Porous AlN ceramic substrates by reaction sintering

A novel approach was undertaken in producing porous AlN microelectronics tapes with high thermal conductivity and low dielectric constant. This method essentially utilised polymer micro-spherical powders that were used as a sacrificial mould to introduce controlled porosity into the green tapes during pyrolysis. The Al₂O₃-rich porous green tapes were then reaction sintered at 1680 °C for 12 h to achieve porous AlN tapes. This work builds upon the previously developed novel reaction sintering process that densified and converted Al₂O₃-rich tapes (Al₂O₃–20 wt.% AlN–5 wt.% Y₂O₃) to AlN tapes at a relatively low sintering temperature of 1680 °C. The sintering behaviour of the porous tapes was investigated, and the effects of the microspheres particle size and volume addition were studied. The microspheres successfully contributed to the significant reduction of tape density by porosity, and this contributed to lowering its dielectric constant. Dielectric constant of the AlN tapes were reduced to about 6.8–7.7 whilst thermal conductivity values were reasonable at about 46–60 W/m K. Coefficient of thermal expansion (CTE) values showed a linear trend according to phase composition, with the porous AlN tapes exhibiting CTE values of (4.4–4.8)×10⁻⁶ °C⁻¹, showing good CTE compatibility with silicon, at 4.0×10⁻⁶ °C⁻¹. The added porosity did not significantly affect the CTE values.     

Optical constants of polyacetylene

Optical constants of neat cis- and trans-(CH)_x and (CD)_x films are determined by measuring infrared, visible and ultraviolet spectra of the film and its thickness directly. The absorption coefficient of cis-(CH)_x at the absorption maximum 18 500 cm⁻¹ was (1.5 ± 0.1) × 10⁵ cm⁻¹ and that of trans-(CH)_x at 15 500 cm⁻¹ was 1.5 × 10⁵ cm⁻¹. The same values as those of (CH)_x were obtained for (CD)_x. The refractive indices and dielectric constants of cis- and trans-(CH)_x were calculated based on absorptionvspectral data.     

White light electroluminescence from PSi devices capped with poly(thiophene)(s) as top contact

For the first time, electrochemically deposited poly(3-methylthiophene) and chemically produced poly(3-methylthiophene) and poly(3-phenoxymethylthiophene) have been employed as top electrical contact on porous silicon light-emitting devices. The polymer-capped devices emitted white light as opposed to the uncapped devices, which emitted orange colour. The polymer-capped devices show much higher rectification ratio (up to 1×10⁵) as opposed to 1×10³ for the uncapped devices at ±10 V. The polymer-capped devices show 10³–10⁴-fold improvement in the luminous efficiency over the uncapped devices. Electrochemically deposited poly(3-methylthiophene) top contacts give a luminous efficiency of 8×10⁻⁵ lm W⁻¹ as opposed to 3×10⁻⁹ lm W⁻¹ obtained from uncapped devices. All the devices were found to fit the space charge limited current model.     

Electronic packaging materials prepared by powder injecting molding and pressure infiltration process

AlSiCp (65 vol.% SiC) electronic packaging materials were manufactured by powder injection molding (PIM) and pressure infiltration process in order to obtain near net-shaped parts. SiCp preformed compacts obtained by pre-sintering process at 1150 K have high strength and good appearance, and the ratio of open porosity to total porosiry is nearly 98%. The relative density of composites is bigger than 99%. The thermal conductivity of AlSiCp composites fabricated by this method is 198 W·m-1·K-1, and the coefficient of thermal expansion (CTE) is 8.0 × 10-6/K (298 K).     

Calorimetric investigation on the Pb---Sm and Sn---Sm alloys

The integral enthalpy of formation of the Sm---Pb and Sm---Sn melts at 1203 K, h^f, was determined by direct reaction calorimetry (drop method) in the Pb and Sn rich sides with the help of a high-temperature Tian-Calvet calorimeter. The results can be fitted respectively with reference to the mole fraction of samarium, x, as follows:

h^f/kJ mol⁻¹=x(1−x)(−109.8−372.0.7x) with0> X_Sm >0.27

and

h^f/kJ mol⁻¹=x(1−x)(−277.0−105.4.x) with0> X_Sm >0.27

for the Sm---Pb and Sm---Sn melts respectively. They yield the following partial enthalpies of samarium at infinite dilution: −109.8 and −277.0 kJ mol⁻¹ respectively.

Such negative values suggest the existence of a strong short-range order in the liquid state. The stoichiometry and the thermal stability of these associations needs additional thermodynamic determinations concerning mainly the free enthalpy of formation. It will be determined by Knudsen-effusion combined with mass spetrometry in a further work.     

