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1.
Mesoporous alumina has been synthesised by the solution combustion synthesis method. While often the synthesis of mesoporous
materials with a relatively narrow pore distribution requires the use of a template, this technique allows a simple and rapid
synthesis of pure alumina without the use of any templating agent: by regulating the synthesis conditions is possible to obtain
from relatively low surface area α-alumina to high surface mesoporous γ-alumina. The porosity of the latter was assessed chiefly
by density functional method and was found to be in the range 2.8–3.5 nm. Low angle X-ray diffraction experiments demonstrated
a partial ordering of the structure, with a periodicity in the range 5.5–6.9 nm. The addition of lanthanum to the alumina
was tested and found to confer a good temperature resistance to the alumina, without modifying the mesoporous structure. After
a prolonged heat treatment at 900 °C the mesoporosity of lanthanum-modified samples was retained, even if the pores size increased
and the specific surface area decreased. 相似文献
2.
The combustion synthesis method is very attractive to produce homogeneous powders of pure or mixed oxidic ceramics without further thermal treatment. Previous work reports the synthesis of monophasic mullite powders by a suspension combustion process using two combustions aids, ammonium nitrate and hydrogen peroxide, and an excess of colloidal silica suspension. The combustion synthesis leads to coarse, agglomerated powders that must be crushed. The effect of attrition milling on both particle size and morphology was studied. Aqueous suspensions of the milled powders were prepared using two dispersants: citric acid and a polyacrylic acid-based polyelectrolyte. The stability of the powders was studied by measuring the zeta potential. The effect of the dispersants on the rheological behaviour of concentrated suspensions was also studied. The sintered densities and the microstructures of specimens obtained by slip casting were studied, demonstrating that mullite powders obtained by a single step combustion process can be processed through conventional shaping routes. 相似文献
3.
In the present work, the influence of sintering temperature and particle size of pristine ZrC particles on the microstructure, mechanical properties, and thermal properties of ZrC/ZTA ceramic composites are investigated. Specimens consolidated by spark plasma sintering at different sintering temperatures from 1500 °C to 1800 °C. XRD results revealed that α-Al 2O 3, t-ZrO 2, ZrC, and a small quantity of m-ZrO 2 phases are present in the composites. The microstructure of μm-ZrC/ZTA is found to be more compact than nm-ZrC/ZTA composites. There is an apparent increase in the average grain size with the increase in temperature. From the micrographs of fracture surfaces, step-wise transgranular fracture structures are observed. Relative densities and Vickers hardness are in proportion to sintering temperature from 1500 °C to 1700 °C. The maximum Vickers hardness of 1919 HV1 is obtained for μm-ZrC/ZTA composites. Indentation fracture toughness displays a gradual rise when the temperature rises from 1500 °C to 1700 °C, then deteriorates at 1800 °C for both nm-ZrC/ZTA and μm-ZrC/ZTA ceramic composites. The maximum fracture toughness values for nm-ZrC/ZTA and μm-ZrC/ZTA are 6.75 MPa m 1/2 and 6.83 MPa m 1/2, respectively. The thermal conductivity of the specimens decreased gradually as the temperature increases from 100 °C to 1000 °C. The obtained results indicated that the 1700 °C is the optimized sintering temperature where μm-ZrC/ZTA composites have excellent performance on microstructure, mechanical properties, and thermal properties than nm-ZrC/ZTA composites. 相似文献
4.
Undoped and 3 mol% yttrium doped ZrO 2–Al 2O 3 composite powders with partially stabilized ZrO 2 (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO 3) 3, ZrOCl 2 and Y(NO 3) 3. The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K c). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al 2O 3 composite. The sintered density and critical stress intensity factor (K c) achieved were compararble to that achieved earlier by hot press and SPS. 相似文献
5.
Titanium nitride has been fabricated by combustion synthesis from a titanium powder compact and liquid nitrogen in a closed vessel. This method drastically increases the pressure around the sample, resulting in combustion propagation. The products obtained in this system are non-homogeneous in morphology and composition along the propagation direction. The nitridation and densification of the products have improved with the increase of the maximum pressure or pressure gradient. With a pressure gradient of 9.5 MPa s −1, the first combustion part of the product is almost unreacted titanium, but the main product at the last combustion part is titanium nitride (TiN 0.75) with a dense body. The Vickers micro-hardness of the product (at 0.98 N force) along the propagation direction is gradually increased from 9.7 to 17.3 GPa. 相似文献
6.
