Microwave dielectric properties of ceramics with compositions along the La2/3TiO3(STAB)–LaAlO3 tie line

The stabilization of La2/3TiO3 in the perovskite structure by 4 mol% LaAlO3 resulted in maximum values for the relative permittivity (k72) and temperature coefficient of resonant frequency (f=123 ppm K-1). The Q-value quality factor of these ceramics were over 5300 measured at 4.5 GHz. Higher contents of LaAlO3 cause a decrease in both the permittivity and f values, accompanied by an increase in the Q-value. Typical values measured at 6.0 GHz for ceramics with the composition 70 mol% La2/3TiO3 – 30 mol% LaAlO3 are k>44, f=+7 ppm K-1 and Q>5500. The results depend on the processing parameters. The sintering temperature increases with increasing LaAlO3 content. An oxidizing sintering atmosphere improves the Q-value, whereas the permittivity and f are not significantly affected.     

Ultrasonic determination of the elastic and nonlinear acoustic properties of transition-metal carbide ceramics: TiC and TaC

Pulse-echo overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic transition-metal carbides TiC and TaC as functions of temperature in the range 135–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. The carbon concentration of each ceramic has been determined using an oxidation method: the carbon-to-metal atomic ratios are both 0.98. In general, the values determined for the adiabatic bulk modulus (B ^S), shear stiffness (), Young's modulus (E), Poisson's ratio () and acoustic Debye temperature (_D) for the TiC and TaC ceramics agree well with the experimental values determined previously. The temperature dependences of the longitudinal stiffness (C _L) and shear stiffness measured for both ceramics show normal behaviour and can be approximated by a conventional model for vibrational anharmonicity. Both the bulk and Young's moduli of the ceramics increase with decreasing temperature and do not show any unusual effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic pressure derivatives of elastic stiffnesses and the acoustic-mode Grüneisen parameters. The values determined at 295 K for the hydrostatic pressure derivatives (C _L/P)_{P = 0}, (/P)_{P = 0} and (B ^S/P)_{P = 0} for TiC and TaC ceramics are positive and typical for a stiff solid. The adiabatic bulk modulus B ^S and its hydrostatic pressure derivative (B ^S/P)_{P = 0} of TiC are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (_L), shear (_S) and mean (^el) acoustic-mode Grüneisen parameters of TiC and TaC ceramics are positive: the zone-centre acoustic phonons stiffen under pressure. The shear _S is much smaller than the longitudinal _L. The relatively larger values estimated for the thermal Grüneisen parameter ^th in comparison to ^el for the TiC and TaC ceramics indicate that the optical phonons have larger Grüneisen parameters. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic TiC and TaC.     

Rejuvenation procedures to recover creep properties of nickel-base superalloys by heat treatment and hot isostatic pressing techniques

The rejuvenation procedures to recover the creep properties of nickel-base superalloys by atmospheric pressure heat treatment and hot isostatic pressing techniques have been reviewed in detail. It is very important that such treatments be applied at an optimum stage in the service life of a turbine blade. In other words, the rejuvenation procedures must be applied early enough to prevent catastrophic failures or irreparable damage and late enough to give a cost-effective benefit. The optimum stage at which to undertake a rejuvenation procedure to extend the creep lives of superalloys is immediately prior to the tertiary stage. By using these techniques it is not possible to extend the creep lives of superalloys indefinitely because of the accumulation of some permanent damage incurred during service conditions.Nomenclature ERF Economic repair factor - P _r Price of repaired and rejuvenated part - P _n Price of new part - L _n Potential operational life of new part - L _r Potential operational life of repaired/rejuvenated part - N Cavity density or number of cavities per unit area (mm^–2) - n _v Number of cavities per unit volume (mm^–3) - Creep strain - ₁ Maximum principal stress (MPa) - ¯ von Mises effective shear stress (MPa) - t _f Time to failure - t _t Time to commencement of tertiary creep - Creep damage tolerance parameter - _f Strain at fracture (or failure) - T _m Absolute melting temperature - ₀ Friction stress - r Spherical radius of cavities - 2x Intercavity spacing - Grain boundary width - P _I Cavity gas pressure - P _H External hydrostatic pressure - Atomic volume - k Boltzmann constant - T Absolute temperature - Surface energy of the cavity - D _b Grain boundary diffusion coefficient - _d Ductility recovery parameter - Strain to reach the same acceleration after recovery annealing - ₀ Strain necessary for standard material to reach a given acceleration of the secondary-creep rate in the tertiary region - _t Strain needed to have produced the reduced cavity volume after rejuvenation annealing - Creep rate - Secondary or minimum creep rate - ₁ Strain previous to the regenerative annealing period - n Total number of strain/regenerative anneal cycles - _v Recovery parameter for cavity volume - V ₀ Original total cavity volume at the start of the recovery - V _t Cavity volume after recovery annealing for a timet     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Thermal transport properties in the normal phase of dilute3He-4He mixtures

