Showing papers by "Solid State Physics Laboratory published in 1983"

Liquid crystalline materials: physical properties and intermolecular interactions

Abstract: The physical properties and the phase behaviour of some nematic liquid crystals are discussed, with emphasis on the influence of short-range antiparallel dipole correlation that occurs in mesogenic compounds with a strong terminal dipole moment. Evidence for this effect, which stems especially from dielectric studies, is summarized. Variations of the dipole correlation with molecular structure can explain the sometimes unexpected phase behaviour and physical properties of these substances. A qualitative model is given in terms of a monomer-dimer equilibrium. During the last decade a growing interest has developed in the 'molecular engineering' of liquid crystalline materials to obtain specific properties. Here three aspects can be distinguished: chemical properties (discussed by others at this meeting); phase behaviour, like mesomorphic temperature range and nematic-smectic behaviour; physical properties, like birefringence, dielectric anisotropy and visco-elastic properties. The demands with respect to the first two points being largely satisfied, attention has shifted to methods of influencing the physical properties. Here we can distinguish two points: the properties of the molecules themselves, and the interactions through which a molecular property influences the macroscopic behaviour. So far attention has been directed mainly towards the first of these two. However, it seems that the interactions mentioned above are far more variable than was commonly thought some years ago. This insight is mainly due to new results from the synthetic chemists, who have presented us with new mesogenic compounds that often show a behaviour that is rather unexpected from a more traditional point of view. In this paper I wish to show that many of these unexpected results can be rationalized in terms of differences in short-range correlation between the molecules. The plan of the paper is as follows. In ? 2 some of the more classical relations between molecular properties and macroscopic physical properties will be summarized. Next, in ? 3, the evidence for short-range correlations will be discussed, emphasizing results from relatively recently synthesized compounds with strongly polar end-groups like CN, saturated ring systems, or both. Finally these results are used to understand at least qualitatively some of the features of the observed phase behaviour (? 4) and of the macroscopic physical properties (? 5).

Flow-measurements of the viscosity coefficients of 2 nematic liquid-crystalline azoxybenzenes

Abstract: To determine the Miesowicz viscosity coefficients a shear-flow set-up has been built, which allows the measurement of η1, η2, η3 and η12. These coefficients are determined by pressing the liquid crystal through a rectangular capillary which forms part of a thermostated set-up placed between the poles of a magnet in order to control the director n. In addition, the flow-alignment angle θ 0 is determined from the change in optical path difference between the ordinary and extraordinary components of polarized light, comparing the situation with n aligned along the flow-direction and with flow-alignment, respectively. Results are given for the complete set of viscosity coefficients of p-methoxy-p'-butylazoxybenzene and p, p'-dibutylazoxybenzene. The results show good agreement with visco-elastic ratios found with light-scattering techniques. Within the experimental accuracy the Onsager-Parodi relation is well fulfilled.

Crystallization of amorphous zr-rich alloys

Abstract: The crystallization of amorphous alloys of the approximate composition Zr 8 M 2 , where M represents Cu, Ni, Co, Fe, Rh and Ir was studied by means of differential scanning calorimetry X-ray diffraction and also, in the case M = Fe by 57 Fe Mossbauer effect spectroscopy. It is shown that the first step in the crystallization of these alloys involves the precipitation of a metastable modification of Zr metal (ω-Zr).

Charge transport in sr1-xlaxf2+x solid-solutions - an ionic thermocurrent study

Abstract: A comprehensive study of the relaxation of space charges in solid solutions of the type ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$ will be reported. In this investigation we have employed the ionic thermocurrent technique. La${\mathrm{F}}_{3}$ concentrations between 0.01 and 42 mol% have been used. We have paid attention to the resemblance between space-charge relaxation and ionic conductivity in heavily rare-earth-doped fluorite-type solid solutions. We will show that the enhancement of the conductivity and the shift of the space-charge relaxation band to lower temperatures with increasing rare-earth concentration can be explained with the same mechanism. Charge transport is supported by two kinds of interstitial jumps: free interstitial jumps occurring relatively far from a rare-earth ion and energetically more favorable dipolelike jumps of interstitial fluoride ions in the neighborhood of a rare-earth ion. We will also discuss the influence of the defect concentration on the degree of dissociation of the dipoles present and the effect of the concentration on the defect structure of the ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$ solid solutions. It will be made clear that the charge transport in rare-earth-doped fluorite-type ionic crystals is supported by a percolation mechanism.

