ALBERT

Notes: A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring the phase lag between a front and rear illumination, at a single chopping frequency. The method is tested using some semiconductor and glass samples.

Notes: Summary In order to study the Mn nutrition ofbidi tobacco plant (Nicotiana tabacum L. aromatic strain 2-1) when it is supplied with nutrient solutions containing different ratios of Mn with Fe, P, K and Ca, a sand-culture experiment was carried out in four different series viz. Fe-Mn, K-Mn, P-Mn and Ca-Mn. No characteristic deficiency symptoms except general loss of green colour and diminished growth were observed at 0.1 ppm Mn in the nutrient solution. Toxicity was observed when Mn in the nutrient solution was 100 ppm and severity of the symptoms decreased with increase in Fe-Mn or K-Mn ratios, but it increased when P-Mn ratio increased while in Ca-Mn series it first decreased and then increased at still higher concentration of Ca on account of chloride ion effect as CaCl2 had to be added to bring about 500 ppm concentration of Ca necessary for the treatment. No symptoms of deficiency or toxicity were observed when Mn in nutrient solution varied from 1 to 10 ppm and Fe-Mn ratio for the leaf varied from 0.4 to 6.1 and its Mn content varied from 190 to 1575 ppm. Slight loss of green colour and plant vigour appeared when Fe-Mn ratio for the leaf was higher than 12.8 even though the Mn content was as high as 90 ppm. Toxic effect due to excessive Mn was felt when Fe-Mn ratio was 0.35 or less and leaf content 1875 ppm or more. Nicotine content was inversely related to the intensity of Mn-toxicity.

Notes: Abstract The I-V characteristics of p-ZnTe films deposited on ψ-Si using different growth conditions have been studied. The devices fabricated at substrate temperatures above 473 K possess bistable impedance states: switching from high-to-low impedance occurs when ZnTe film is positive biased, while switching from low-to-high impedance state occurs when ZnTe film is negative biased. It is observed that zero-based capacitance, open circuit photovoltage and short circuit current density are increased when the device is switched to the low impedance state and revert to their initial values when switched back to the high-impedance state. It is believed that an electronic switching mechanism is responsible, most probably the filling and the emptying of traps in the wide band gap ZnTe films.

Notes: Abstract A detailed study of ultrasonic relaxation in the entire vitreous range of the glass system MoO3-P2O5 (0 to 83 mol% MoO3) is presented. Pulse echo techniques were employed in the frequency range 15 to 135 MHz and temperature range 4 to 300 K. Ultrasonic absorption peaks were observed at various temperatures between 74 and 128 K depending upon glass composition and operating frequency. The composition dependence of the position and overall shape of the loss peaks was analysed in terms of an assumed loss of the standard linear solid type, with low dispersion, and a broad distribution of Arrhenius-type relaxation times with temperature-independent relaxation strengths (two-well formalism with a broad distribution of asymmetries). After a brief review of two-well formalism, a simple central force model of the microscopic origins of two-well systems is presented, to demonstrate that (in contrast to what has often been assumed previously) there is no need to postulate bond directionality effects to account for “acoustically active” two-well systems in glassy materials. Also given is a quantitative model of the deformation potential arising in phenomenological models of acoustic loss produced by two-well systems. The shape of the loss peaks (i.e. the shape of the relaxation spectra), and the mean activation energy determined from the frequency dependence of the loss-peak temperature (V p), are both strongly composition-sensitive. Furthermore, they are found to correlate with the elastic properties of the glass system and the assumed bond force constants. The conclusion thus arrived at is that the acoustic loss is a property of the vitreous network as a whole rather than due to a second-order effect like the presence of dangling bonds or polar groups, or to network holes etc. Finally a microscopic phenomenological model is developed to account for the composition dependence of the observations. In particular, semi-theoretical formulae are used to expressV p and the fractional number of two-well systems per oxygen atom,N, as a function of the bulk modulus and a mean cation-anion stretching force constant. Considerable success was thereby achieved in explaining the contrasting behaviour ofV p andN.

