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1.
《Ceramics International》2016,42(5):5802-5809
Porous yittria-stabilized zirconia is an important advanced ceramic material for technological applications. One of the most important characteristics of this material is low thermal conductivity, which is greatly influenced by the presence of pores into the microstructure. In fact, air trapped in the pores represents a better thermal insulator. The role of the pore volume fraction on porous material characteristics has been extensively studied. On the other hand, the influence of the structure disorder, the pore size range and pore size distribution have been studied much less. In this study, an intermingled fractal model capable of relating thermal properties of ceramic materials and their pore microstructure has been proposed. Model predictions are found confirming the experimental data fairly well, even better than the others models available in the literature.  相似文献   

2.
《Ceramics International》2017,43(12):9184-9190
Advanced porous ceramics are a remarkable class of materials with important applications in engineering fields. Porosity features have received wide attention for their capability to influence all properties. In this paper, the correlation between pore structure and heat transfer has been studied. Different analytical procedures found in literature as well as an Intermingled Fractal Units’ model are proposed. Models predictions are compared with experimental data. It has been observed that IFU is particularly suitable to predict thermal conductivity values very close to experimental ones. This fact is related to its capability to replicate porous microstructures in terms of pore volume fraction, pore size range and pore size distribution.  相似文献   

3.
Yttria Stabilized Zirconia ceramics with well-controlled porosity, pore size and shape were prepared using well-calibrated poly-methyl-methacrylate (PMMA) micro-beads (MB) as a pore-forming agent. The microstructure was observed by Scanning Electron Microscopy. Impedance spectroscopy was used to evaluate the effect of pore morphology (pore size, pore size distribution, pore shape and interconnectivity) on the electrical properties of YSZ ceramics. Archie's law based analyzes to express the dependence of conductivity on porosity have shown that Archie's law is independent of pore size for a pore diameter of between 1 and 7 μm. The Bruggeman model could be used to predict the bulk conductivity if the porosity was less than 25%, thus showing that the impedance response included the effect of sinuousness and constriction induced by pores. Therefore, the tortuosity factor calculated from the bulk conductivity was higher than that predicted by the Bruggeman model for porosities greater than 25% and spherical pores wide (>20 μm). Another point relates to the comparison between tortuosity factors obtained for pore samples fabricated with pore-forming PMMA or by sub-sintering.  相似文献   

4.
《Ceramics International》2019,45(15):18255-18264
Thermal Barrier Coatings (TBCs) play a significant role in improving the efficiency of gas turbines by increasing their operating temperatures. The TBCs in advanced turbine engines are prone to silicate particles attack while operating at high temperatures. The silicate particles impinge on the hot TBC surfaces and melt to form calcia-magnesia-aluminosilicate (CMAS) glass deposits leading to coating premature failure. Fine powder of CMAS with the composition matching the desert sand has been synthesized by solution combustion technique. The present study also demonstrates the preparation of flowable yttria-stabilized zirconia (YSZ) and cluster paired YSZ (YSZ-Ln2O3, Ln = Dy and Gd) powders by single-step solution combustion technique. The as-synthesized powders have been plasma sprayed and the interaction of the free standing TBCs with CMAS at high-temperatures (1200 °C, 1270 °C and 1340 °C for 24 h) has been investigated. X-ray diffraction analysis of CMAS attacked TBCs revealed a reduction in phase transformation of tetragonal to monoclinic zirconia for YSZ-Ln2O3 (m-ZrO2: 44%) coatings than YSZ (m-ZrO2: 67%). The field emission scanning electron microscopic images show improved CMAS resistance for YSZ-Ln2O3 coatings than YSZ coatings.  相似文献   

5.
Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y2O3)-stabilized zirconia (ZrO2) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m−1 K−1 and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m−1 K−1, predicted by the minimum thermal conductivity model.  相似文献   

