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1.
正A novel composite ceramics with excellen mechanical and dielectric properties was fabricated by means of low temperature hot-pressing using mechanically alloyed SiBON powders as raw materials.The influences of SiO_2 content on phase microstructure,mechanical and dielectric properties of the SiBON ceramics were investigated.The  相似文献   

2.
The electrical properties of (Nb,Li)-doped SnO2 ceramics as a new varistor material were investigated.The sample 97.95% SnO20.50%Li2O0.05%Nb2O5( mol fraction)sintered at 1450℃ possess the highest density(p=6.77g/cm^3) and nonlinear electrical coefficient(α=11.6).The substitution of Sn^4 with Li^ increases the concentration of oxygen vacancies,together with the formation of solid solution ,which will increase the sintering rate greatly and decrease the optimized sintering temperature.The substitution of Sn^4 with Li^ and the variation of temperature play very important effects on the densities,dielectric constant,nonlinear electrical properties and other characteristics of the samples.The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature.A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.  相似文献   

3.
The electrical properties of (Nb, Li)-doped SnO2 ceramics as a new varistor material were investigated. The sample 97.95%SnO2·0.50%Li2O·0.05%Nb2O5 (mol fraction) sintered at 1450= possess the highest density (ρ=6.77 g/cm3) and nonlinear electrical coefficient (α=11.6). The substitution of Sn4+ with Li+ increases the concentration of oxygen vacancies, together with the formation of solid solution, which will increase the sintering rate greatly and decrease the optimized sintering temperature. The substitution of Sn4+ with Li+ and the variation of temperature play very important effects on the densities, dielectric constant, nonlinear electrical properties and other characteristics of the samples. The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature. A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.  相似文献   

4.
A series of Na_2O–Ca O–Si O_2 glass ceramics containing different content of Nd~(3+)ions were prepared by the method of high temperature melting and subsequent crystallization.The absorption,excitation and emission spectra of these glass ceramics were investigated;effects of Nd~(3+)content and crystallization behavior on the laser properties of this material had been studied.The results show that the emission bands originating from the ~4F_(3/2) state of Nd~(3+)were firstly enhanced with the increase of the Nd_2O_3 doping content and the crystallinity degree,and then decreased with more doping content and deepened crystallization.The possible reasons of this phenomenon were analyzed.Research will be favored to promote the development of glass ceramics laser materials for space solar energy.  相似文献   

5.
High performance X8R dielectric ceramics were prepared by dopingBi2O3 to BaTiO3-based ceramics.The effect of small amounts(≤1.2 mol%) ofBi2O3 additive on the microstructure and dielectric properties of BaTiO3-based ceramics have been investigated.The Bi2O3 ,acting as a sintering additive,can effectively lower the sintering temperature of BaTiO3-based ceramics from 1300 to 1130 °C.The bulk density of BaTiO3-based ceramics increased and reached the maximum value with increasingBi2O3 content.The dielectric con...  相似文献   

6.
The influences of equimolar substitution of yttria by gadolinia on the phase constituent and stability for Y_2O_3—Ta_2O_5—ZrO_2 ceramics have been investigated.The ceramics with the Gd content lower than 8 nnol%exhibit pure tetragonal phase as well as high tetragonal phase stability.However,the high Gd containing ceramics consist of t-ZrO_2,m-ZrO_2 and monoclinic GdTaO_4 and show complicated phase evolution behaviors.The fractions of monoclinic ZrO_2 and GdTaO_4 increase with increasing Gd content,indicating that the excessive substitution of Gd for Y results in a reduction in the tetragonal phase stability.In addition,the lower Gd containing ceramics show an almost invariable tetragonality,while the higher Gd containing ceramics reveal a composition-dependent tetragonality.Accordingly,an association of the tetragonal phase stability and tetragonality with the Gd concentration is established,which provides us a clue to understand the phase stability of these ceramics.  相似文献   