Electrical properties of polyimide thin films formed by the vapor deposition polymerization method

Thin films of polyimide (PI) were fabricated by a vapor deposition polymerization method (VDPM) and studied for their insulator characteristics in semiconductor devices. Polyamic acid (PAA) thin films fabricated by vapor deposition polymerization (VDP) from PMDA (pyromellitic dianhydride) and DDE (4,4′-diaminodiphenyl ether) were converted to PI thin films by thermal curing. The appropriate curing temperature was 300 °C. From TG-DTA (Thermo Gravimetry-Differential Thermal Analysis), the PI thin films can endure 230 °C for 20 000 h. They exhibited a relative permittivity of 3.9–3.5 and a dissipation loss factor of 0.008 at frequency of 10 kHz in the temperature range from 25 to 200 °C. The resistivity was approximately 3.2×10¹⁵ Ωcm and the dielectric breakdown strength was 4.61 MV/cm.     

Growth and characterization of tetragonal bismuth ferrite–lead titanate thin films

Thin films of tetragonal bismuth ferrite–lead titanate (1 − x)BiFeO₃–xPbTiO₃ with x = 0.9–0.7 were prepared by pulsed laser deposition (PLD). The films exhibit a dense columnar grain growth. XRD analysis reveals that the films have a perovskite structure and exhibit a preferred (1 1 1) texture. The film microstructure was studied using SEM. The ferroelectric properties of the films are discussed in the light of polarization-field hysteresis behaviour and impedance spectroscopy. The remanent polarization values ranged between 2P_r  45 and 60 μC cm⁻² at a field amplitude of 500 kV cm⁻¹ and −10 °C, while the dielectric permittivity of the films ranged between 375 and 1096 at a frequency of 2 kHz.     

Studies on the structural and ionic transport properties at elevated temperatures of La1−xMnO3±δ synthesized by a wet chemical method

Structural transformation and ionic transport properties are investigated on wet-chemically synthesized La_1−xMnO₃ (x=0.0–0.18) compositions. Powders annealed in oxygen/air at 1000–1080 K exhibit cubic symmetry and transform to rhombohedral on annealing at 1173–1573 K in air/oxygen. Annealing above 1773 K in air or in argon/helium at 1473 K stabilized distorted rhombohedral or orthorhombic symmetry. Structural transformations are confirmed from XRD and TEM studies. The total conductivity of sintered disks, measured by four-probe technique, ranges from 5 S cm⁻¹ at 298 K to 105 S cm⁻¹ at 1273 K. The ionic conductivity measured by blocking electrode technique ranges from 1.0×10⁻⁶ S cm⁻¹ at 700 K to 2.0×10⁻³ S cm⁻¹ at 1273 K. The ionic transference number of these compositions ranges from 3.0×10⁻⁵ to 5.0×10⁻⁵ at 1273 K. The activation energy deduced from experimental data for ionic conduction and ionic migration is 1.03–1.10 and 0.80–1.00 eV, respectively. The activation energy of formation, association and migration of vacancies ranges from 1.07 to 1.44 eV.     

Superplastic behavior of a 7055 aluminum alloy

It is shown that a high strength 7055 aluminum alloy with partially recrystallized initial structure exhibits superplastic behavior in the temperature interval 400–490 °C within a wide strain rate range from 8.3×10⁻⁵ to 3.3×10⁻² s⁻¹. Maximum total elongation of about 960% and strain rate sensitivity coefficient, m, of 0.6 were obtained at a temperature of 450 °C and a strain rate of 3.3×10⁻⁴ s⁻¹.     

Anelastic properties of aluminium thin films on silicon cantilevers

Thin films (thickness 40 to 250 nm) of Al on microstructurized Si substrates have been investigated by the vibrating-reed technique (typical frequencies 100 Hz to 10 kHz) with strain amplitudes in the range of 10⁻⁷ to 10⁻⁴ and for temperatures up to 850 K. The combined evaluation of flexural and torsional vibrations permits to separate the complex shear modulus and biaxial modulus of the thin layer, which helps to identify the damping mechanisms. For Al thin films with thickness <200 nm, in addition to the well-known damping peak due to grain boundary sliding (peak temperature about 370 K), a further maximum of damping has been observed around 600 K, the nature of which is discussed.     