The nanopowders of zirconia toughened alumina (ZTA) as soft spherical granules were directly synthesized by combining co-precipitation with spray drying (CPSD). The co-precipitation of alumina (Al 2O 3) and yttria-stabilized zirconia (YSZ) was performed together in one step to obtain the ZTA precipitate. Spray drying, which is the most preferred industrial processing way to produce the nanopowders as spherical granules, was used to atomize, dry and granulate the suspension like milk prepared from the synthesized ZTA precipitate under previously optimized spray drying conditions. However, the important and complex processing parameters of spray drying such as the solid-liquid ratio and the feeding rate of the suspension, the inlet temperature and the flow rate of the hot air have to be optimized depending on the moll mass and volume of the component in the prepared suspension. The ZTA nanopowders containing 4–20 wt% YSZ synthesized by the CPSD method have a crystalline structure of alumina and YSZ, an average nanoparticle size between 26.64 and 46.70 nm with a very high specific surface area (SSA) between 77.43 and 112.41 m 2 g −1 in a soft spherical granule form, which were determined by XRD, BET and SEM, respectively. The preparation and drying conditions of the synthesized precipitate, the solid-liquid ratio of the suspension and the molar mass ratio of YSZ in Al 2O 3 matrix have a significant effect on the crystallinity, morphology, particle size, SSA, and granule form of the synthesized ZTA nanopowders. 相似文献
7.
Eu-doped Ca-α-SiAlON phosphors, featuring high phase purity, uniform particle size of 3–5 μm and good luminescent properties with a yellow emission spectrum under blue light excitation, were prepared by a highly efficient combustion synthesis (CS) method. A certain amount of NaCl was applied as an innovative additive to regulate and control the properties of synthesized phosphors. Further, the effects of NaCl additive in the CS system were systematically investigated and rational proposed. It was found that the effect on accelerating nitridation and crystallization played a dominant role in the reaction as the content of NaCl was less than 6 wt%, while the effect of absorbing reaction heat through vaporization was dominant with the further-increased content of NaCl. The intensity of the emission spectrum for the sample doped with 6 wt% of NaCl was remarkably enhanced, nearly 40% more than the sample which was not NaCl-doped. Moreover, a continuous blue-shift phenomenon in emission spectra was observed with the increased content of NaCl. 相似文献
8.
Herein, we put forward a simple combustion synthesis strategy for the highly efficient preparation of Eu-doped Ca-α-SiAlON yellow phosphors with different composition of Ca (m/2−x)Eu xSi 12−m-nAl m+nO nN 16−n. The as-synthesized phosphors were endowed with outstanding photoluminescence behavior of yellow emission peaking at ~580 nm under excitation of near-ultraviolet (UV) or blue light. In the designed experiments, products with different compositions, which were determined by the varying m and n values, were obtained by adjusting the proportion of starting reactants. Further, the dependence of composition on the overall properties of products was systematically studied. It was found that the m and n values could have distinct impact on the phase composition, microstructure and photoluminescence properties of as-synthesized phosphors. The most prominent enhancement of spectral intensity was achieved in the sample with composition of m=1.5 and n=0.8 in this research system. The resultant Ca-α-SiAlON:Eu 2+ phosphors were simultaneously featured with high purity, favorable uniformity and equiaxial morphology. A continuous red shift phenomenon in emission wavelength with the increase of m value and an inverse blue shift with the increase of n value were both observed and rationally uncovered. 相似文献
9.
High surface area maghemite, γ-Fe 2O 3, nanoparticles were prepared via carbon-templated solution combustion synthesis, which is a two-step approach. Step one involves the combustion synthesis of maghemite nanoparticles embedded in an amorphous carbon matrix, by using a fuel rich reaction mixture of triethylenetetramine and iron nitrate. Step two consists of residual carbon removal by treating the previously-obtained composite precursor with hydrogen peroxide, which releases the maghemite nanopowder. This approach avoids carbon removal by thermal treatment, thus preserving the nanometric size of maghemite nanoparticles (8–12?nm), yielding a high specific surface area of 191.9?m 2/g. At the same time, the final maghemite nanoparticles presented a superparamagnetic behavior and a saturation magnetization of 26.2?emu/g, in relation to the small particle size. 相似文献
10.
In this work a methodology to reconstruct three-dimensional microstructures, representative of real biphasic ceramics using Neper free software is proposed. Finite element analysis in Ansys was implemented in order to calculate the effective elastic modulus of the simulated microstructures.Fine grained and dense zirconia toughened alumina (ZTA) materials with 5 and 40 vol.% of Yttria Stabilized Zirconia (YTZP) have been chosen to validate the proposed methodology. First, the effects of the size of the representative volume elements (RVEs) and the characteristics of the grain shapes are analysed. Second, the compliance with the isotropic condition is also verified.Agreement between the numerical and experimental values of the elastic modulus of the considered ZTA materials has been found. For these materials, zirconia fractions higher than 10 vol.% lead to bi-continuous microstructures which make the elastic properties deviate from the Voigt limit due to the increased number of contacts between zirconia grains. 相似文献
11.
