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1.
Based on the concept of intrinsic elastic moduli an overview of modulus–porosity relations is given, which includes exponential and power-law expressions as well as the Hasselman relation and a relation recently proposed by Pabst and Gregorová. The formal structure of these relations is compared and the physical meaning of the parameters discussed. It is recalled that certain popular relations violate the Hashin–Shtrikman upper bounds and are, therefore, useless (Spriggs relation, Ishai–Cohen relation). Coble–Kingery relations are recalled in their correct form and an improved version of the Gibson–Ashby relation for the shear modulus is proposed. Selected relations are applied to describe the porosity dependence of the relative tensile moduli of alumina, zirconia, silicon nitride and silicon carbide prepared with corn-starch as a pore-forming agent. Porous ceramics with this type of (matrix-inclusion-based) microstructure are shown to follow approximately a modified exponential relation and can be fitted by a master curve with critical porosity 68.4%.  相似文献   

2.
Alumina–zirconia composite ceramics (AZ composites) have been prepared in the whole range of compositions from pure alumina to zirconia (in steps of 10 vol.%) by slip casting, followed by sintering at 1350 °C and microstructural characterization via the Archimedes method (relative densities 0.93–0.99). Young's modulus has been measured at room temperature via the impulse excitation technique (IET) and, after appropriate porosity correction (linear, power-law, exponential), found to be in good agreement with the Hashin–Shtrikman bounds. The damping factor (internal friction), which has been measured for dense AZ composites (also via IET at room temperature), is found to increase with increasing zirconia content. Damping factors measured for porous AZ composites with porosities 25–71%, prepared with corn starch as a pore former, have been found to depend only slightly on porosity, unless the porosities are extremely high (>70%). At these porosities, however, where the Young's moduli approach zero, the damping factors exhibit a steep increase.  相似文献   

3.
The elastic moduli (Young's moduli) of alumina and zirconia ceramics with porosities ranging from almost dense (2–3%) to highly porous (46–52%), the latter prepared with starch as a pore-forming agent, have been measured via impulse excitation and four-point bending tests from room temperature up to more than 1200 °C. It is shown that, independent of the temperature and the material, the porosity dependence of the Young's modulus is well predicted by our exponential relation and that, irrespective of porosity, the temperature dependence follows a master curve that is characteristic of the material (for alumina exhibiting a decrease with a gradually growing tangent slope and for zirconia exhibiting a steep decrease with an inflection point at moderately elevated temperatures below 400 °C). Differences between isothermal (static) and adiabatic (dynamic) values are negligible as long as the materials are purely elastic (i.e. at temperatures below approximately 1000 °C).  相似文献   

4.
The effective thermal conductivity, elastic modulus, and coefficient of thermal expansion of epoxy resins filled with ceramic fillers like silica, alumina, and aluminum nitride were determined. The data obtained was compared with theoretical and semitheoretical equations in the literature that are used to predict the properties of two phase mixtures. It was found that Agari's model provided a good estimate of the composite thermal conductivity. The Hashin‐Shtrikman lower bound for composite modulus fits the modulus data fairly well at low concentrations of the filler. Also, it was found that the coefficients of thermal expansion of the filled composites lie in between Schapery's upper and lower bounds. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3396–3403, 1999  相似文献   

5.
Based on the Eshelby solution for the single-inclusion problem and Wu's specification of this solution to spheroidal pores, we show that the Eshelby–Wu coefficients for Young's modulus, in contrast to their counterparts for the bulk and shear moduli, are quite insensitive to changes of the Poisson ratio. Therefore the Eshelby–Wu coefficients of Young's modulus can be described (to a very good approximation) by a unique function of the aspect ratio, which is calculated in this paper and for which a master curve is obtained by segment-wise fitting. Also the implementation of the Eshelby–Wu coefficients into the well-known effective medium approximations (Maxwell, self-consistent, differential) and our exponential relation is discussed. Irrespective of the model into which the Eshelby–Wu coefficients are implemented, prolate pore shape affects the porosity dependence of Young's modulus only very weakly, whereas oblate pore shape can lead to an arbitrary reduction of Young's modulus.  相似文献   