6.
Zirconia doped with yttrium, widely known as yttria-stabilized zirconia (YSZ), has found recent applications in advanced electronic and energy devices, particularly when deposited in thin film form by atomic layer deposition (ALD). Although ample studies reported the thermal conductivity of YSZ films and coatings, these data were typically limited to Y2O3 concentrations around 8 mol% and thicknesses greater than 1 μm, which were primarily targeted for thermal barrier coating applications. Here, we present the first experimental report of the thermal conductivity of YSZ thin films (∼50 nm), deposited by plasma-enhanced ALD (PEALD), with variable Y2O3 content (0–36.9 mol%). Time-domain thermoreflectance measures the effective thermal conductivity of the film and its interfaces, independently confirmed with frequency-domain thermoreflectance. The effective thermal conductivity decreases from 1.85 to 1.22 W m−1 K−1 with increasing Y2O3 doping concentration from 0 to 7.7 mol%, predominantly due to increased phonon scattering by oxygen vacancies, and exhibits relatively weak concentration dependence above 7.7 mol%. The effective thermal conductivities of our PEALD YSZ films are higher by ∼15%–128% than those reported previously for thermal ALD YSZ films with similar composition. We attribute this to the relatively larger grain sizes (∼23–27 nm) of our films.  相似文献   

7.
This work provides a novel and green approach to preparing porous forsterite ceramics by a transient liquid phase diffusion process based on fused magnesia and quartz powders without detrimental additives. The size of quartz particles markedly affected the sintering behaviors, phase composition, microstructure and properties of the porous forsterite ceramics. Fine quartz particles (D50, 3.87?µm) accelerated the rate of the forsterite formation at elevated temperatures and promoted solid-state sintering behavior of the porous ceramics. Conversely, coarse quartz particles (D50, 25.38?µm) reduced the rate of the solid state reaction and a large amount of unreacted SiO2 and enstatite (MgSiO3) phases transformed into a transient liquid-phase during the firing process. This effect resulted in a high porosity (approximately 58.89%) and formation of many large pores (mean pore size of 42.36?µm). These features contributed to the excellent thermal isolation properties of the prepared porous forsterite ceramics. The strength of the obtained porous ceramics (about 23.6?MPa) is relatively high compared with those of conventional ceramics.  相似文献   

8.
An efficient computation method to study flow and transport process of small molecules in porous media using a dual site-bond lattice model, DBSM, is described. The microscopic properties of the porous network take into account the influence of local heterogeneities during the simulations. The numerical experiments demonstrated the combined effect of pore size distribution and connectivity distribution on the mass transport properties and the structural tortuosity. The results indicate that the pore size distribution and percolation phenomena related with pore shielding effects, influence significantly the tortuosity and the effective diffusivity of the porous network. Also, the simulations raise the important role of the connectivity distribution among the various pores in the gas diffusive properties of the poorly connected networks.  相似文献   

9.
In the present study, atmospheric plasma-sprayed yttria-stabilized zirconia coatings were post-remelted by a continuous diode laser to improve the hot corrosion resistance for as-sprayed coatings. The coating surfaces were covered with a salt mixture (V2O5 and Na2SO4) and then subjected to a hot corrosion test at 1100?°C in air. The influence of laser parameters including power and scanning rate on the coating microstructure and corrosion resistance was investigated. Results showed that the hot corrosion resistance can be improved by producing a dense and smooth surface and reducing the coating permeability to the molten salt. The transformation of the hot corrosion mechanisms was clarified on the basis of the observed corrosion behaviors. A laser power of 1500?W and scanning rate of 9?mm/s can produce minimal surface roughness with few segmented cracks, which can provide improved performance of the hot corrosion resistance.  相似文献   

10.
This study focuses on the role of the water content on the effective thermal conductivity of porous ceramics placed in different conditions of relative humidity. Fully stabilized zirconia samples with variation in the capacity to take up water were prepared by varying the temperature of the thermal treatment. The pore volume fraction of the dried samples decreases from 56% down to 30%. Thermal conductivity measurements were made on samples placed in a chamber where the relative humidity was fixed between 3% and 99%. For all samples, the experimental values of the effective thermal conductivity increase significantly with the water content. Experimental results agree closely to analytical predictions based on the upper limit of the Hashin and Shtrikman expressions for calculating the thermal conductivity of the pores (constituted by air and water) and Landauer's effective medium expression for calculating the effective thermal conductivity of the material.  相似文献   

11.
The potentially useful role of lanthanum zirconate (La2Zr2O7, LZO) porous bulk ceramics has been rarely explored thus far, much less the optimisation of its pore structure. In this study, LZO porous ceramics were successfully fabricated using a tert-butyl alcohol (TBA)-based gelcasting method, and the pore structures were tailored by varying the initial solid loading of the slurry. The as-prepared ceramics exhibited an interconnected pore structure with high porosity (67.9 %–84.2 %), low thermal conductivity (0.083–0.207 W/(m·K)), and relatively high compressive strength (1.56–7.89 MPa). The LZO porous ceramics with porosity of 84.2 % showed thermal conductivity as low as 0.083 W/(m·K) at room temperature and 0.141 W/(m·K) at 1200 °C, which is much lower than the counterparts fabricated from particle-stabilized foams owing to its unique pore structure with a smaller size, exhibiting better thermal insulating performance.  相似文献   