Notes: Abstract A detailed study of ultrasonic relaxation in the entire vitreous range of the glass system MoO3-P2O5 (0 to 83 mol% MoO3) is presented. Pulse echo techniques were employed in the frequency range 15 to 135 MHz and temperature range 4 to 300 K. Ultrasonic absorption peaks were observed at various temperatures between 74 and 128 K depending upon glass composition and operating frequency. The composition dependence of the position and overall shape of the loss peaks was analysed in terms of an assumed loss of the standard linear solid type, with low dispersion, and a broad distribution of Arrhenius-type relaxation times with temperature-independent relaxation strengths (two-well formalism with a broad distribution of asymmetries). After a brief review of two-well formalism, a simple central force model of the microscopic origins of two-well systems is presented, to demonstrate that (in contrast to what has often been assumed previously) there is no need to postulate bond directionality effects to account for “acoustically active” two-well systems in glassy materials. Also given is a quantitative model of the deformation potential arising in phenomenological models of acoustic loss produced by two-well systems. The shape of the loss peaks (i.e. the shape of the relaxation spectra), and the mean activation energy determined from the frequency dependence of the loss-peak temperature (V p), are both strongly composition-sensitive. Furthermore, they are found to correlate with the elastic properties of the glass system and the assumed bond force constants. The conclusion thus arrived at is that the acoustic loss is a property of the vitreous network as a whole rather than due to a second-order effect like the presence of dangling bonds or polar groups, or to network holes etc. Finally a microscopic phenomenological model is developed to account for the composition dependence of the observations. In particular, semi-theoretical formulae are used to expressV p and the fractional number of two-well systems per oxygen atom,N, as a function of the bulk modulus and a mean cation-anion stretching force constant. Considerable success was thereby achieved in explaining the contrasting behaviour ofV p andN.

Notes: Abstract The preparation of glasses with compositions spanning the entire vitreous range obtainable from melting phosphorous pentoxide with molybdenum trioxide, and phosphoric acid with ammonium molybdate, in open crucibles, is described. The second order elastic constants of these Mo-P-O glass systems have been obtained from ultrasonic wave velocity measurements. The most striking features of the results are two discontinuities in the compositional gradient of each elastic modulus, at compositions of ∼53 and 63 mol% MoO3 content. These compositions approximate to those of molybdenum metaphosphate and molybdenum pyrophosphate crystals. A second noteworthy feature is that the elastic moduli display a substantial overall increase with molybdenum content, 36% for bulk and 83% for shear over the entire composition range, even though the Mo-O bond stretching force constant is less than half that of the P-O bond. A detailed qualitative interpretation of these features is made, in terms of the proposed compositional dependence of the crosslink densities of the molybdenum and phosphorus atoms, and the atomic ring sizes.

Notes: Abstract The temperature dependence of 15 MHz ultrasonic bulk wave velocity in the range 4 to 600 K has been measured in the entire range of glasses that can be prepared by mixing MoO3 with P2O5 in open crucibles. In all cases the temperature gradients are negative. However, when the contribution to the velocity variation caused by (anelastic) two-well relaxational effects is subtracted, the residual velocity variation is always positive with respect to temperature. It is suggested that this effect, which has been reported to be present in other glasses but not in crystals, is still caused by two-well systems, but is elastic in nature. It is argued that under certain conditions the expansion of a glass due to increases in temperature softens the vibrations of particles in transverse two-well systems. In the simple case of wave propagation along a chain this leads to a positive temperature gradient of the shear modulus, and in an isotropic system positive gradients will similarly be caused in all elastic moduli. A detailed model is presented to justify the proposal quantitatively by identifying the conditions under which such behaviour could take place. One cause would be the presence of an appreciable number of two-well systems of barrier height ≪kT at room temperature. For the MoO3-P2O5 system it is estimated that 25% of cation-cation spacings contracted by between 0 to 1 % from equilibrium (crystalline) values, to produce barrier heights ⩽ 0.005 eV, would explain the observations. On the other hand an appreciable fourth-power term in the transverse force potentials (i.e. high third-order bending force constants) would cause a similar effect even if a much smaller number of barriers were present, but of height 〉 kT indeed, such a mode softening term has exactly the opposite effect on the temperature gradients of velocity to what it has in single-well systems.