EPR and Ionic Thermocurrents Experiments on Cubic Solid Solutions Sr1-xEuxF2+x

Abstract: In this paper we report about the results of EPR and ionic thermocurrents (ITC) experiments on cubic solid solutions of the type Sr 1- x Eu x F 2+ x . A part of the Eu-ions is in the divalent state; these ions can be detected easily with EPR. From the EPR results we conclude that for 0≦ x ≦0.115 the long range order is still cubic and the lattice parameter does not vary appreciably. The interactions between distant Eu 3+ -F - i dipoles and the Eu 2+ probes is estimated; from this we conclude that for \(x{\gtrsim}0.044\) clustering of trivalent Eu-ions cannot be excluded. The ITC results are in line with those obtained from EPR. In addition we note that for small values of x nearest neighbor (NN) Eu 3+ -F - i complexes are the dominant defects. For x >5.10 -3 NNN dipoles are present also. Apart from the reorientation bands associated with the above mentioned defects we observe with ITC a depolarization peak, ascribed to space charge phenomena. The behavior of the dipole peaks and the space charge peak is d...

Ionic thermocurrents and ionic conductivity of solid solutions of SrF2 and YbF3

Abstract: We report dielectric [ionic thermocurrent (ITC)] experiments and ionic conductivity of cubic solid solutions of the type ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}{\mathrm{F}}_{2+x}$. These combined experiments provide us with new information concerning the ionic conductivity mechanisms which play an important role in solid solutions ${\mathrm{Sr}}_{1\ensuremath{-}x}{R}_{x}{\mathrm{F}}_{2+x}$ ($R$ is a trivalent rare-earth ion). From the results it will be concluded that the depolarization peak, which is associated with space charges in a polarized sample, can be connected with bulk ionic conductivity. An interesting feature of the ITC results and the ionic conductivity data is that there are major fluctuations of the dielectric properties and the ionic conductivity as a function of the concentration of ${\mathrm{Yb}}^{3+}$ impurities. These fluctuations are ascribed to clustering of ${\mathrm{Yb}}^{3+}$ impurities. Fluctuations of this type have not been observed in earlier investigations, where solid solutions ${\mathrm{Sr}}_{1\ensuremath{-}x}{R}_{x}{\mathrm{F}}_{2+x}$ were treated. It appears that clustering is important for ${R}^{3+}$ ions of the final part of the series of lanthanide ions. For ${R}^{3+}$ ions of the first part of the series of lanthanide ions clustering turns out to be unimportant.

Defect structure and defect-defect interactions in solid-solutions of af2 and rf3 doped with gd-3+ probes

Abstract: In this paper we present results of an investigation of cubic ${\mathrm{Gd}}^{3+}$ ions in solid ${A}_{1\ensuremath{-}x}{R}_{x}{\mathrm{F}}_{2+x}$ ($A$ is an alkaline-earth ion and $R$ is a rare-earth ion) solutions. The values of $x$ were chosen between 0 and 0.06 and the experimental technique used is EPR. We have observed that with increasing concentrations of the rare-earth impurities the widths of the EPR lines increase. This can be interpreted in terms of new contributions to the spin Hamiltonian originating from defect centers present in the crystal. These new contributions are second-degree crystal-field terms and can be approximated by a point-charge description of the imperfect crystal. An interesting feature of our results is that clustering does not appear to be appreciable defect clustering if we confine ourselves to the alkaline-earth elements Ca and Sr and the rare earths La, Ce, Pr, and Nd. For the heavy (small) rare-earth ions Dy, Er, and Yb the situation may be different, but these ions have not been investigated here. It appears that the defect structure of the systems ${A}_{1\ensuremath{-}x}{R}_{x}{\mathrm{F}}_{2+x}$ depend upon the relative sizes of the ions ${A}^{2+}$, ${R}^{3+}$, and ${\mathrm{F}}^{\ensuremath{-}}$ and slight differences between the formation energies of different defects may cause drastic changes in the defect structure of the materials. This may be the reason why the trivalent Gd ions have cubic symmetry as soon as sufficient concentrations of large trivalent ions are present.

