M. Friesel

Proton and deuteron conductivity in CsHSO4 and CsDSO4 by in situ isotopic exchange

Abstract The protonic conductivity of CsHSO4 and CsDSO4 was measured with in situ isotopic exchange using van der Pauw and 2-electrode geometries for the high- and low-temperature phases, respectively. The proton/deuteron conductivity ratio of around 2.3 in the high temperature phase is a result mainly of a 0.02 eV higher activation energy of deuteron conductivity, assigned to a difference in zero-point energy. The low temperature phases exhibited no large isotope effects and hence no indications of proton tunneling.

Tracer solute diffusion of Nb, Zr, Cr, Fe, and Ni in γ-TiAl: effect of preferential site occupation

Abstract Solute diffusion of Nb, Zr, Fe, Cr, and Ni was measured in the phase γ-TiAl within a wide temperature interval. Both the SIMS method (93Nb, 58Fe, and 52Cr isotopes) and the radiotracer technique (95Nb, 95Zr, and 59Fe isotopes) were applied. Linear Arrhenius dependencies were found for Nb, Zr, and Cr diffusion, whereas Fe diffusion in TiAl revealed a substantially curved Arrhenius dependence similarly to Ti self-diffusion in this compound. The observed solute diffusion behavior is interpreted in terms of the preferential site occupation in the compound and by the interplay of the Ti sublattice diffusion mechanism and the intersublattice jumps of the solute atoms. Unlike in α2-Ti3Al, neither Fe diffusion nor Ni diffusion is extremely fast in γ-TiAl. This feature is explained by the different modes of forming solid solutions (substitutional or interstitial) in both compounds, which seems to be related to the different Ti atom arrangements around the interstitial octahedral sites in both compounds.

Improved SrTiO3 multilayers for microwave application: Growth and properties

Au/SrTiO3 (STO), Au/YBa2Cu3O7−x (YBCO)/STO, Au/STO/CeO2/STO, and Au/STO/MgO/STO multilayer structures on LaAlO3 substrates have been fabricated. Their properties are investigated at 1 MHz and at microwave frequencies (40 MHz–40 GHz) as a function of applied voltage, and in the temperature range between 20 and 300 K. The STO thin films showed comparatively high values of dielectric constant (up to 1800 at 50 K) with low measured film losses, tan δ<0.005, (limit of the measurement setup), at 20 GHz and 20 K, indicating their applicability for microwave devices. The dielectric constant is shown to be independent of frequency to at least 40 GHz, while the losses exhibit no or weak frequency dependence. Relaxation of the STO film in oxygen atmosphere before deposition of YBCO is shown to reduce interactions between the YBCO and STO films. After an “ex situ” high-temperature (900 °C) treatment in flowing oxygen a reduction of the STO dielectric constant as well as tan δ has been observed. Using cerium dioxide a...

Tracer diffusion behavior of Ga as an Al-substituting element in Ti3Al and TiAl intermetallic compounds

Abstract Tracer diffusion of 69 Ga was measured in pure α-Ti, α 2 -Ti 3 Al (33 at% Al), and γ-TiAl (54 at% Al) over wide temperature intervals up to the upper limits of stability of the corresponding phases. The concentration profiles were determined by secondary ion mass spectroscopy (SIMS). The diffusion coefficients revealed a good Arrhenius behavior with the frequency factors D 0 = 2.1 +0.7 −0.5 ×10 −3 ; 6.3 +9.7 −3.8 ×10 −5 ; and 4.4 +1.9 −1.3 ×10 −5 m 2 s −1 and the activation enthalpies Q =295.5±2.3; 315±10; and 293±4 kJ mol −1 for the Ga solute diffusion in α-Ti, Ti 3 Al and TiAl, respectively. The systematics of Ga diffusion behavior in these materials is discussed in dependence on the topology of the Ti sublattice in the compounds. The experimental data of Ga diffusion suggest that Ga behaves as an Al-substituting element in titanium aluminides. The mechanisms of diffusion of Al-substituting solutes in ordered titanium aluminides are considered. It is shown that Ga diffuses in Ti 3 Al most likely as anti-structure-like defects via the Ti sublattice by nearest neighbor jumps. The experimental data for the Ga diffusion in TiAl can be explained by a predominant contribution of anti-structure bridge jumps.

Pressure dependence of the transition to the proton conducting phase of CsHSO4, CsHSeO4 and RbHSeO4 studied by differential scanning calorimetry

Abstract The temperature and enthalpy of the transition to the proton conducting α-phase have been determined for CsHSO 4 , CsHSeO 4 and RbHSeO 4 in the pressure range up to 1 GPa by means of differential scanning calorimetry (DSC). At normal pressure the transition occurs at 415, 400 and 447 K, respectively, the transition enthalpy is 5.53, 5.74 and 10.5 kJ/mole, respectively, and the volume increase is of the order of 0.3% for CsHSO 4 , while there is instead a volume decrease of the order of 1.3% for RbHSO 4 . For CsHSO 4 the transition temperature has a flat maximum near 0.6 GPa and 422 K. For CsHSeO 4 the transition temperature is close to 400 K in the pressure range below about 0.56 GPa, while d T /d P is about 55 K/GPa at higher pressures. A new high-pressure pressure phase exists only in a very limited temperature and pressure range. For RbHSeO 4 the phase diagram has either a triple point or a minimum in the vicinity of 430 K and 0.45 GPa. The volume increase is of the order of 2% for the transformation to the α-phase at 0.55 GPa.