7.
Two kinds of different structured alumina—titania composite powders were used to prepare alumina matrix ceramics by microwave sintering.One was powder mixture of alumina and titania at a micron—submicron level,in which fused-and-crushed alumina particles(micrometers) was clad with submicron-sized titania.The other was powder mixture of alumina and titania at nanometer—nanometer level,in which nano-sized alumina and nano-sized titania particles were homogeneously mixed by ball-milling and spray dried to prepare spherical alumina—titania composite powders.The effect of the microstructure of composite powders on microstructure and properties of microwave sintered alumina matrix ceramics were investigated.Nano-sized composite(NC) powder showed enhanced sintering behavior compared with micro-sized composite(MC) powders.The asprepared NC ceramic had much denser,finer and more homogenous microstructure than MC ceramic.The mechanical properties of NC ceramic were significantly higher than that of MC ceramic,e.g.the flexural strength,Vickers hardness and fracture toughness of NC ceramic were 85.3%,130.3%and 25.7%higher than that of MC ceramic,respectively.The improved mechanical properties of NC ceramic compared with that of MC ceramic were attributed to the enhanced densification and the finer and more homogeneous microstructure through the use of the nanostructured composite powders.  相似文献   

8.
The B2O3-doped MgTiO3 powders and ceramics have been prepared by sol-gel method using Mg(NO3)2·6H2O, Ti(C4H9O)4 and H3BO3 as the starting materials. The sintering behavior and microwave dielectric properties of ceramics prepared from powders with different particle sizes were investigated. The gels were calcined at 650, 700, 750, 800, 850 and 900 C and the derived particle sizes of powders were 20-30 nm, 30-40 nm, 40-60 nm, 60-90 nm, 90-120 nm and 120-150 nm, respectively. The nanoparticles with the size of 30-60 nm benefited the sintering process with high surface energy whereas nanoparticles with the size of 20-30 nm damaged the microwave dielectric properties due to the pores in the ceramics. The addition of B2O3 used as a liquid sintering aid reduced the sintering temperature of MgTiO3 ceramic, which was supposed to enter the MgTiO3 lattice and resulted in the formation of (MgTi)2(BO3)O phase. The B2O3-doped MgTiO3 ceramic sintered at 1100 C and prepared from the nanoparticles of 40-60 nm had compact structure and exhibited good microwave dielectric properties: εr=17.63, Q × f=33,768 GHz and τ f= 48×10 6 C 1.  相似文献   

9.
Si-B-O-N powder without B-O bonds synthesized by polymeric precursor were hot-pressed into ceramics at different tempera-tures.The variations of microstructure and mechanical properties of Si-B-O-N ceramics have been investigated.Crystallization of Si-B-O-N ceramics occurred at about 1400℃.Density, elastic modulus,and flexural strength of the ceramics increased with the increasing sintering temperatures, and reached to their maximum values at 1600℃ .By contrast, hardness and frac-ture toughness of the ceramics monotonically changed with increasing sintering temperatures.Hardness decreased,while the fracture toughness increased.The principal toughening mechanisms including crack deflection, crack bridging and plate grain pulling-out effects are discussed.  相似文献   

10.
A plasma electrolytic nitrocarburising(PEN/C) process was performed on stainless steel 316L to improve the surface properties for using as medical implants.A bath was optimised to reduce the required voltage to 150 volts.Aqueous urea-based solutions with 10% NH 4 Cl were prepared with slightly different amounts of Na 2 CO 3 to optimise the electrolyte composition.The surface and the cross-section morphologies were studied by scanning electron microscopy.The microstructure and the chemical composition of samples were investigated by X-ray diffraction(XRD) and energy dispersive X-ray(EDX) techniques.The microstructure of the outer layer of the coatings was found to be a complex oxide containing Cr and Fe.The wear properties of the samples were examined by using a pin on disk wear test with Ringer s solution and were compared with their wear properties in the ambient atmosphere.The Ringer s solution acted as a lubricant,reducing friction coefficient.Hardness and roughness were also studied.The bath with the composition of 10% NH 4 Cl and 3% Na 2 CO 3 exhibited the best tribological properties.The results showed that the tribological properties of treated samples were improved and the wear mechanism was abrasion of the pin.  相似文献   