We present steady-state measurements of the thermal diffusion ratiok _T and of the heat conductivity for three dilute mixtures of³He in⁴He with concentrations 9×10^–3X(³He)5×10^–2 at saturated vapor pressure in the normal phase close to the superfluid transition. The data are compared with predictions by Dohm and Folk from the renormalization group (RG) theory. From auxiliary determinations of thermodynamic derivatives for these mixtures, we obtain the separation factor =–(k _T /T)×(/X) _T,P /(/T) _X,P above T over the range wherek _T is positive. Here is the mass density. From the transients of X(t) as a function of time, we obtain an estimation for the mass diffusion coefficientD and compare the results with predictions by Dohm and Folk and with results from other experiments.     

Amplifier based on a field-effect triode with negative current feedback

Conclusions A field-effect triode amplifier with series negative current feedback allows a voltage gain of the order of 200–300 to be obtained for a load resistance R_s1 M. The coefficient K_u begins to decrease noticeably only for a feedback resistance above 500 .The current gain reaches (8–10)·10³. Increasing the resistances R_s and R_L to hundreds of ohms has practically no effect on K_i. For a further increase of R_s and R_L the coefficient K_i decreases.The power gain reaches its maximum value (of the order of 10⁴ or more) for R_s100 and R_L=10–100 k. An increase in R_s leads to a reduction of K_pmax and to a shift of the extremum of the function K_p=f(R_L) into the range of higher values of R_L.A large input resistance of the amplifier (tens of megohms and higher) is obtained when R_s increases to 10–100 M. The maximum input resistance is obtained for R_L and R_s and may exceed values of from hundreds of megohms to several gigaohms. The minimum input resistance is hundreds of kilohms for R_L and R_s0.The minimum input resistance (5–10 k or less) is ensured for R_g and R_L0. An increase of the output resistance to hundreds of megohms or higher occurs for R_g and R_s.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 67–70, September, 1971.     

Paramagnetic resonance in a “dirty” spin-glass

For a spin-glass with nonmagnetic defects (n _m ^1/3l 1, where n _m is the magnetic impurity concentration and l is the mean free path) an absorption function () is derived. Three ranges of temperature and external magnetic field are considered. In the vicinity of the transition the value of () d is estimated as a function of temperature and field.     

Microstructure and mechanical properties of metastable lath martensite wires in the Fe-Ni-Cr-Al-C system produced by melt spinning in rotating water

