首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The thermal shock resistance of miniaturized multilayer ceramic capacitors (MLCs), of sizes 0402, 0603, 0805 and 1206, was investigated by comparing the leakage currents before and after thermal shock. It was generally found that smaller capacitors have a higher thermal shock resistance than larger ones. The 0402 MLC possesses a thermal shock resistance in excess of 420 C. The linear interdependence of thermal shock resistance and reciprocal of half thickness, as predicted by conventional thermal shock analysis, was not observed. Instead, the thermal shock resistance of an MLC was found to be inversely proportional to the total area of its ceramic surface. This confirms that pre-existing flaws on the ceramic surface dominate the crack initiation process and are therefore primarily responsible for determining the thermal shock resistance of an MLC.  相似文献   

2.
Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm2, and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.  相似文献   

3.
Under consideration is the effect of nonideality of the components in a gas mixture on the process of their separation by thermal diffusion. It is demonstrated that in the expressions for the heat flux and the mass flux, the thermodiffusion ratio and the characteristic of diffusional thermal conductivity the effect of nonideality appears in the heat of mixing.Notation p pressure - density - length of the mean free path for molecules during transport of particles - length of the mean free path for particles during a transfer of the mean velocity - n molecule concentration - M molecular weight - I particle flux - J mass flux - m mass of a molecule - t time - Dij coefficient of interdiffusion for a binary mixture - D i T coefficient of thermal diffusion - KT thermodiffusion ratio - T thermodiffusion constant - xi molar fraction of the i-th component in the mixture (r), intermolecular interaction potential - r intermolecular distance - collision integrals - T temperature - T* referred temperature - R universal gas constant - k Boltzmann constant - Ñ Avogadro's number - v mean velocity of molecules - ¯V diffusion rate - i, trans thermal conductivity associated with translatory degrees of freedom - fi(r, v, t) velocity distribution function of molecules - viscosity - i chemical potential of the i-th component - ci mass fraction - o thermal conductivity at the initial instant of time - thermal conductivity in the steady state - DT diffusional component of thermal conductivity - g and h molar thermodynamic functions - ¯g and ¯h specific thermodynamic functions - cp specific heat - Jq heat flux - Jq reduced heat flux - B second virial coefficient - U* transport energy - coefficient of thermal expansion - coefficient of isothermal compression - fi activity coefficient for the i-th mixture component Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 829–839, May, 1981.  相似文献   

4.
A new method to measure the thermal diffusivity of liquids is presented. It requires determination of the time dependence of the thermal expansion of the liquid when it is subjected to a heat source at the top of the cell containing the liquid. The high accuracy of the method (about 3%) is due to an essential reduction of convective currents and also to the absence of temperature detectors, which generally introduce unwanted perturbations on the thermal Field.Nomenclature Thermal conductivity - c Specific heat - Density - c = specific heat x density - h Newton coefficient - Thermal diffusivity - T, 0 Temperature - tV Electric signal - Calibration coefficient - exp, th Volume change of the liquid  相似文献   

5.
The problem of freezing of pure water in a round pipe is treated with due regard for convection under asymmetric thermal boundary conditions in the absence of motion along the pipe. The problem is solved numerically using the control volume approach, SIMPLER algorithm, and the enthalpy method. Results are obtained for three Grashof (Gr) and six Biot (Bi) numbers: Gr = 1.55 × 106, Bi = 0.305 (0 < ), Bi = 0.044 ( < 2); Gr = 1.24 × 107, Bi = 0.610 (0 < ), Bi = 0.087 ( < 2); Gr = 9.89 × 107, Bi = 1.220 (0 < ), Bi = 0.174 ( < 2). The correctness of calculation of the problem disregarding free-convection flows is analyzed.  相似文献   

6.
An analytical solution of the thermal conductivity problem with boundary conditions of the third kind and arbitrary coordinate and time dependence of the Biot number is found in the form of a converging series of quadratures.Notation , z dimensionless coordinates - dimensionless temperature - Q dimensionless volume heat-liberation density per unit time - Fo=/2 Fourier number - Bi1(, Fo)=(, Fo) · / Biot number - thermal diffusivity coefficient - plate thickness - time - (, Fo) heat-liberation coefficient - thermal conductivity coefficient - i summation index - Jo zero order Bessel function of the first kind Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 3, pp. 536–540, September, 1981.  相似文献   

7.
The white phosphor matrix 4SrO·7Al2O3 has been synthesized by firing the appropriate mixture of SrCO3, Al(OH)3 and H3BO3 in the molar ratios 1:3.5:0.135 at 1300°C for 4–7 h. The crystal structure of 4SrO·7Al2O3 has been determined as a orthorhombic Pmma space group with a=24.7451(2)Å, b=8.4735(6)Å, c=4.8808(1)Å, V=1023.41(3)Å3, Z=2, and D=3.66 g cm–3 by the Rietveld analysis. The refinement figures of merit are Rp=8.26, Rwp=11.60, Rbragg=4.44 and s=2.61 for 844 reflections with 2<119.94°. And the corresponding X-ray powder diffraction data are presented for search/match analysis.  相似文献   