11.
Lead free polycrystalline ceramics (K0.5Na0.5)Nb(1-x)MoxO3 (x = 0, 0.02, 0.04, 0.06 and 0.08) have been synthesized via solid state reaction method. The formation of single phase perovskite structure up to 6 mol% of Mo^6+ has been confirmed by X-ray diffraction pattern. Impedance spectroscopy reveals that bulk resistance decreases with increasing temperature, which indicates negative temperature coefficient of resistance (NTCR) behaviour of the compounds. The diffuse reflectance spectroscopy results indicate a red shift of the band gap energy of K0.5Na0.5NbO3 (KNN, from 4.28 to 3.61 eV) with increasing Mo^6+ concentration due to structural modification. The photoluminescence spectra of doped samples are composed of two emission bands at room temperature. One emission band is near band edge ultraviolet (UV) emission (354 nm) and other is visible emission band (-397 nm) which may explore the possibility of these ceramics to be used in optical device applications.  相似文献   

12.
Structural and various electrical properties of polycrystalline Ca0.05+xNd0.5-x(Ti0.5Fe0.5)O3,prepared by standard solid state reaction technique,were studied.Formation of single phase orthorhombic structure of the compositions was confirmed by X-ray diffraction study.The composition dependence of lattice parameters, density and microstructural study show that they vary significantly with Ca content.The dielectric measurements were carried out at room temperature as function of frequency and composition.The experimental results reveal that the dielectric constant(ε’) increases with increasing Ca content.Similar behavior is observed for the dielectric loss(tanδ) and ac conductivity(σac).In complex impedance analysis it is observed that the real part(Z’) vs imaginary part(Z") graph exhibits a tendency of formation of a single semicircular arc for each composition of samples.Different parameters were determined by fitting the experimental data in Cole—Cole empirical formula.A dominance of grain boundary resistance(Rgb) is observed.The Rgb decreases with increasing Ca content.The highε’ observed in present samples are suitable for fabrication of devices.  相似文献   

13.
The frequency dependent dielectric properties of barium magnesium tantalate(BMT),Ba(Mg_(1/3)Ta(2/3))O_3 and barium zinc tantalate(BZT),Ba(Zn_(1/3)Ta_(2/3))O_3 synthesized by solid state reaction technique have been investigated at various temperatures by impedance spectroscopy.BMT and BZT possess cubic structure with lattice parameter a = 0.708 and 0.451 nm,respectively.The resonance peaks due to dielectric relaxation processes are observed in the loss tangent of these oxides.The relaxation in the samples is polydispersive in nature.The temperature dependence of dc conductivity,the most probable relaxation frequency(ω_m) obtained from tanδ vs logw plots and ω_m obtained from imaginary parts of the complex electrical modulus vs logw plots follow the Arrhenius behavior.According to these Arrhenius plots the activation energies of BMT and BZT are about 0.54 and 0.40 eV,respectively.Thus the results indicate that samples are semiconducting in nature.The frequency-dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms.Both these formalisms show qualitative similarities in relaxation time.Our study points that for complex perovskite oxides with general formula A(B'B")O_3,the dielectric properties significantly depend on the atomic radii of both A and B type cations.BMT and BZT exhibit enhancement in dielectric property compared to their niobate counterparts.They may find several technological applications such as in capacitors,resonators and filters owing to their high dielectric constant and low loss tangent.  相似文献   

14.
Al-Si eutectic growth mechanism was investigated in a directionally-solidified AI-1 3 wt% Si alloy with different strontium (Sr) and magnesium (Mg) additions, growth velocities and temperature gradients. Macro- and micro- scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying AI-Si eutectic front and the final morphology of eutectic grains in the solidified A1-13 wt% Si alloys. By varying (tailoring) these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition (CET) of eutectic structure, which can be interpreted on the basis of Hunt's Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.  相似文献   

15.
The effect of titanium addition on the microstructure and mechanical properties of brass Cu4OZn has been studied via the powder metallurgy (P/M) route. The water-atomized Cu4OZn-1.0 wt% Ti alloy powder was consolidated at different temperatures in the range of 400-600℃ using spark plasma sintering (SPS) and hot extrusion subsequently. Results show that the super-saturated solid solution titanium element in rapidly cooled brass Cu4OZn powder created high chemical potential for a precipitate reaction, showing significant grain refinement effects on the consolidated Cu4OZn matrix. Consequently, excellent mechanical properties were obtained by precipitation hardening and work hardening after sintering and extrusion, with yield strength of 390 MPa, ultimate tensile strength of 617 MPa, and Vickers micro-hardness of 192 HV, which are 28.7%, 23.4%, and 23.9% higher values than those of extruded Cu4OZn brass, respectively.  相似文献   