Temporal and spatial evolution of the laser-induced plasma in methane

Abstract: This paper gives an account of an experimental study on the temporal and spatial evolution of the plasma in methane produced by the impact of TEA-CO2 laser radiation. The preliminary analysis of this plasma by mass spectroscopy and ir spectroscopy show that acetylene and hydrogen are the dominant gaseous products. This is conform with other types of methane pyrolysis which include fast quenching. The temporal evolution of the plasma formation as well as the shock-wave generation and expansion are investigated by Schlieren high-speed photography and Mach-Zehnder interferometry. The temporal and spatial variation of the distributions of the C2, (a 3π u ), CH(X 2 π r ), and C3(1∑ g ) radicals in the plasma are measured with the aid of laser-induced fluorescence. These experimental methods yield a detailed picture of the temporal evolution of the laser-induced plasma and give an insight into the reaction kinetics. The dominant reaction process is fast heating in the shock wave and a subsequent rapid quenching.

The effect of built-in drift field and emitter recombinations on FCVD of a p-n junction diode

Abstract: This paper discusses the Forward Current induced open circuit Voltage Decay (FCVD) of a p-n junction diode including the effects of recombinations in the emitter as well as the built-in drift fields in the base and in the emitter. The analysis is based on the quasi-static approximation (QSA) of the carrier profiles in the emitter. It is shown that the emitter effects on FCVD is completely determined by JEO, the dark saturation current in the emitter. The value of JEO in general, depends on the heavy doping effects in the emitter, the drift field in the emitter, emitter thickness and surface recombination velocity at the emitter surface. It is shown that for a diode with retarding drift field in the base, emitter recombinations play a very significant role in FCVD. The decay time constant for large values of time in this case is given by τ eff = τ B /[1 + ƒ B 2 − (a − ƒ B ) 2 ], where a = J EO /J BO , ƒ B is the drift field parameter in the base. The higher value of a, the faster is the voltage decay. For accelerating fields in the base, the time constant for large values of time is independent of emitter recombinations and is given by τ eff = τ B /(1 + ƒ B 2 ) . However, the decay rate for small values of time is strongly affected by emitter recombinations for both types of the field; the higher the emitter recombinations, the faster is the initial rate of the voltage decay. For extremely strong drift fields in the base, QSA in the emitter is not valid. The coupled continuity equations are solved with the conditions ƒ B 2 ⪢ τ B /τ E and an analytic expression for FCVD is derived. It is seen that FCVD for strong base fields is determined solely by emitter lifetime τE except for small values of time of the order of a few τE.

Effect of clustering on the space-charge relaxation phenomena in fluorite-type solid solutions Sr1-xDyxF2+x and Sr1-xErxF2+x

Abstract: In this paper we present the results of ionic thermocurrent (ITC) and dielectric loss experiments on two systems of solid solutions: ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{F}}_{2+x}$ and ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Er}}_{x}{\mathrm{F}}_{2+x}$. The materials investigated have the fluorite structure, and the value of $x$ has been varied in the range $0\ensuremath{\le}x\ensuremath{\le}0.4$. In contrast with results published in earlier papers on solid solutions ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$, we find, for the above-mentioned materials, that clustering plays an important role. It appears that clustering becomes more and more important with decreasing ionic radius of the trivalent lanthanide. We have observed that for Yb-, Er-, and Dy-doped crystals the concentration of next-nearest-neighbor (NNN) dipoles decreases for $R{\mathrm{F}}_{3}$ concentrations higher than 0.4, 0.6, and 1.0 mol%, respectively. The ITC peak associated with space-charge relaxation shows a complicated behavior as a function of the $R{\mathrm{F}}_{3}$ concentration ($R=\mathrm{Dy}\mathrm{or}\mathrm{Er}$). For low concentrations the space-charge relaxation peak shifts to lower temperatures with increasing concentration until a minimum value for the temperature is reached. For ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Er}}_{x}{\mathrm{F}}_{2+x}$ and ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Dy}}_{x}{\mathrm{F}}_{2+x}$ the concentrations for which this minimum occurs are 0.6 and 1.0 mol%, respectively; i.e., the same concentrations as those mentioned above, where the NNN dipole concentration has its maximum value. The position of the high-temperature (HT) band turns out to be related to the concentration of dipoles in the low-concentration range. In heavily doped materials the position of the HT band probably depends upon the number of clusters present. It will be concluded that clustering is very complex and that it is not possible to draw unambiguous conclusions from our experimental results concerning the defect structure of heavily doped materials. In spite of this we have presented a few possible descriptions which may be applicable.