6.
《Ceramics International》2017,43(18):16107-16113
Accurately establishing the relationship between the network architecture characteristics and performance of fibrous porous ceramics is instructive for structural design and performance control. In the present work, fibrous, high porous (82.87–90.02%), low density (0.247–0.512 g/cm3) and low elastic modulus (50.62–188.56 MPa) mullite ceramics were fabricated by freeze casting. The three dimensional network architectures were characterized by X-ray tomography technique and quantitatively analyzed by 3D image analysis software (imorph, www.imorph.fr). The radius (5.04 µm), types, lengths (64.72–96.49 µm) and orientations (0.87–1.45, anisotropy parameter) of fiber segments in the network architecture were investigated. The extracted results were employed to predict the Young's modulus of the mullite fibrous porous ceramics according to a model based on the bending and axial compression of single fiber segment. The predicted Young's modulus agreed well with the experimental results. The differences of Young's modulus and Poisson ratio between the prediction and the model of Markaki and Clyne were compared. The comparison showed that the difference became larger when the aspect ratio of the fiber segment was less than 6 due to the effect of axial compression. The predicted Poisson ratio had a certain dependence on fiber segment aspect ratio and got close to the constant (1/π) reported by Markaki and Clyne with the increase of fiber segment aspect ratio.  相似文献   

7.
Investigating Young's modulus at elevated temperatures supports the understanding of microstructural changes as a function of application temperature. A sintered alumina and three carbon‐bonded alumina materials with carbon contents of 20 and 30 wt% and alumina grain size of 0.6–3 mm were investigated. Young's modulus was measured in a temperature range from 25°C to 1450°C by the impulse excitation technique. The Young's modulus of carbon‐bonded materials increases up to 140% at 1450°C. After one cycle, a decrease of the Young's modulus up to 50% is registered at room temperature. There is a strong hysteresis behavior during one cycle. Thermal expansion measurements show highest expansion for the highest graphite content material. The expansion of alumina grains and graphite flakes, resulting in microcrack generation during cooling and microcrack healing during heating, is reflected in the registered values of the Young's modulus as a function of the temperature. It is assumed, that higher graphite amounts as well as coarse grains lead to lower sintering effects of the microstructure at elevated temperatures and as a result lower values of the Young's modulus have been registered.  相似文献   

8.
The origin of the nonlinear behavior of the Young's modulus (E) of carbon‐bonded alumina at high temperatures was addressed, based on the microstructural changes observed during processing and their thermo‐mechanical properties. Impulse excitation technique, thermogravimetric analysis, porosity measurement, and scanning electron microscopy were conducted in order to highlight and explain the E behavior. The finite element model of a virtual microstructure was simulated and the results attained are in good agreement with the experimental data. The tests revealed that the Young's modulus of a cured sample heated from room temperature up to 500°C was governed by the release of volatiles. Above this temperature, the thermal expansion mismatch among alumina, graphite, and the carbon matrix is dominant resulting in an increase in the effective Young's modulus. During cooling, crack networks and gaps between alumina particles and the carbon matrix were developed. The former were induced by volatile release and by the graphite's highly anisotropic thermal expansion. The latter was derived by the thermal expansion mismatch between the alumina and the carbon matrix. The closure of the gaps and cracks governed the expansion behavior during the second heating cycle and a nonlinear effective Young's modulus increase as a function of temperature was observed.  相似文献   

9.
10.
Zirconia-toughened alumina composites containing 0–30 vol% of 3Y-TZP were fabricated by sintering at 1600 °C for 2 h in air. The effect of the 3Y-TZP content on the mechanical properties and microstructure of the alumina ceramics was investigated. The fracture toughness and biaxial flexural strength increased as the 3Y-TZP content increased. The Young's modulus decreased with 3Y-TZP content according to the rule of mixture, while the hardness showed the contrary tendency. The Weibull modulus of the Al2O3 with 20 vol% 3Y-TZP composite is higher than that of alumina. The residual hoop compressive stress developed in ZTA ceramic composites probably accounts for the enhancement of strength and fracture toughness, as well as for the higher tendency of crack deflection. No monoclinic phase and strength degradation were found after low temperature degradation (LTD) testing. The excellent LTD resistance can be explained by the increased constraining force on zirconia embedded in alumina matrix.  相似文献   