Nitride based ceramics are considered as a kind of promising material for structural and functional integration due to their robust structure, extreme environmental resistance and electromagnetically transparency. It is still challenging to prepare nitride based ceramics with homogeneous and controllable microstructure because of their low self-diffusion coefficient and difficulty in sintering. Here, we developed a gelcasting-SHS process by combining gelcasting forming and self-propagating high temperature synthesis (SHS) for the preparation of porous BN-Si 3N 4 composite ceramics. First, carbon residue problem in the gelcasting -SHS process was studied. Based on the result, porous BN-Si 3N 4 composite ceramics with high porosity (69.42 ~ 86.48%), high strength (21.7 ~ 81.0 MPa) and low dielectric constants (1.42 ~ 2.87) were synthesized. In addition, the thermal shock resistance of porous BN-Si 3N 4 composite ceramics until 1000 ℃ was evaluated. 相似文献
12.
Zinc Oxide nanostructures have been hybridized on carbon fiber bundles though joule-heating carbon fiber coated with metallic zinc powder. Upon sufficient heat transfer from the carbon fiber in ambient atmospheric conditions, the metallic Zinc coating will undergo combustion. The combustion reaction will yield a variety of ZnO nanoparticles on the carbon fiber bundles. This simple cost-effective technique provides a fast, non-catalytic, and economic approach for the hybridization of ZnO nanowires on advanced textiles. The integration of ZnO nanostructures on the carbon fiber bundles can open up new avenues for multifunctional composites or smart materials. The ZnO nanostructures on the carbon fiber bundle have been characterized through SEM, XRD, and TEM. The new method and minimum heating rates to cause ignition of Zn powders are discussed. 相似文献
13.
燃烧合成法是一种高效能、低消耗的陶瓷材料合成方法。简要介绍了燃烧合成方法特点,评述了近年来燃烧合成氮化硅陶瓷晶须的研究进展,详细总结了原料选择、多种添加剂(如铁、稀土氧化物、铵盐等)对氮化硅晶须最终形貌和性能的影响, 总结了工艺参数,尤其是氮气压力和堆积密度对晶须生长的影响,并详细讨论了在燃烧合成过程中晶须的生长机理。 相似文献
14.
The ceramic nanocomposites (CNCs) like zirconia toughened alumina (ZTA) ceramics are important futuristic materials for structural and functional applications in advanced strategic systems, structural components, biomedical prostheses and devices. In all structural materials including the ZTA CNCs, the very early stages of plastic deformation i.e., the incipient plasticity events (IPE) are most important to be understood so that the microstructure and mechanical properties can be tuned to suit a given end application. Here we report for the first time the mechanisms of IPE in the nanoindentation experiments conducted at 10–1000 mN loads in the 40 ZTA CNCs. Here 40 ZTA CNC stands for 40 vol% of 3 mol% Yttria partially stabilized zirconia toughened alumina (40ZTA) CNC. The role of load ranges in variations of the IPE related parameters in the 40 ZTA CNCs is also studied. Further, an attempt is made to assess how the amount of zirconia content in ZTA CNCs affects the variations of the IPE related parameters. Through the extensive usage of field emission scanning electron microscopy (FESEM) and theoretical estimations, efforts are also directed to check out the linkage, if any, between the localized shear deformation and/or microcracking with the IPE events that occur in the present CNCs. In addition, a new concept of damage resistance is introduced for the first time in the present work to explain the presence of a strong indentation size effect (ISE) in the 40 ZTA CNCs. Finally, an attempt is also directed to understand how the indentation load (P) controls the relative size of interaction zones of dislocation loops as well as the damage resistance and thereby, engineer the acuteness of the ISE in ZTA CNCs. The implication of these findings in futuristic design of especially the ZTA CNCs for various applications is also discussed. 相似文献
15.
This article focuses on the nano-crystalline tricobalt tetraoxide (Co 3O 4) synthesis using the combustion method. The fabrication process involves the combustion a mixture of cobalt nitrate as oxidized and expanded polystyrene (EPS) as a novel fuel. The effects of fuel/oxidizer ratio on the phases formed, Co 3O 4 as a major phase and CoO as an impurity, and the Co 3O 4 crystallite size have been addressed. Based on the thermal analyses data (TGA and DTA), which follow the phase changes during the heat treatment of the parent fuel/oxidizer precursor, the different parent precursors have been calcined at 500?°C. Characterization of the formed solids has been performed using various tools (XRD, FT-IR, SEM, TEM, and XPS). It was concluded that the EPS content, in the combustion precursor mixtures, is a key parameter that controls the phase features of the prepared nanomaterials. CoO impurity was detected for the solids with EPS?>?4?wt%. The crystallite size, morphology, and the surface elemental concentration are influenced by changing the EPS content. The activity of the different powders towards hydrogen peroxide decomposition has been evaluated at the 35–50?°C temperature range. 相似文献
16.