The influence of heavy doping effects on the reverse recovery storage time of a diode

Abstract: During the reverse recovery process in a modern Si p-n junction diode, the value of JEO/JBO (the ratio of emitter to base dark saturation currents) increases and the recombination of carriers in the emitter becomes important due to heavy doping effects. A theory is developed to take these effects into account. The emitter and the base components of the current during the reverse recovery phase are found to vary with time. However, their sum remains equal to the constant reverse current JR, which flows in the external circuit. The ratio of the total quantity of charge present in the base to that present in the emitter is found to increase rapidly with time. Values of the storage time ts for different values of JEO/JBO are calculated. In a typical case, the storage time is reduced by a factor 5 in a diode with JEO/JBO = 2. In such cases, the values of lifetime τB calculated using measured ts values and the Kingston's formula, become inaccurate. Theoretical expression for the total charge QBS left in the base at t = ts in a base dominated diode is derived. An earlier semi-empirical formula known as Kuno's formula is derived theoretically. It is found that the formula is valid both for the base dominated diode as well as in a diode with large contribution of the emitter but only when JR/JF is small. According to this formula ts vs 1n(1 + JF/JR) plot is approximately a straight line with slope approximately equal to τB in both cases. For large values of JR/JF when ts values are small, the correct formula shows that the plot is highly curved. An analysis of this part of the curve yields a value of JEO/JBO.

The Variation of Optical Energy Gaps with Composition in Pb1xHgxS Alloy Films

Abstract: Studies of optical properties of flash evaporated Pb1−xHgxS films, in the composition range 0.07 ≦ x ≦ 0.93, show that the alloy films are formed by alloying of PbS with α-HgS. The lead rich films show an indirect gap and a direct optical gap whereas the mercury rich alloy films show only the direct optical gap. The optical gap increases linearly in polycrstalline films with increasing Hg concentration. The single crystal films, grown at 100 °C (0.07 ≦ x ≦ 0.50) have indirect and direct optical band gaps; the optical gap although increasing linearly with x has values smaller than the films grown at 25 °C. Untersuchungen der optischen Eigenschaften von schnell aufgedampften Pb1−xHgxS-Schichten im Zusammensetzungsbereich 0,07 ≦ x ≦ 0,93 zeigen, das die Legierungsschichten durch Legierung von PbS mit α-HgS gebildet werden. Die bleireichen Schichten zeigen eine indirekte Bandlucke und eine direkte optische Bandlucke, wahrend die quecksilberreichen Legierungsschichten nur das direkte optische Gap zeigen. Die optische Bandlucke in polykristallinen Schichten nimmt linear mit steigender Hg-Konzentration zu. Die Einkristallschichten, bei 100 °C gewachsen mit 0,07 ≦ x ≦ 0,50 besitzen eine indirekte und eine direkte optische Bandlucke, die optische Bandlucke besitzt kleinere Werte als die der bei 25 °C gewachsenen Schichten, obgleich sie linear mit x zunimmt.

Flow-alignment and the sign of κ2 in nematic liquid crystals

Abstract: Results are presented for the flow-alignment angle θ 0 of various nematics. Measurements for two compounds that differ mainly in the presence of a strongly polar terminal cyano-group indicate that this has no influence on θ 0 . Results for a homologous series give a decrease of θ 0 with increasing alkyl chain lengths, until finally no flow-alignment exists due to a change in sign of κ 2 . This is attributed to pre-transitional smectic order Presentation des resultats pour θ 0 , angle d'alignement des nematiques par l'ecoulement. Des mesures sur deux composes qui ne different que dans la presence d'un groupe cyano terminal fortement polaire, n'indiquent aucune influence de ce groupe sur θ 0 . Des resultats pour une serie homologue montrent que θ 0 decroit avec la longueur des chaines d'alkyl, jusqu'a perdre l'alignement en raison d'un changement du signe de κ 2 . Ces effets sont attribues a l'ordre smectique pretransitionnel

Reverse recovery in p-n junction diodes with built-in drift fields

Abstract: This paper extends the earlier analysis of Moll, Krakauer and Shen of the reverse recovery of a symmetrical p-n junction diode with built-in retarding drift field in the base to the diodes in which the effect of bulk recombinations in the base is important. It is shown that the effect of bulk recombination becomes important if J f J R ⪆ 0.5 (JF and JR are the forward and the reverse currents respectively) even when the drift field is very strong. For very strong retarding fields, ts vs ln (1 + J F J R ) plot is linear and has a slope very close to the minority carrier lifetime τB in the base even for small values of time. Hence, τB can be conveniently determined by this method for such diodes. Expressions for the charge left in the base at the end of the storage phase and for the decay of the current in the second phase are also derived and discussed.