12.
叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷   总被引:1,自引:0,他引:1  
以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.  相似文献   

13.
High-performance, porous, Y-TZP nanoceramics with hierarchical heterogeneities originating from uniform, intra- and inter-particle packing were prepared by colloidal processing and partial sintering of a mesoporous powder. The powder consisted of 100–150-nanometer-sized secondary particles initially composed of smaller, loosely aggregated, primary, nanoscale crystallites. Green bodies were prepared by centrifugal slip casting of weakly flocculated suspensions. During the initial stage of sintering, necking between the secondary particles was accompanied by intra-particle pore coalescence, while the pores originating from the secondary particle packing remained intact. Such microstructures with porosity levels between 46 and 18.3% and pore areas between 18 and 4 m2/g led to attractive properties, i.e., much reduced thermal conductivities of 0.63–1.88 W m?1 K?1, high bending strengths of 70–540 MPa and lowered elastic moduli of 32–156 GPa, making them potentially ideal as thermal insulators and/or load-bearing porous biomaterials owing to the possibility of further impregnation with bioactive ingredients.  相似文献   

14.
Thermal insulation applications of porous SiC ceramics require low thermal conductivity and high mechanical strength. However, low thermal conductivity and high mechanical strength possess a trade-off relationship, because improving the mechanical strength requires decreasing the porosity, which increases the thermal conductivity. In this study, we established a new strategy for improving both the mechanical strengths and thermal resistances of porous SiC ceramics with micron-sized pores by applying a double-layer coating with successively decreasing pore sizes (submicron- and nano-sized pores). This resulted in a unique gradient pore structure. The double-layer coating increased the flexural strengths and decreased the thermal conductivities of the porous SiC ceramics by 24–70 % and 29–49 % depending on the porosity (48–62 %), improving both their mechanical strengths and thermal resistances. This strategy may be applicable to other porous ceramics for thermal insulation applications.  相似文献   

15.
The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size Rp to the phonon mean free path for zero-porosity, l0. The thermal conductivity for l0/Rp > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l0/Rp < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.  相似文献   

16.
《Ceramics International》2017,43(18):16722-16726
The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al2O3 in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ0 exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.  相似文献   

17.
Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.  相似文献   

18.
《Ceramics International》2022,48(8):10480-10485
Advanced nano-porous super thermal insulation materials are widely used in spacecraft, soler-thermal shielding, heat exchangers, photocatalytic carriers due to their low thermal conductivity. In this work, adopting dry preparation technology, nano-Al2O3, nano-SiO2, SiC and glass fibers as raw materials, novel nanometer alumina-silica insulation board (NAIB) were prepared. The preparation process was simple, safe, and reliable. In addition, the NAIB exhibited a high porosity (91.3–92.3%), small pore size (39.83–44.15 nm), low bulk density (0.22–0.26 g/cm3), better volumetric stability, and low thermal conductivity (0.031–0.050 W/(m·K) (200–800 °C)), respectively. The as-prepared NAIB could render them suitable for application as high-temperature thermal insulation materials.  相似文献   

19.
SiC structures with porosities ranging between 20–60% have been fabricated using two methods emulsification and freeze casting. While emulsification results in foam-like isotropic materials with interconnected pores, freeze casting can be used to fabricate highly anisotropic materials with characteristic layered architectures. The parameters that control the pore size and final porosity have been identified (solid content in the initial suspensions, emulsification times or speed of the freezing front). We have found that liquid state sintering (suing Al2O3 and Y2O3 as additives) at 1800 °C on a powder (SiC/Al2O3) bed provides optimum consolidation for the porous structures. The mechanical strength of the materials depends on their density. Freeze casted materials fabricated with bimodal particle size distributions (a controlled mixture of micro and nanoparticles) exhibit higher compressive strengths that can reach values of up to 280 MPa for materials with densities of 0.47.  相似文献   

20.
To lower the sintering temperature of Al2O3 microfiltration membrane support, Al2O3 powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al2O3 grains depends on their agglomeration state: if fine Al2O3 grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al2O3 grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al2O3 powder has less effect on the pore size distribution of the support if only coarse Al2O3 grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.  相似文献   

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