Metastable lath martensite ( _L ) phase wires with high strengths have been produced in the Fe-Ni-Cr-Al-C alloy system by melt spinning in rotating water. These wires have a circular cross section and a white lustre and the wire diameter is in the range of 100 to 140m. The width and length of each lath in the _L phase are as small as about 0.3 and 2m, respectively. The _y, _f and _p are about 900 and 1650 MPa and 2.0% for the _L wires. The subsequent annealing causes an increase in _p as well as _y and _f and the attained values are about 1000 and 1700 MPa and 3.0% for Fe-10Ni-10Cr-6.5 Al-1.0C wire annealed at 773 K for 1 h owing to the precipitation strengthening of a very fine unidentified carbide and to a high density of dislocations and lath boundaries in the _L phase. Further annealing causes a significant decrease in _p through decomposition of _L to+M₇C₃+M₂₃C₆. Therefore, the high strength combined with relatively good ductility for the _L wires is interpreted as due to the suppression of the phase transformation of _L to a mixed structure of+M₇C₃+M₂₃C₆ by melt quenching.     

Use of adjoint functions in investigations of heat conduction and transfer processes

An examination is made of the use of adjoint functions in heat conduction and convection theory. Formulas of perturbation theory are obtained for steady and unsteady cases, an interpretation of the physical meaning of adjoint temperature is given, and some applications of the theory are discussed.Notation (r,) thermal conductivity - t(r,) temperature - t *(r,) adjoint temperature - q_V(r,) density of heat release sources - p(r,) a parameter of adjoint equation - r generalized coordinate - time - (r_s, ) heat transfer coefficient - I linear functional of temperature - (r,;r₀,₀) and *(r,; r₀,₀) Green's function for t(r, ) and t *(r, ) - C(r,) volume specific heat - W(r, ) vector distribution of flow velocities - V, S volume and surface areas of body - R radius of HRE - r, radial and angular coordinates - F_in, F_out inlet and outlet flow areas of channel     

Crystalline structure and dielectric properties of Ba(Ti1−y Ce y )O3

Crystalline structure, microstructure and dielectric properties for Ba(Ti_1–y Ce _y )O₃ (0 y 0.5) ceramics have been studied. Dense ceramics with the relative density higher than 95% and grain size of 0.7–1.5 m have been obtained. The limit of the solid solubility is determined as y = 0.3 by the X-ray diffraction, SEM, and dielectric properties measurements. The crystalline symmetry is tetragonal for the sample with y = 0.02 and cubic for y 0.06 at room temperature, and the unit cell increases with increasing Ce content in the solid solution range. Correspondingly, the dielectric response exhibits three dielectric peaks for y = 0.02, and one pinched dielectric peak with ferroelectric relaxor behavior for y 0.06.     

Phase relation in titanium-aluminide alloy — an X-ray study

Results of X-ray diffraction studies on titanium aluminides stabilized by niobium, vanadium and molybdenum are reported to establish a phase relation in the Ti-25Al-10Nb-3V-1Mo at% (Ti-25-10-3-1) alloy. It is shown that the composition of the phases probably deviated slightly from ideal stoichiometry Ti₃Al for ₂ and Ti₂AlNb for -type; its partial ordering in of the -phase type and the phase relation is 64% -type and 36% ₂ phase.     

Effect of the elastic factor on the hydrodynamic stability of a structurally viscous medium

The effect of relaxation phenomena on the hydrodynamic stability of the plane gradient flow of a structurally viscous medium is investigated using linear theory.Notation _ij stress tensor deviator - U_i components of the velocity vector - x_i coordinates - t time - P pressure - =₀L/^*V plasticity parameter - _o limiting shear stress - andc dimensionless wave number and the perturbation frequency - Re=VL/* Reynolds number - density - F_ij deformation rate tensor Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 5, pp. 868–871, November, 1978.     

Crystalline structure and dielectric properties of (Sr1−1.5x Bi x ) TiO3 ceramics

The crystalline structure, microstructure and dielectric properties of the (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.267) ceramics were studied. Cubic solid solutions were determined for x 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2–4 m was obtained for (Sr_1–1.5x Bi _x )TiO₃ (0 x 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO₃ samples. The occurrence of the impurity modes and the ferroelectric relaxor mode and their evolution are demonstrated as a function of Bi concentration.     