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Ionic conductivities of polymeric solid electrolyte films containing rare earth ions

Polymeric solid electrolyte films containing rare earth metal ions (Ce³⁺, La³⁺, Yb³⁺) have been prepared, with a view to applying them to solid-state electrochemical devices. The films, composed of poly(ethylene oxide)-grafted poly(methylmethacrylate) (PEO–PMMA), were prepared by dissolving rare earth salts with appropriate amounts of poly(ethylene glycol) dimethyl ether (PEG) that have ethylene oxide units in their structure. The ionic conductance behaviour of the polymeric composite electrolyte systems was investigated by AC impedance and DC polarization methods in an ambient temperature range. The oligo(ethylene oxide) units in the polymer matrix and the PEG components ensured the dissociation of the salts and the high mobility of the resulting ionic species in the solid films. About 10⁻⁵ S cm⁻¹ or above of ionic conductivity was obtained for a PEO–PMMA/PEG/Ce(ClO₄)₃ system at room temperature. The addition of a liquid plasticizer in the composite improved the conductivity by about two orders of magnitude.     

Thermoelectric properties of Ni- and Zn-doped Nd2CuO4

The electrical resistivity, Seebeck coefficient, and thermal conductivity of Nd₂(Cu_0.98M_0.02)O₄ (M: Ni and Zn) have been measured in the temperature range from room temperature to about 1000 K. Ni- and Zn-doping decreases the electrical resistivity and the absolute values of the Seebeck coefficient. The thermal conductivity decreases with increasing temperature, showing phonon conduction, and also decreases by doping. The power factor of Nd₂(Cu_0.98Ni_0.02)O₄ reaches 1.02×10⁻⁴ W m⁻¹ K⁻² and the figure of merit is 1.35×10⁻⁵ K⁻¹ at 320 K. The relatively low figure of merit compared with that of the state-of-the-art thermoelectric materials is due to the high thermal conductivity.     

Bulk and lattice thermal expansion in Ce1−xSrxO2−x (0.0≤x≤0.10)

In this communication, we report on the bulk and lattice thermal expansion studies on a number of compounds, within the homogeneity range of solid solutions, in a series with the general composition Ce_1−xSr_xO_2−x (0.0≤x≤0.10). The XRD pattern of each product was refined to determine the solid solubility of SrO into the lattice of CeO₂, and the homogeneity range. The composition with maximum solid solubility limit of SrO in CeO₂ lattice, under the slow cooled conditions, was delineated as Ce_0.91Sr_0.09O_1.91 (i.e. 9 mol.% of SrO). The bulk thermal expansion measurements from ambient to 1123 K, as investigated by a dilatometer, revealed that the _l (293 to 1123 K) values for the compositions within the homogeneity range increase from 11.58×10⁻⁶ to 12.13×10⁻⁶ K⁻¹ on increasing the Sr²⁺ content from 0 mol.% (i.e. CeO₂) to 9 mol.%, i.e. the upper solubility limit of SrO into the lattice of CeO₂. A similar trend was observed in the lattice thermal expansion coefficients _a (293 to 1473 K) as obtained by a high temperature-XRD.     

The electronic structure of organometallic complexes of the f elements XLI: Crystal field strength of η-cyclooctatetraenyl ligand estimated on the basis of the preliminary crystal field parameters of (COT)Nd[HB(3,5-Me2pz)3]

The absorption spectrum of (η⁸-COT)Nd[HB(3,5-Me₂pz)₃] (COT=cyclooctatetraenyl, pz=pyrazol-1-yl) has been measured at room temperature, at approximately 90 K and in parts at ca. 30 K. The bands were assigned by applying the selection rules for C_8v symmetry and by performing crystal field (CF) calculations assuming the CF parameters of the Nd complex were the same as for the previously analyzed (COT)Pr[HB(3,5-Me₂pz)₃]. The parameters of an empirical Hamiltonian were fitted to the energies of 27 levels to give an r.m.s. deviation of 25.5 cm⁻¹. Neglecting the influence of [HB(3,5-Me₂pz)₃] ligand, the CF strength of the [COT]⁻⁻ ligand could be estimated from the CF parameters obtained.     

Conductivity enhancement in transparetn ZnO films via Al-dopping produced by CW-CO2 laser-induced evaporation

Highly conductive transparent aluminium-doped ZnO (ZnO:A1) films were successfully deposited by CW-CO₂ laser-induced evaporation. Optimisation of evaporation parameters was based on laser power, substrate temperature, O₂ partial pressure in the vacuum chamber and amount of Al in the ZnO source pellet. ZnO:A1 films with an electrical resistivity as low as 6.6 × 10⁻²Ω·cm and an optical transmission of 80% at 500nm were obtained at laser power of 15 W, substrate temperature of about 200°C, O₂ partial pressure of 6—7 × 10⁻⁴ Torr and 5wt.% Al. Conductivity of ZnO films can be increased one order via Al-doping in ZnO films. The films obtained by laser-induced evaporation have compared quite favorably with the high quality films obtained by sputtering.     

