A study of silicon nitride ceramic radomes, which includes preparation of the material and optimal design of the radome wall structure, is presented in this paper. Multilayer radome wall structure with high dielectric...
A study of silicon nitride ceramic radomes, which includes preparation of the material and optimal design of the radome wall structure, is presented in this paper. Multilayer radome wall structure with high dielectric constant skins and a low dielectric constant core layer is used for broadband application. As a candidate material for both the skins and core layer, silicon nitride ceramics of controlled dielectric constant in the range 3.0∼7.5 were prepared by adding different content of sintering aids such as magnesia, alumina, silica and zirconium phosphate binder and choosing suitable sintering methods. A computer aided design (CAD) for the wall structure of silicon nitride multilayer ceramic radome based on microwave equivalent network method is carried out according to design requirements. By optimizing the thickness of skins and core layer, the power transmission efficiency of such a multilayer Si3N4 ceramic radome is calculated. The calculated results suggest that when the dielectric constant of skins lies in the range 6∼7.5 and core layer in the range 3.5∼4, the power transmission efficiency is above 85% with frequency of 2∼18 GHz while the thickness of skins is less than 0.03λ and the thickness ratio of skins to core layer is less than 1:15.
The temperature field of two types of porous graded ZrO2 subjected to thermal shock loading is analyzed in this paper using the finite‐element method. The numerical simulation results indicate that the structure of Z...
The temperature field of two types of porous graded ZrO2 subjected to thermal shock loading is analyzed in this paper using the finite‐element method. The numerical simulation results indicate that the structure of ZrO2 with an un‐symmetric porous microstructure is superior to symmetric one and that, ignoring the influence of temperature dependent properties, the effects of thermo‐mechanical coupling can be ignored if the imposed heat flux density is less than 4 WM/m2.
In the present study, Si3N4 ceramics with pore gradient structure were prepared by spark plasma sintering (SPS) technique. Silicon nitride ceramics with different controlled porosities were prepared by using ZrP2O7 as...
In the present study, Si3N4 ceramics with pore gradient structure were prepared by spark plasma sintering (SPS) technique. Silicon nitride ceramics with different controlled porosities were prepared by using ZrP2O7 as a binder material and heat treated at 1100 °C in a pressureless nitrogen atmosphere. Si3N4 ceramics with controlled porosity of 34–47% were obtained. The distribution of porous structure was homogenous. Fully dense Si3N4 ceramics could be sintered at 1400∼1600 °C by using MgO and alumina Al2O3 as the sintering aids. Pore gradient structure was formed by laminating the Si3N4 porous ceramics and powder mixture was used to obtain fully dense ceramics, and then sintering at 1400–1600 °C. Microstructure of sintered samples was observed by scanning electronic microscope (SEM) and the change of phase compositions was analyzed by X‐ray diffraction (XRD). The results showed that these samples exhibited a good porous graded structure with a highly porous layer and a dense surface layer. The major phase of the Si3N4 ceramics was still α phase.
Laser cladding has become an extensively attractive technique in the field of biomaterials. Significant effort has been devoted to combine mechanical properties of metal with biologic properties of bioceramic by this ...
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ISBN:
(纸本)9780912035895
Laser cladding has become an extensively attractive technique in the field of biomaterials. Significant effort has been devoted to combine mechanical properties of metal with biologic properties of bioceramic by this technique. The paper described the gradient bioceramic coatings on titanium alloy substrate (Ti-6Al-4V) obtained by laser cladding. The surface and cross-sections morphology of the composites was observed by scanning electron microscope (SEM). The element distribution of calcium phosphate coatings were investigated by electron probe microscopic analyzer (EPMA). Phase composition of the coatings was analyzed by X-ray diffraction (XRD). Microhardness tests also were performed. These analyses showed that the obtained coatings surface was homogeneous with well phase composition, and the coatings structure was gradual variation as well as a good microhardness distribution.
In the present work, three kinds of alloys with nominal compositions (at. %) of Ni-25Al, Ni-15Al-10Si and Ni-5Al-20Si have been fabricated successfully by a 10 kW continuous wave CO2 laser under optimum processing con...
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ISBN:
(纸本)9780912035895
In the present work, three kinds of alloys with nominal compositions (at. %) of Ni-25Al, Ni-15Al-10Si and Ni-5Al-20Si have been fabricated successfully by a 10 kW continuous wave CO2 laser under optimum processing conditions. X-ray diffraction (XRD) results showed that the phases of all alloys consisted mainly of Ni-Al type intermetallic compounds. The microstructures of three alloys were characterized using scanning electron microscopy (SEM). The hardness was measured on a HX-1000TM Vickers microhardness tester. The average hardness of Ni-15Al-10Si alloy was the highest value of 914 HV compared with Ni-25Al and Ni-5Al-20Si alloys.