Phase equilibria and thermodynamic properties in the system Ni-Mo-O

Coexisting phases in the Ni-Mo-O ternary system at 1373 K have been identified by X-ray diffraction, optical microscopy and scanning electron microscopy. The samples were equilibrated in evacuated quartz capsules. Only one ternary phase, NiMoO₄, was found to exist in the system. The reversible e.m.f. values of the following solid-state galvanic cells were measured in the temperature range 900 to 1500 K: (I) Pt, Ni + NiO/(CaO) ZrO₂/NiO + MoO₂ + NiMoO₄, Pt; (II) Pt, Mo + MoO₂/(CaO) ZrO₂/O₂, Pt; and (III) Pt, Mo + MoO₂/(CaO) ZrO₂/Ni-Mo + MoO₂, Pt. The Gibbs energies of formation of NiMoO₄ and MoO₂ and activities in Ni-Mo alloys were derived from the e.m.f. data. For the reaction NiO + MoO₂ + 2(02) NiMoO₄ we obtain G _r ⁰ = -201 195 + 69.70T (±400) J mol^–1; for Mo + (02) MoO₂ we obtain G _f ⁰ = –578880 + 168.5T (+500) J mol^–1. Based on the information from phase identification studies and thermodynamic stabilities, the isothermal section and oxygen potential diagram for the Ni-Mo-O system at 1373 K have been developed.     

The preparation of alumina fibre by sol-gel processing

Attempts have been made to prepare alumina fibre from the colloidal sol and polymerized alkoxides. The aluminium chloride or aluminium nitrate systems were found to be potential methods for producing continuous alumina fibre: the aluminium nitrate system had a better sintering behaviour than the aluminium chloride system. The aluminium isopropoxide system, however, was unsuitable for preparing alumina fibre but was suitable for the preparation of monoliths, membranes, powders, and multicomponent ceramics. The thermal changes of these precursors were studied by transmission electron microscopy, Fourier-transform infrared spectroscopy and X-ray diffraction. The results demonstrated the different routes of phase transformation as the temperature increases. The aluminium chloride system exhibits two routes for phase transformation: (a) boehmite -Al₂O₃, and (b) gibbsite -Al₂O₃.     

Structure, Electronic Spectrum, and Chemical Bonding of Fullerene-like Nanoparticles Based on MB2(M = Mg, Al, Sc, Ti) Layered Diborides

The structure, electronic spectrum, and interatomic interaction parameters of _n B_2n (n = 10, 30, 90, 120, 160; M = Mg, Al, Sc, Ti) fullerene-like molecules based on MB₂ layered diborides are assessed using quantum-chemical modeling and are analyzed in relation to the atomic configuration, size, and chemical composition of _n B_2n . The electronic structure of concentric nanoparticles consisting of _n B_2n cage molecules having identical (₁₀B₂₀@₉₀B₁₈₀) or different (₁₀B₂₀@₉₀B₁₈₀, where M, M = Mg, Al) compositions is considered. The results are compared with the electronic properties of crystalline MB₂ phases.     

Microstructure and crystallographic texture of rolled polycrystalline Fe3Al

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.     

Transient thermal stress analysis of multilayered hollow cylinder

Summary This paper deals with the transient response of one-dimensional axisymmetric quasistatic coupled thermoelastic problems. Laplace transform and finite difference methods are used to analyze the problems. Using the Laplace transform with respect to time, the general solutions of the governing equations are obtained in the transform domain. The solution is obtained by using the matrix similarity transformation and inverse Laplace transform. We obtain solutions for the temperature and thermal stress distribution in a transient state. Moreover, the computational procedures established in this article can solve the generalized thermoelasticity problem for a multilayered hollow cylinder with orthotropic material properties.Nomenclature Lame's constant - density - C _v specific heat - k _r ,k radial and circumferential thermal conductivity - _r , linear radial and circumferential thermal expansion coefficient - E _r ,E radial and circumferential Young's modulus - v _r Poisson's ratio - ₀ reference temperature - ,T dimensional and nondimensional temperature - r ^*,r dimensional and nondimensional radial coordinate - ,t dimensional and nondimensional time - _r ^* , _r dimensional and nondimensional radial stress - ^* , dimensional and nondimensional circumferential stress - U, u dimensional and nondimensional radial component of displacement     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