Novel alkaline-free Er-doped fluorophosphate glasses for broadband optical fiber lasers and amplifiers

Two sets of Er³⁺-doped alkaline-free glass systems, MgF₂–BaF₂–Ba(PO₃)₂–Al(PO₃)₃ (MBBA) and Bi(PO₃)₃–Ba(PO₃)₂–BaF₂–MgF₂ (BBBM), have been prepared and investigated with the aim of using them as active media. Radiative lifetimes (τ_rad) and branching ratios (β) have been obtained for the excited states of Er³⁺. The absorption spectra were recorded to obtain the intensity parameters (Ω_t) which are found to be Ω₂ = 4.47 × 10⁻²⁰ cm², Ω₄ = 1.31 × 10⁻²⁰ cm², Ω₆ = 0.81 × 10⁻²⁰ cm² for the MBBA system and Ω₂ = 4.03 × 10⁻²⁰ cm², Ω₄ = 1.34 × 10⁻²⁰ cm², Ω₆ = 0.53 × 10⁻²⁰ for the BBBM system, respectively. The emission cross-section for the ⁴I_13/2 → ⁴I_15/2 transition is determined by the Fuchtbauer–Ladenburg method and found to be 2.35 × 10⁻²⁰ cm² and 3.54 × 10⁻²⁰ cm² for the MBBA and BBBM system, respectively. Comparison of the measured values to those of Er³⁺ transitions in other glass hosts suggests that our new glass systems are good candidates for broadband compact optical fiber and waveguide amplifier applications.     

Computer-aided thermodynamic study of solid Co---Pd alloys by Knudsen cell mass spectrometry, and calculation of the phase diagram

F.c.c. solid Co---Pd alloys have been investigated thermodynamically by means of computer-aided Knudsen cell mass spectrometry. Thermodynamic evaluation has been performed by applying the “digital intensity ratio” method. The thermodynamic excess properties can be described algebraically by means of thermodynamically adapted power series with two adjustable parameters, i.e. C₁^G (−20 810 + 9.608T) J mol⁻¹) and C₂^G (−30 720 + 6.78T) J mol⁻¹). At 1470 K, f.c.c. solid Co---Pd alloys are characterized by negative molar excess Gibbs energies G^E, exothermic molar heats of mixing (H^E) and small negative molar excess entropies S^E. At 1470 K, the minimum G^E value is −4600 J mol⁻¹ (61.9 at.% Pd), the minimum H^E value is −9400 J mol⁻¹ (59.5 at.% Pd) and the minimum S^E value is −3.3 J mol⁻¹ K⁻¹ (55.9 at.% Pd). The thermodynamic activities of Co show small positive deviations from the ideal case for the Co-rich alloys (x_Pd < 0.34), and negative deviations from Raoults' law for alloys with higher Pd contents. The Pd activities a_Pd show negative deviations from the ideal case for all compositions. The phase diagram has been computed by means of a generally applicable procedure for the calculation of the equilibrium compositions of coexisting phases. This was achieved using the results of this work, thermodynamic data from earlier mass spectrometric studies on the liquid phase, and literature data for the heat capacities and enthalpies of Co and Pd.     

Crystal fields in (La1−xGdx)OCl:Eu solid solutions

The formation of cation solid solution in the (La_1−xGd_x)OCl:Eu³⁺ (0≤x_Gd≤1; Δx_Gd=0.1) series was studied by photoluminescence spectroscopy. The luminescence from the ⁵D_0–2 to the ⁷F_0–4 levels of the Eu³⁺ ion in the (La_1−xGd_x)OCl series was recorded at 77 K by using argon ion laser excitation (457.9 nm). The interpretation of the spectra according to the C_4v site symmetry of the Eu³⁺ ion in the tetragonal PbFCl-type structure yielded nearly complete sets (18 to 19 levels) for the ⁷F_0–4 levels. Simulations of the Stark level schemes were carried out with the aid of a phenomenological c.f. theory utilizing the five non-zero c.f. B_q^k parameters (B₀², B₀⁴, B₄⁴, B₀⁶ and B₄⁶) allowed for the C_4v site symmetry. By using the calculated c.f. parameter sets a quantitative measure was obtained to monitor the formation of cation solid solutions. The strength of the c.f. effect was estimated with the c.f. strength parameters S and S^k (k=2, 4 and 6). The c.f. parameter sets reproduced the experimental ⁷F_J (J=0–4) energy level schemes with the rms deviations between 4 and 11 cm⁻¹. The individual parameters as well as the c.f. strength parameters were found to evolve in a smooth manner indicating complete solid solubility in the (La_1−xGd_x)OCl series. Some local distortions from the C_4v symmetry — probably of long range—leading to the splitting of the ⁷F₁ doubly degenerate E level were observed, however.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).