When the column and pore orientation was inclined (by tilting the substrate) the thermal conductivity was observed to fall. By alternating the inclination angle as growth progressed, coatings containing zig-zag columns and pores could be synthesized. Using this technique, YSZ coatings with thermal conductivities as low as 0.8 W/m K were obtained. The observed thermal conductivity reduction arises from the longer thermal diffusion path of the zig-zag pore micro-structures.     

Casting/mold thermal contact heat transfer during solidification of Al-Cu-Si alloy (LM 21) plates in thick and thin molds

Heat flow at the casting/mold interface was assessed and studied during solidification of Al-Cu-Si (LM 21) alloy in preheated cast iron molds of two different thicknesses, coated with graphite and alumina based dressings. The casting and the mold were instrumented with thermocouples connected to a computer controlled temperature data acquisition system. The thermal history at nodal locations in the mold and casting obtained during experimentation was used to estimate the heat flux by solving the one-dimensional inverse heat conduction problem. The cooling rate and solidification time were measured using the computer-aided cooling curve analysis data. The estimated heat flux transients showed a peak due to the formation of a stable solid shell, which has a higher thermal conductivity compared with the liquid metal in contact with the mold wall prior to the occurrence of the peak. The high values of heat flux transients obtained with thin molds were attributed to mold distortion due to thermal stresses. For thin molds, assumption of Newtonian heating yielded reliable interfacial heat transfer coefficients as compared with one-dimensional inverse modeling. The time of occurrence of peak heat flux increased with a decrease in the mold wall thickness and increase in the casting thickness.     

多孔材料孔界面分形结构小角度X射线散射实验

简要介绍了用X射线小角散射分析技术测定多孔固体材料中孔界面分形 维数的理论,同时报道了应用这一理论对多种炭素材料和水泥石材料中孔界面分形维数的测定结果。结果表明,所有试样中孔界面均具有分形结构特征。     

Microstructural characterization of porous thermal barrier coatings by laser flash technique

An inversion procedure has been developed to obtain microstructural parameters describing the porosity morphology of porous thermal barrier coatings (TBCs) from the thermal diffusivity measured in different environments by a laser flash technique. The model base for the inversion procedure combines the Knudsen effect (i.e. the thermal conductivity variation of gases within a pore as a function of pore size and shape) with the asymmetric Bruggeman model that describes the effect of porosity on the thermal conductivity of a porous medium in terms of pore shape, orientation and content. A simplified approach in the inversion procedure has been proposed, and the reliability has been checked by simulating different microstructures within the TBC. The inversion procedure has been also applied to experimental thermal diffusivity values of a TBC which were obtained by filling pores with He, N_2, Ar and in vacuo.     

Microstructure and thermal conductivity of porous ZrO2 ceramics

ZrO₂ ceramics usually have nonuniform microstructures due to the agglomeration of fine ZrO₂ powders. In this paper, the thermal conductivity of porous ZrO₂ ceramics with different microstructures was investigated by the laser-flash method. It was found that the thermal conductivity of porous ZrO₂ ceramics with uniform microstructure was clearly lower than that of ceramics with nonuniform microstructure. Scanning electron microscopy observations showed that porous ceramics with nonuniform microstructures have wider pore and grain size distributions, relative to those with uniform microstructures. Theoretical analyses were done to model the thermal conductivity of porous ceramics using an effective medium approach. These analyses revealed that matrix grain size distribution has a significant impact on the thermal conductivity of porous ceramics, while the effect of pore size distribution is negligible.     

Infrared-optical properties and heat transfer coefficients of semitransparent thermal barrier coatings

Thermal barrier coatings (TBC) which are used in aircraft and land-based gas turbines for thermal insulation of thermally highly loaded components are usually semitransparent in the infrared spectral region at higher temperatures. Thus, at turbine surface conditions the total heat transfer coefficient of TBCs increases above the heat transfer coefficient caused by solid heat conduction alone.The solid thermal conductivity of electron beam physical vapor deposited (EB-PVD) partially yttria stabilized zirconia (YSZ) coatings derived from laser flash measurements were correlated with the microstructure of the coatings, which was adjusted by defined heat treatments. To obtain the contribution of the radiative transfer on the total heat transfer coefficient, infrared-optical characterizations were carried out at ambient and elevated temperatures.A theoretical model was developed which can be used to describe the heat transfer through semitransparent, absorbing and scattering media. Finally, the total heat transfer, caused by solid thermal conduction, radiative transfer and an interaction of both is derived for the coatings prepared in this work. Additionally the measurement method BBC (black body boundary conditions) which is suitable to determine spectral transmittance and emittance at elevated temperatures is introduced.     

热处理工艺对Al-Mg-Si系合金导热性能的影响

研究了热处理工艺对Al-Mg-Si合金导热性能的影响,并从理论上分析了热处理(均匀化、固溶、时效)的作用机理。结果表明,均匀化处理有助于改善合金的显微组织。采用560℃×6h均匀化制度后组织细小均匀,导热性能较好;随着固溶温度的升高,合金材料的导热性能有下降的趋势,在510℃时强度与导热性能达到较佳配合;人工时效后组织中的缺陷得到有效修复,使导热性能加强,使用200℃×6h时效制度较好。