An isothermal theory of anisotropic rods

Summary To provide an isothermal, deterministic theory of anisotropic rods is the primary objective of this paper. Our starting point is the 3-D linear theory of micropolar elastodynamics. First, the governing equations of the theory are established by the use of a suitable averaging procedure together with a separation of variables solution for kinematic variables. Next, without making the usual definiteness assumption for the strain energy density, a dynamic uniqueness theorem is constructed for the solutions of the governing equations. Logarithmic convexity arguments are then used to enumerate a set of conditions sufficient for uniqueness. The theory includes the effects of warping and shearing deformations, and in fact, it incorporates as many higher order effects as deemed necessary in any special case. Also, the application of the theory is illustrated in a sample example.Nomenclature Euclidean 3-space - X_k, X₁z, X a system of right-handed Cartesian coordinates in, rod axis, lateral coordinates; k=1, (=2, 3) - , , a regular region of space in , its closure and boundary surface - L _d,L complementary subsets of, on which deformations and stresses are prescribed, respectively;L _d L =L,L _d L = 0 - L length of rod - A area of cross-section of rod - C a Jordan curve which boundsA - V, L entire volume of rod and its boundary surface - L ₁,L _t lateral surface of rod, surface portion ofL ₁ on which stresses are prescribed - A _r,L ₁ right and left faces of rod - dv, dA, ds element of volume and area onA, and line element alongC - n_i,v _i unit exterior normal vectors toL andC - t time - t_kl, m_kl components of stress and couple stress tensors - f_i, l_i body force and body couple vectors per unit volume - , J_kl mass density, components of microinertia tensor - u_i, _i displacement and microrotation vectors - prescribed steady temperature increment - _kl, _klm components of Kronecker's delta and alternating tensor - t_i, m_i stress and couple stress vectors - _kl, e_kl components of strain and infinitesimal strain tensors - C_klmn, D_klmn components of isothermal elastic stiffnesses - B_kl thermal coefficients of material - , Lamé's elasticity constants - , , , elastic moduli of microisotropic continuum - B coefficient of linear thermal expansion - d_i, d_kl u_i or _i, _kl or e_kl and/or _kl - T _kl ^(m,n) , M _kl ^(m,n) components of stress and couple stress resultants of order (m, n) - T _k ^(m,n) M _k ^(m,n) components of stress and couple stress vector resultants of order (m, n) - d _k ^(m,n) , d _kl ^(m,n) components of deformation (u _k ^(m,n) , _k ^(m,n) ) and strain ( _kl ^(m,n) , e _kl ^(m,n) , _kl ^(m,n) ) of order (m, n) - F _i ^(m,n) , L _i ^(m,n) body force and body couple resultants of order (m, n) - U _i ^(m,n) , _i ^(m,n) displacement and microrotation resultants of order (m, n) - P _i ^(m,n) , Q _i ^(m,n) effective loads of order (m, n) - N , M, , P, Q, w, T ₁₁ ^(0,0) ,M ₁₁ ^(0,0) , P ₁ ^(0,0) , Q ₁ ^(0,0) , U ₁ ^(0,0) , ₁ ^(0,0) - v₀ rod velocity, (E/)^1/2 - K, W kinetic and potential energy densities - V, U kinetic and potential energies per unit length of rod - total energy of rod - C^(m,n) functions with derivatives of order up to and including (m) and (n) with respect to space coordinates and time, respectively - G (t) logarithmic convexity function - ()^. time differentiation, /t () - () partial differentiation with respect to the axial coordinate, /z () - E,v Young's modulus, Poisson's ratio