CuIn 1‐xGa xSe 2 and CuIn(S ySe 1‐y) 2 bulk materials with relative densities of 93.4%‐96.2% have been prepared by combustion synthesis, from elementary reactants and in a reaction time of a few seconds. The samples have a chalcopyrite lattice structure, and the lattice parameters decrease with increasing x and y. The substitution of In with Ga and Se with S in CuInSe 2 causes an increase in the bandgap. In combustion synthesis, the high temperature accelerates the reaction and results in fast densification, the high heating rate simplifies the reaction path and avoids the formation of intermediate compounds, and the gas pressure depresses the evaporation of reactants. As a fast, furnace‐free, and scalable technique, combustion synthesis may offer a low‐cost way to produce CuInSe 2‐based materials and bring new possibilities to their commercial applications. 相似文献
17.
Combustion synthesis has been applied to LaMnO 3 production with a view to boosting its activity towards natural gas combustion by enhancing its specific surface area. With a highly exothermic and self-sustaining reaction, this oxide can be quickly prepared from an aqueous solution of metal nitrates (oxidisers) and urea (fuel). The favourable conditions for LaMnO3 formation were sought: only fuel-rich mixtures are effective, but carbonaceous deposits are formed when too much urea is used. In the field of operating conditions in which the combustion synthesis reaction takes place, the specific surface areas were not dramatically higher than those obtained with traditional methods; moreover, even short thermal treatments have been found to rapidly deactivate the catalysts by rapid sintering. With a view to tackling these problems, NH4NO3 was chosen as an additive for its low costs, highly exothermic decomposition and because it generates gaseous products only, without altering the proportion of the other elements in the catalysts. With ammonium nitrate, specific area was enhanced from 4 m2/g up to about 20 m2/g. A short thermal treatment at 900 °C partially deactivates also the NH4NO3-derived catalysts. It was found that NH4NO3-boosted mixtures produce materials whose activity, after a similar thermal treatment, behave practically as the perovskites obtained by the “citrates” method. Combustion synthesis is though rather cheap—in terms of reactants employed—and quick, given that the process requires few minutes at low temperature without successive calcination. However, the main drawback of this method is that hazardous or polluting compounds are emitted during the synthesis (mainly NH3 or NOx). The MgO introduction, which should act both as a structural promoter and as a sulphur poisoning limiting agent, has proved to be harmful: since MgO does not physically interpose between perovskite grains, it does not offer resistance to deactivation induced by high temperatures. 相似文献
18.
The influence of mechanical activation of powder mixtures of Si and C, via high energy attrition milling (up to 12 h), on combustion synthesis of SiC was experimentally investigated. β-SiC fine powder was successfully fabricated in 1.0 MPa N 2 atmosphere without other additional treatments, such as preheating, electric action, or chemical activation. Relatively weak peaks of α-SiC, α-Si 3N 4 and Si 2ON 2 were also found in the final products. The experimental results and their theoretical treatment showed that mechanical activation via high energy ball-milling provides to the initial Si/C powder mixture extra energy, which is needed to increase the reactivity of powder mixture and to make possible the ignition and the sustaining of combustion reaction to form SiC. 相似文献
19.
The objective of this work was to study two-step sintering as a means of controlling the microstructure of coarse Al 2O 3 matrix composites containing submicrometric and nanometric inclusions of ZrO 2 ranging from 0–30 wt. % by weight based on commercially available powders and evaluate its hydrothermal degradation as function of a water vapour pressure and its mechanical properties. The results showed that two-step sintering allowed a more efficient microstructural control than single-step sintering, resulting in good mechanical properties. The highest flexural strength was achieved for ZTA samples sintered in two-stage sintering conditions TSS2 with T1 = 1560 °C for 3 h, T2 = 1460 °C for 8 h. The studied composites showed good resistance to hydrothermal degradation compared to composites sintered in single step sintering conditions. 相似文献
20.
Uniform quasi-aligned AlN nanowhiskers grown in the reacting Al particles have been successfully prepared in high content by combustion synthesis using NH 4Cl as a morphology-controlled promoting additive. FE-SEM and TEM images show that the nanowhiskers, which are single-crystalline hexagonal wurtzite AlN growing along [0 0 1] direction, have diameters in the range of 80–170 nm and a length of several to several tens of micrometers. The effect of NH 4Cl on the growth of nanowhiskers was discussed. It was found that NH 4Cl not only controlled the products’ morphology, but also changed the combustion behavior and nitridation mechanism in the combustion synthesis process. 相似文献
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