Sintering behavior, microstructure and microwave dielectric properties of Ba1+x(Mg1/3Nb2/3)O3 (-0.01 [less-than or equal to] x [less-than or equal to] 0.01) were investigated. An improvement of the degree of 1:2 order...
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Sintering behavior, microstructure and microwave dielectric properties of Ba1+x(Mg1/3Nb2/3)O3 (-0.01 [less-than or equal to] x [less-than or equal to] 0.01) were investigated. An improvement of the degree of 1:2 ordering and sinterability was found in Ba-deficient Ba(Mg1/3 Nb2/3)O3 because there existed Ba vacancies. Excessive Ba inhibits the sintering and the degree of 1:2 ordering. It was proposed that excessive Ba exist in grain boundary and inhibit the movement of grain boundary. Therefore, the sintering was restrained. Ba-deficiency had little effect on dielectric constant of Ba(Mg1/3Nb2/3)O3 ceramics. With Ba-deficiency increasing, Qf value decreased drastically.
In this paper the inflaming retarding effects of epoxy resins blended with basic magnesium carbonate [Mg(OH)2·4MgCO3·5H2O] were synthesized. When the add-on of basic magnesium carbonate reached 56.5%, the LO...
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In this paper the inflaming retarding effects of epoxy resins blended with basic magnesium carbonate [Mg(OH)2·4MgCO3·5H2O] were synthesized. When the add-on of basic magnesium carbonate reached 56.5%, the LOI of the epoxy resins increased to 29.7%. The flame retardant mechanisms of basic magnesium carbonate for epoxy resins was investigated by XRD, TG-DSC and SEM. Results showed that through the ways of diluting, cooling, isolating and their synergies, basic magnesium carbonate achieves the flame retarding target.
The air plasma spray (APS) technology was used to prepare WC/NiCrAl and WC/NiCrBSi ceramic/metal composite wear preventive coating on cast iron substrate. The microstructure, microhardness, bond strength and slurry we...
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The air plasma spray (APS) technology was used to prepare WC/NiCrAl and WC/NiCrBSi ceramic/metal composite wear preventive coating on cast iron substrate. The microstructure, microhardness, bond strength and slurry wear preventive performance of coating was measured. The results revealed that all of the coating prepared by APS has good qualities. The coating with NiCrBSi as its combining metal is better than that with NiCrAl on wear preventive performance, and coating containing more WC is supreme than that containing less WC Finally, wear mechanics and the principle of the coating's wear preventive property was concluded.
Based on a high power CO2 laser beam passing by pyramid polygon mirror, the bioceramic coatings of gradient composition were fabricated on titanium alloy substrate (Ti-6A1-4V). The relations among laser processing par...
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Based on a high power CO2 laser beam passing by pyramid polygon mirror, the bioceramic coatings of gradient composition were fabricated on titanium alloy substrate (Ti-6A1-4V). The relations among laser processing parameters, microstructure and biocompatibility of the gradient bioceramic coatings were investigated. The results indicated that the contents of rare earth oxide additions had an immediate effect on the formation of bioactive phases. The gradient bioceramic coatings showed favorable biocompatibility in vivo after they were implanted into canine femur for 45, 90, and 180 days, respectively. The bioceramic coatings of Ca/P=1.4 and 0.6wt.% Y 2O3 totally combined with new bones merely implanted for 45 days. Furthermore, the MTT (Methyl Thiazolyl Tetrazolium) colorimetry results of cell proliferation demonstrated that the cell growth distinctly increased on the gradient bioceramic coatings by laser cladding compared with the un-treated titanium alloy substrate.
Water transport in membrane plays an important role in PEM fuel cell performance. Therefore research on water transport in membrane is extremely essential. This article established a 3D single fuel cell model with ser...
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Water transport in membrane plays an important role in PEM fuel cell performance. Therefore research on water transport in membrane is extremely essential. This article established a 3D single fuel cell model with serpentine flow field. The influences of the operating temperature, membrane thickness, air stoichiometry, relative humidity, operating pressure and current density on the water content in membrane under low current density are studied, and then the influence of the above parameters on the performance of fuel cell is obtained. The results show that: As the temperature rises, the water content will increased and then lower. Reducing the proton exchange membrane thickness and air stoichiometry, increasing the relative humidity, pressure and current density operation, the water content in the membrane will increase, the conductivity will increase and thus will improve the performance of the fuel cell.
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