8.
The formation of tricalcium phosphate (TCP) from dicalcium phosphate and calcium carbonate was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). DTA showed three distinct thermal events attributed to dehydration of dicalcium phosphate dihydrate (brushite, DCPD) to dicalcium phosphate anhydrous (monetite, DCPA), the formation of beta-calcium pyrophosphate (-Ca2P2O7), and the calcination of calcium carbonate. TGA showed three weight losses corresponding to the three thermal events, respectively. XRD analysis showed that -TCP formed, beginning at about 900°C, by the reaction of -Ca2P2O7 with CaO and -TCP changed to -TCP above 1200°C. Further examination of the formation of TCP from calcium hydroxide (Ca(OH)2) and DCPA showed that -TCP would form only after the decomposition of Ca(OH)2 to CaO and from the reaction of CaO with -Ca2P2O7 at a fairly low temperature of 800°C. In addition, by naturally cooling -TCP, formed at 1300°C within the heating furnace, to room temperature, it was difficult to obtain a pure phase of -TCP. The proposed mechanism of the reaction to form TCP may include the dehydration of brushite to monetite, dehydration of monetite to beta-calcium pyrophosphate, decomposition of calcium carbonate, the formation of -TCP, and phase transition.  相似文献   

9.
The mechanical behaviour of the multi-phase ( + /) alloy Ni-20 at % Al-30 at % Fe and alloys similar to its constituent and / phases, Ni-30 at % Al-20 at % Fe and Ni-12 at % Al-40 at % Fe, respectively, were investigated. When tested in tension at 300 K, the alloys exhibited 20%, 2% and 28% elongation, respectively. At elevated test temperatures (700, 900 and 1100 K), the multi-phase alloy exhibited increased ductility, reaching an elongation in excess of 70% at 1100 K without necking or fracture. Similarly, the alloy demonstrated increased ductility with increasing test temperatures. In contrast, the / alloy showed greatly reduced ductility with increasing temperature and was quite brittle both at 900 and 1100 K. Thus, whilst at room temperature the / phase improved the ductility of the + / aggregate, at elevated temperatures the phase alleviated the brittleness of the / phase, thereby preventing any embrittlement of the multi-phase alloy over the temperature range 300–1100 K. Also, whilst the phase improved the room-temperature strength of the multi-phase alloy, at elevated temperatures where the phase is known to be weak, the / phase improved the strength of the multi-phase alloy up to 900 K, beyond which the strength deteriorated due to disordering and lack of anomalous strengthening in the / component.  相似文献   

10.
The compositions of high magnesium content -magnesium sialon crystals within the 3M/4X plane of the Mg-Si-Al-O-N system indicate some form of metal atom ordering wihtin the structure. Although it is not possible using X-ray diffraction to detect weak additional reflections arising from the ordering between magnesium, aluminium, and silicon atoms, such weak reflections are revealed on electron diffraction photographs for a phase, resulting in a tripling of the hexagonal latticec-dimension. These show to have a structure very similar to rhombohedral willemite (Zn2SiO4), with the (Mg, Al) metal atoms ordered with respect to the silicon atoms in a 21 ratio. However, there are some additional weak diffraction spots with indices not obeying the rhombohedral condition of –h+k+l=3n. It is proposed that the structure of this is identical to that of willemite and the extra spots are a result of some form of twinning, which implies the existence of ordered microdomains.  相似文献   

11.
The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni4(Mo, Al, V); it contained 90° rotational domains of Ni3(Mo, Al, V) with a DO22 structure and Ni2(Mo, Al, V) with a Pt2Mo structure.  相似文献   

12.
The diffusible hydrogen in Cr-Mo steels are observed with autoradiography technique. Specimens with the diffusible hydrogen are prepared by an electrochemical cathodic charging method and those without the diffusible hydrogen by annealing at 373 K after charging hydrogen. TEM autoradiographs suggests, by the developed silver grains, that the hydrogen trapping sites are the grain boundary and internal interface of ferrite-cementite and ferrite-lath structure. After keeping the sample at 373 K, the silver grains disappeared. Most of hydrogen trapping sites release almost all the hydrogen at 373 K. It is clear that these sites of high-strength steels supplies the diffusible hydrogen. Hydrogen absorption characteristics of quench hardening tempering Cr-Mo steels have been evaluated by thermal desorption spectrometry (TDS). From tritium electron microscopic autoradiography and TDS analysis, the lower temperature (360 K–370 K) peaks show the diffusing hydrogen which is released a few days. The diffusible hydrogen from trapping sites such as the internal interface of ferrite-cementite or ferrite-lath structure are distinguished to the diffusing hydrogen.  相似文献   