11.
Freeze gelcasting of hydrogenated vegetable oil-in-aqueous alumina slurry (HVO-in-AAS) emulsions has been studied for the preparation of macroporous ceramics. The emulsions with HVO to AAS volume ratios in the range of 1.34–2.69 prepared from a 30 vol.% AAS containing carrageenan using sodium dodecyl sulphate emulsifying agent at 85 °C undergo gelation on cooling to room temperature due to the solidification of HVO and physical cross-linking of carrageenan. Macroporous ceramics obtained by drying, HVO removal followed by sintering at 1500 °C of the gelled emulsion bodies had porosity in the range of 70.7–84% and contain cells of spherical to polygonal shape. The average cell size (13.24–3.6 μm) decreased and the cell interconnectivity increased with an increase in HVO to AAS volume ratio and mixing speed. The macroporous alumina bodies had high compressive strength (6.5–39.6 MPa) and Young's modulus (350–2352 MPa).  相似文献   

12.
Zirconia toughened alumina (ZTA) nanocomposites are attractive structural materials which combine the high hardness and Young's modulus of the alumina matrix with an additional toughening effect by the zirconia dispersion.In this study two approaches to prepare ZTA are compared. For the first approach, an ultrafine alumina powder was coated with 5 vol% zirconia by a wet chemical method. For the second one, the reference material was prepared by intensively mixing and milling the same alumina with nanoscale zirconia powder. Samples were consolidated at 1350–1600 °C by hot pressing and their mechanical properties, microstructure and transformation behavior were compared. Toughness increments derived from different toughening mechanisms are also briefly discussed. Besides better sinterability, the mixed material exhibited a finer grain size of both matrix and dispersion and thus higher hardness and strength. The alumina matrix was under compressive hydrostatic residual cooling stress, whereas zirconia was under tensile one. The coated material, however, showed higher transformability, deeper transformation zones and thus higher fracture toughness. In addition, it contained more monoclinic zirconia so the matrix was under tension.  相似文献   

13.
Multi-walled carbon nanotubes (MWCNTs) are often reported as additives improving mechanical and functional properties of ceramic composites. However, despite tremendous efforts in the field in the past 20 years, the results are still inconclusive. This paper studies room temperature properties of the composites with polycrystalline alumina matrix reinforced with 0.5–2 vol.% MWCNTs (composites AC) and zirconia toughened alumina with 5 vol.% of yttria partially stabilised zirconia (3Y-PSZ) containing 0.5–2 vol.% of MWCNTs (composites AZC). Dense composites were prepared through wet mixing of the respective powders with functionalised MWCNTs, followed by freeze granulation, and hot-pressing of granulated powders. Room temperature bending strength, Young's modulus, indentation fracture toughness, thermal and electrical conductivity of the composites were studied, and related to their composition and microstructure. Slight increase of Young's modulus, indentation fracture toughness, bending strength, and thermal conductivity was observed at the MWCNTs contents ≤1 vol.%. At higher MWCNTs contents the properties were impaired by agglomeration of the MWCNTs. The DC electrical conductivity increased with increasing volume fraction of the MWCNTs.  相似文献   

14.
《Ceramics International》2016,42(8):10058-10065
Mechanical behaviour of SiC ceramics synthesized from two artificial precursors from plants – coir fibreboard and bamboo pulp fibreboard – was studied and the mechanical properties – flexural strength, Young's modulus, fracture toughness and hardness – of the ceramics synthesized from the two kinds of artificial precursors were compared. The effect of processing of the artificial precursors, as reflected in the microstructure of the ceramics synthesized from them, was taken into consideration in the analysis of the mechanical property data; these data could also be possible to be explained by the empirical models of fracture mechanics. The results of the study established the possibility of application of the investigated SiC materials as structural ceramics.  相似文献   

15.
《Ceramics International》2022,48(10):14224-14230
A SiC whisker-bonded mullite foamed ceramic was prepared by using white clay, industrial alumina and silicon powder as raw materials without solid carbon sources. The XRD, SEM, EDS, and Factsage® software were used to investigate the effect of sintering temperature on the phase composition, microstructure, compressive strength, and Young's modulus of foamed ceramics. Additionally, the synthesis reaction of in-situ SiC whiskers and the effect of their formation on the properties of ceramics were studied. The results showed that the in-situ SiC whiskers with dendrite shapes were formed after firing above 1300 °C at the expense of Si/SiO vapors as well as CO vapor, though there were no solid carbon sources in raw materials, which provided a new idea for the synthesis of SiC whiskers. The formation of SiC whiskers was helpful for improving the compressive strength and Young's modulus of mullite foamed ceramics remarkably. Furthermore, the reinforcement mechanism has been investigated systematically.  相似文献   