16.
In this research, fabrication of a(Ti,Hf)-rich NiTiHf alloy by using vacuum induction melting(VIM) process and a graphitic crucible was investigated. For this purpose, casts with the nominal composition of Ni49Ti36Hf15 were prepared in graphitic crucible and mold. Optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) tests were employed to characterize the samples. Results demonstrated that microstructure of the first cast was composted of a B2 austenite phase as well as a great amount of two differently formed(Ti,Hf)C carbides.Moreover, no austenite 4 martensite transformation peak was detected in the DSC curve of this sample,indicating a drastic decline in the transformation temperatures. In the succeeding cast, however, owing to the formation of carbide layers on the inner surfaces of the graphitic crucible and mold during the initial casting process, the amounts of carbides decreased remarkably. This cast exhibited transformation temperatures above100° C, while XRD pattern denoted the presence of B190 monoclinic martensite phase at room temperature.All in all, results confirmed that VIM process using graphitic mold and crucible can be considered as an appropriate method for the fabrication of(Ti,Hf)-rich NiTiHf high temperature shape memory alloys.  相似文献   

17.
The role of double oxide film (bifilm) defects in the formation of gas porosity in commercial purity and Srcontaining AI alloys was investigated by means of a reduced pressure test (RPT) technique. The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt. The melt was then subjected to different "hydrogen addition" and "holding in liquid state" regimes before RPT samples were taken. The RPT samples were then characterized by determining their porosity parameters and examining the internal surfaces of the pores formed in them by scanning electron microscopy. The results indicated oxide defects as the initiation sites for the growth of gas porosity, both in commercial purity and Sr-containing AI alloys. The results also rejected reduction of the surface tension of the melt, increase in the volumetric shrinkage and reduction in interdendritic feeding as the possible causes of an increase in the porosity content of the AI castings modified with strontium. The change in the composition of the oxide layers of double oxide film defects was suggested to be responsible for this behaviour.  相似文献   

18.
The effect of dispersion with different weight fractions of Al2O3 particles in metallic matrices (AI/Mg/Ti/Cu) fabricated by powder metallurgy was investigated. In the case of 15 wt% Al2O3 reinforced composites, peak hardness was attained which subsequently decreased with increasing the content of Al2O3. A correlation between the microhardness and nanomechanical properties at submicron scale was examined for all the composites. Specific strength and specific modulus were measured in order to figure out the performance of the composites.  相似文献   

19.
A SiO_2-TiO_2 template with ordered tubular mesochannels has been prepared by the sol—gel method.Au nanorods are deposited in the tubular mesochannels of the SiO_2—TiO_2 template,and the shape of Au is changed from nanorods to nanospheres by ultraviolet irradiation during thermal deposition.The photocatalytic activity of mesoporous SiO_2—TiO_2 with/without Au nanorods/nanospheres is evaluated.Deposition of Au in the mesoporous SiO_2—TiO_2 template enhances the photocatalysis of TiO_2.Interestingly,the sample containing Au nanorods exhibits higher photocatalytic activity than that with Au nanospheres.Photocatalysis by exciting surface plasmon resonance is not detected in the composite samples regardless of the shape of the deposited Au nanoparticles.  相似文献   

20.
A β-NiAl alloy with normal purity, a S-doped and a Dy and S co-doped b-NiAl alloys were prepared by arc-melting and their corresponding S contents were less than 20×10 6, 33×10-6 and 22×10-6, respectively. The isothermal oxidation behavior of the alloys at 1200° C was investigated and the extent of S segregation at the scalee alloy interface was determined by scanning Auger microscopy. S-doping had no significant effect on the phase transformation rate from q- to a-Al2O3, while the addition of Dy retarded this process. For the Sdoped alloy, scale rumpling occurred only after 2 h thermal exposure and numerous large voids were observed at the scalee alloy interface where S segregated. In contrast to this, the oxide scale formed on the Dy and S co-doped alloy still remained flat even after 50 h isothermal oxidation and only small voids existed at the interface where S segregation was not detected.  相似文献   

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