13.
Molecular approaches are discussed to the density (), viscoeleastic (), and rheological () behavior of the viscosity(,,) of concentrated colloidal suspensions with 0.3 < < 0.6, where, is the volume fraction, the applied frequency, and ; the shear rate. These theories are based on the calculation of the pair distribution functionP 2(r,,), wherer is the relative position of a pair of colloidal particles. The linear viscoelastic behavior(,,=0) follows from an equation forP 2(r,,) derived from the Smoluchowski equation for small, generalized to large by introducing the spatial ordering and (cage) diffusion typical for concentrated suspensions. The rheological behavior(,,=0) follows from an equation forP 2(r,) of a dense hard-sphere fluid derived from the Liouville equation. This leads to a hard-sphere viscosityhs(,) which yields the colloidal one(,) by the scaling relation(,) 0=hs(,) B, where 0 is the solvent viscosity. B is the dilute hard-sphere (Boltzmann ) viscosity and the's are appropriately scaled,(,) and(,) agree well with experiment. A unified theore for(,,) is clearly needed and pursued.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994. Boulder, Colorado, U.S.A.  相似文献   

14.
The results of structural and phase hardening of pipes made of Zr + 2.5% Nb alloy show that ultrahigh-frequency thermal treatment of pipes (fast heating to the temperature of existence of the -phase followed by sharp cooling and annealing in the high-temperature range of the -phase) destroys the texture and forms a fine-grained structure (the grain diameter is about 1 m) with numerous transitional twins and a high density of precipitations of the secondary -niobium phase ( 1016cm–3). In this state, the alloy is rather strong and plastic (at room temperature, u 650 MPa, 0.2 550 MPa, and 20% both in the longitudinal and transverse directions). The efficiency of hardening by ultrahigh-frequency thermal treatment is not reduced with increase in the temperature of testing up to 500°C. Corrosion tests of channel pipes made of Zr + 2.5% Nb alloy subjected to ultrahigh-frequency thermal treatment in water containing various amounts of oxygen (from 0.1–0.3 to 600 mg/kg) at temperatures of 285–350°C for 700–6600 h under static conditions and in reactor water of the Ignalina Nuclear Power Plant for 5000 h under dynamic conditions showed that the corrosion resistance of this alloy is on a par with the corrosion resistance of the material of assembly channels of high-power channel reactors subjected to a standard treatment.  相似文献   

15.
Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L12), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L10 type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L10 martensite is examined.  相似文献   

16.
The nitrogen-sensing properties of LaAl12O18N are described for the first time. Positive e.m.f. measurements at high temperature across the cells -Nb (N), -Nb2N, -Al2O3, LaAl12O18NLaAl11O18 (or LaAl12O18N)LaAl12O18N, -Al2O3, -Nb2N, -Nb4N3, were in agreement with PN2(-Nb2N, -Nb4N3)>PN2(-Nb(N), -Nb2N) for the idealized stoichiometric cell reaction -Nb4N3+2-Nb(N)3-Nb2N, thus demonstrating the nitrogen-sensing property of these cells. The e.m.f. for a variety of cells with electrodes containing -V2N, -Nb2N, -Ta2N and Ti2N, were consistent with the predicted equilibrium nitrogen partial pressures.  相似文献   

17.
An imperfectly B2 ordered Fe3Al aggregate was cast, thermomechanically hot rolled and finally annealed at 870 K. Subsequently, the specimen was rolled at 800–830 K to a strain of 80%. The microstructure and the crystallographic texture of the rolled polycrystalline sample was investigated within the range =20–80%. The microstructure consisted of flat, elongated grains. In numerous grains straight slip lines were detected. Even after =80% recrystallization was not observed. The rolling texture of Fe3Al considerably deviates from that of non-ordered body centered cubic (b.c.c.) alloys and pure b.c.c. metals. The {111}uvw texture fibre (7-fibre) was very pronounced, while the {hkl}110 fibre (-fibre) was very weak. The {112}110 orientation which represents the strongest texture component in non-ordered b.c.c. alloys did not occur at all. The textures are discussed in terms of the {110}111, {112}111, {112}111 and {123}111 slip systems. The contribution of crystallographic slip of the various types of potential slip systems was simulated by means of the Taylor theory.  相似文献   

18.
Phase decomposition behaviour of liquid-quenched (bcc) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of to 1 (Ti-rich, bcc) + 2 (Ti-lean, bcc) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed 1 + 2 two-phase field expands towards higher temperatures than the predicted binodal line. The coherent 1 + 2 two-phase state exhibits the so-called 100 modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the -phase. We obtained time-temperature-transformation (TTT) diagrams of -type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is coherent 1 + 2 incoherent 1 + 2 incoherent 1 + 2 + grain boundary precipitates stable state of + TiCr2 or + TiCr2. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.  相似文献   

19.
Solid-state phase transformation of BaB2O4 during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.  相似文献   

20.
We have studied the magnetic state from a viewpoint of crystallographic features of the 1-D chain compound Ca0.824CuO2. A possible spin-hole arrangement in the magnetically coexisting state was determined by analyzing the local structural distortion in the CuO2 chain by means of a modulated-crystal-structure analysis. The essential periodic sequence expected is (: up- and down-spin, : hole), which can be regarded as a kind of spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号