Diffusion of tin in germanium studied by secondary ion mass spectrometry

The diffusion of tin in intrinsic germanium has been studied in the temperature range from 555 to 930 °C by secondary ion mass spectrometry. Tin has been indiffused under vacuum conditions both from the gas phase and from thin films. In both cases, pure metal was used as a source of tin. In the studied temperature range the diffusion coefficient of tin as a function of temperature can be expressed as D(cm2/s)=8.4×102 exp[(−3.26 eV)/kT]. Based on the similarities between tin diffusion and germanium self‐diffusion, it is concluded that tin diffuses in germanium via the monovacancy mechanism.

Bulk phase transitions of cesium hydrogen sulphate initiated by surface processes, grinding or external pressure

Abstract The conditions have been studied for the existence at room temperature of three phases of CsHSO 4 , here called the β-, γ-phases. Crystallization of an aqueous solution produces the δ-phase, while the metastable β-phase can be obtained after annealing at an elevated temperature. Both these phases can be transferred by a mechanical treatment, grinding, to the γ-phase, which has the lowest energy of them all. Transformation to the δ-phase is instead favoured by applying either hydrostatic or uniaxial pressure. Transitions between all three phases can be controlled by modifying the water vapour pressure above the salt, i.e. a surface process triggers phase transitions in the bulk.

Pressure dependence of phase transitions in CsHSO4 studied with differential scanning calorimetry

The pressure dependence of three solid-solid and solid-liquid phase transitions in CsHSO4 has been investigated with differential scanning calorimetry at pressures up to 0.9 GPa with gaseous argon as the pressure-transmitting medium. The transitions that at ambient pressure occur at 333 K and 375 K seem to coincide at a pressure of about 0.15 MPa and probably exhibit above 0.1 GPa a solid-solid critical point. The ΔV value of the phase transition that occurs at 333 K (at ambient pressure) is calculated from the measured ΔH values and the constant dT/dp slope and is compared with rontgenographic determinations. The phase transition that occurs at 415 K (at ambient pressure) exhibits a linear temperature increase up to about 0.55 GPa and a flat maximum at about 0.6 GPa. The melting curve was determined up to 0.7 GPa and is described by Simons equation.

Nitrogen incorporation in GaNAs layers grown by molecular beam epitaxy

GaNAs/GaAs quantum wells with high N concentrations, grown by molecular beam epitaxy, have been investigated by secondary-ion mass spectrometry (SIMS), high resolution x-ray diffraction (XRD), and photoluminescence (PL) measurements. The substitutional N concentration in an 18 nm thick strained GaNAs layer varies from 1.4% to 5.9% when the growth rate is reduced from 1 to 0.2 µm/h. By further reducing the growth rate, more N can be incorporated but relaxation occurs. Both the total N concentration, deduced from SIMS measurements, and the substitutional N concentration, deduced from XRD measurements, increase with reduced growth rate. By comparing the SIMS and XRD results, we found that a large amount of N was not in substitutional position when the substitutional N concentration is high (>4%). The experimental results also show that there is no detectable change of total and substitutional N concentrations, within the instrument resolutions, after rapid thermal annealing at 700 °C for 30 s. However, PL measurements show a strong blueshift of the emission wavelength after annealing and the PL intensity increases by more than one order of magnitude.

Pressure dependence of high temperature phase transitions in solid CsH2PO4 and RbH2PO4

Temperatures and enthalpies of phase transitions have been determined by differential scanning calorimetry (DSC) for CsH2PO4 and RbH2PO4 in the pressure range up to 1 GPa and 0.75 GPa, respectively. For temperatures between ambient and 400 K the cesium salt has two monoclinic phases at normal pressure, while the rubidium salt has one tetragonal and two monoclinic phases. For both salts the transition between monoclinic phases is reversible, although very sluggish for cooling. Adsorbed water has an influence on the reversibility of the transition between tetragonal and monoclinic RbH2PO4. For CsH2PO4 the transition temperature is 380 ± 2 K at normal pressure and about 440 K at 1 GPa, and the transition enthalpy decreases when the pressure is increased. For RbH2PO4 the temperature of the structural transition increases from 352 ± 3 K at normal pressure to about 370 K at 0.75 GPa, while the enthalpy decreases. The transition between monoclinic phases (380 ± 3 K at normal pressure) appears to have a tricritical point at about 0.15 GPa and 374 K, where, the enthalpy approaches zero while the gradient dT/dp has a sharp change. An alternative interpretation would be a triple point, but this is excluded since no evidence of a similar behaviour is noticed for the transition line of the lower transition. A brief comparison is made of the results for the two dihydrogen phosphates with what is reported in the literature concerning RbD2PO4.