16.
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   

17.
The elastic constants and conductivity of partially sintered single-phase and two-phase ceramics (exemplified by alumina ceramics and alumina-zirconia composites, respectively) with different grain size ratio (from 1:1 to 1:4) are investigated by numerical modeling. The relative elastic moduli of partially sintered two-phase ceramics are shown to be relatively similar to those of single-phase ceramics, whereas the relative conductivity is significantly lower, because of the higher phase contrast. The more the grain size ratio deviates from unity, the higher is the initial packing fraction, and the lower are the relative elastic moduli and conductivity of the partially sintered ceramics. The porosity dependence of the Poisson ratio shows a decreasing trend which is only very weakly affected by the grain size ratio. Correlations between relative Young’s modulus and relative conductivity lie between upper and lower cross-property bounds. For single-phase materials the correlation lies below, for two-phase materials above, the Pabst-Gregorová cross-property relation.  相似文献   

18.
A new aqueous alumina gelcasting system using hexamethylenediamine (HMDA) and paraformaldehyde monomers has been studied. The 500 vol% aqueous alumina slurries ‘A’ and ‘B’ containing paraformaldehyde and HMDA, respectively, undergo gelation after thorough mixing of the two due to the polymerization of HMDA and formaldehyde. The gelation time of the slurries cast in a mold is in the range of 7–2.4 min at HMDA to formaldehyde mole ratio in the range of 1.1–1.5. The faster reaction between HMDA and formaldehyde prevents the formaldehyde emission during the processing. The minimum HMDA to formaldehyde mole ratio required for the formation of a mechanically stable gel is 1.1. The compressive strengths and Young's modulus of the wet and dry alumina bodies increased with an increase in HMDA to formaldehyde mole ratio. Though the wet gelcast alumina bodies had low compressive strength (11.2–88.7 kPa) and Young's modulus (0.17–5.9 MPa) the dried ones showed high strength (6–11.7 MPa) and Young's modulus (209–364 MPa). The binder removal by slow heating to a temperature below 500 °C followed by sintering at 1600 °C produced alumina ceramics with ~97% of theoretical density.  相似文献   

19.
《Ceramics International》2017,43(5):4252-4262
The present study investigated pressed high-purity alumina refractories containing either white fused or tabular (sintered) alumina aggregates under comparable conditions. Using factorial experiments especially the effects of the pressing pressure, the particle size distribution model and the particle morphology were evaluated. White fused alumina exhibited a higher refractoriness under load as well as a lower total compression and creep rate in creep in compression experiments. However, tabular alumina had a higher cold crushing strength and Young's modulus before and after thermal shock. Yet, no significant effect regarding the relative loss of the Young's modulus due to thermal shock was determined. Generally, a higher pressing pressure reduced the apparent porosity and increased the cold crushing strength, the Young's modulus and the refractoriness under load. The batches according to a recently suggested modified Andreasen particle size distribution model contained a considerably higher amount of the coarsest particle fraction, while the medium particle size fractions were reduced. Surprisingly, for both alumina raw materials the modified Andreasen model resulted in a virtually identical apparent density and a slightly lower apparent porosity compared to the conventional Andreasen model. Furthermore, the thermomechanical properties were essentially unaffected, while the cold crushing strength and the Young's modulus were somewhat lower. For both raw materials the addition of blocky coarse grain fractions yielded a lower apparent porosity and higher apparent density compared to angular grains due to improved particle packing. Remarkably, the creep in compression and the creep rate were reduced as well. Consequently, the modified Andreasen model together with a designed particle morphology might allow the fabrication of shaped alumina products with a much higher content of coarse grained particles resulting in at least similar or even improved physical, mechanical and thermomechanical properties irrespective of the used alumina raw material.  相似文献   

20.
The physical and mechanical properties of two barium alkali silicate glasses were determined as a function of temperature. Their Young's modulus and Poisson's ratio were determined by resonant ultrasound spectroscopy; their viscosity, thermal expansion, and glass transition temperature were determined using a thermomechanical analyzer. The wetting behavior of the two glasses on alumina and 8 mol% yttria stabilized zirconia (8YSZ) substrates was determined by measuring contact angles in air as a function of temperature and time. Values of Young's modulus for both glasses were in good agreement with those predicted by the Makishima and MacKenzie model. The physical and mechanical properties of these glasses are discussed in the context of their potential use for sealing applications in solid-oxide fuel cells.  相似文献   

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