Optical limiting and its mechanism in potash alum

Optical power limiting is the increase of optical absorption with light intensity. Pure crystalline potash alum was grown and optical absorption in dependences of the light intensity and thickness of the crystal was studied using laser light. The transmission of the crystals was found to increase with increasing light intensity. However, at higher light intensity, an increase of optical absorption with intensity was observed. The power limiting characteristics, e.g. linear transmission/dynamic range and saturation point, were found to be varied with thickness of the crystal. The findings resulting from this investigation might have potential applications in optical limiting.     

Crystallization of potash alum: effect of power ultrasound.

The influence of power ultrasound on the crystallization of potash alum was investigated. Experiments have been carried out in a batch stirred vessel. It was found that ultrasonic waves decrease the supersaturation limits and modify the morphology of the crystals produced. The average crystal size decreases with an increase of ultrasonic power. To investigate also the action of ultrasound on already existing crystals, crystals produced in silent conditions were suspended in saturated potash alum solution at various ultrasonic powers. The results show that ultrasound has also an abrasive effect on potash alum crystals for high power inputs.     

Growth and decrescence of two-dimensional crystals: A Markov rate process

A Markov rate process whose transitions are captures and escapes of single atoms from the edge of a two-dimensional crystal is introduced. The stochastic equilibrium states of this process describe steady crystal growth, crystal-fluid equilibrium, and steady crystal decrescence. Exact and asymptotic growth rates are found. This extends recent results which dealt only with capture events. One application is to the growth of lamellar crystals from polymers.     

Growth,structure, thermal,electrical and optical properties of potassium aluminum sulfate dodecahydrate (potash alum) single crystal

Single crystal of potassium aluminum sulfate dodecahydrate (potash alum) was grown by the slow evaporation technique of supersaturated aqueous solution at room temperature. The obtained crystal was characterized by using powder X-ray diffraction, DSC, TGA, electrical conductivity, optical transmittance and optical reflectance measurements. Structural and thermal analyses were performed on potash alum crystal. The electrical conductivity of potash alum as electrolyte sample was 1.894 × 10⁻⁶ S/cm. The optical transition and optical band gap energy of potash alum crystal were studied for the first time. Direct and indirect optical band gap energies were 5.75 and 4.50 eV respectively. Extinction coefficient, refractive index, real and imaginary dielectric constants and optical conductivity were calculated for potash alum crystal at a range of photon energies between 1.4 and 6.2 eV. Normal dispersion parameters of potash alum crystal were calculated for the first time.     

Low-temperature electrolytic coloration and spectral property of ammonium alum crystals

Ammonium alum crystals are colored electrolytically using a pointed cathode and a flat anode at low temperatures and under various voltages. SO₃^?, SO₂^? and O₃^? hole-trapped centers are produced in colored ammonium alum crystals. Characteristic absorption bands of SO₃^?, SO₂^? and O₃^? hole-trapped centers are observed in absorption spectra of colored ammonium alum crystals. Production and conversion of hole-trapped centers are explained. Current–time curves for electrolytic coloration of ammonium alum crystal and their relationship with electrolytic coloration process are given.     

Characterization of ultrasonic transducers using cholesteric liquid crystals

This paper relates to two methods for the characterization of ultrasonic transducers by means of a visualization technique which exploits the thermotropic properties of cholesteric liquid crystals. Examples illustrating these methods are used to demonstrate the validity of the results obtained whilst maintaining the advantages of a simple and economic technique.     

Growth of polymer crystals during annealing

Observations by transmission electron microscopy are reported on the processes involved in polymer crystal growth during annealing. The observations suggest that crystal growth occurs by two processes. One process involves the melting of those regions of the crystals in which the melting point is lower than the annealing temperature. The polymer melt due to the melting process gradually becomes incorporated into the unmolten crystals, resulting in crystal growth. The alternative process is solid-state crystal growth by the migration of the amorphous region between crystallites.     

Growth and characterization of p-type AgInS2 crystals

AgInS₂ crystals have been grown at below melting point by using a Hot-Press method. The samples grown at 400∼600 °C under 10 MPa pressure were found to contain AgIn₅S₈. Occurrence of this phase decreased with increasing growth temperatures and at 700 °C a sample containing only the AgInS₂ phase was successfully obtained. Furthermore, the elemental compositions were found to become increasingly stoichiometric with increasing temperature. We also found that conduction type could be changed from n to p-type by Sb-doping, perhaps as a result of the substitution defect of Sb atoms on the S site.     

Spontaneous-emission rate in microcavities: application to two-dimensional photonic crystals

We present a simple, efficient procedure to compute the spontaneous-emission rate from short-time finite-difference time-domain (FDTD) data of the electromagnetic field energy in microcavities of arbitrary geometry. As an illustration, we apply this procedure to two-dimensional photonic crystals. For comparison, we calculate the local radiative density of states employing an unconditionally stable FDTD method, that is without solving the eigenvalue problem and integrating over the (first) Brillouin zone. We demonstrate that both methods yield the same predictions about the enhancement or suppression of the spontaneous-emission rate by photonic crystals.     

About the dissociation rate of ionic crystals

A new method of calculation of dissociation rates of ionic crystals is developed. Crystals of ideal composition and those with a homogeneity range are considered. Using available data from the literature, the obtained equations are tested for the dissociation rate of copper iodide in vacuum at 200–300 °C. The agreement between the calculated and the experimental values is found to be good.     

The electrical properties and growth conditions of CdS crystals

Large crystals of cadmium sulphide have been grown in various partial pressures of cadmium and sulphur vapour. The concentration and ionisation energy of the shallow donors in the different crystals have been determined by measuring the Hall coefficient and electrical conductivity of semiconducting samples, and by comparing the magnitudes of the drift and Hall mobilities in photoconducting (semi-insulating) samples. The variation of the donor and acceptor content can be explained in broad terms in relation to the parameters of crystal growth. Hall measurements indicate that the ionisation energy of the shallow donors is 0·021 ± 0·002eV. The drift mobility experiment suggests that two shallow traps are present with energy depths of 0·015 and 0·037 eV. The variation of Hall mobility with temperature over the range 30–300°K can be explained in terms of polar optical mode, piezoelectric and ionised impurity scattering. The best value of effective mass obtained was 0·19m.     

Electroreduction of N-methylphthalimide in room temperature ionic liquids under insonated and silent conditions

The selective electroreduction N-methylphthalimide to 3-hydroxy-2-methyl-isoindolin-1-one has been performed in ionic liquids using phenol as a proton donor under silent and ultrasonic conditions. A significant increase in the rate of electroreduction is shown using ultrasonic activation and in addition high current efficiencies were observed. Some decomposition of the ionic liquid was found to have occurred under exposure to ultrasound.     

Anisotropy of UV-reflectivity in wurtzite crystals: a comparison between GaN and CdSe

The imaginary parts of the dielectric functions ϵ₂(ω) are calculated for wurtzite CdSe and GaN within the one-electron approximation using the linear muffin-tin-orbital method and analyzed in terms of separate band-to-band contributions and critical points. Differences and similarities are pointed out. The effects of excitonic enhancement at critical points are discussed.     

Growth kinetics of polymer crystals in bulk

Temperature-dependent measurements of spherulite growth rates carried out for i-polystyrene, poly(ε -caprolactone) and linear polyethylene show that the controlling activation barrier diverges at a temperature which is 14K, 22K and 12K, respectively, below the equilibrium melting points. We discuss the existence of such a “zero growth temperature” T _zg in the framework of a recently introduced thermodynamic multiphase scheme and identify T _zg with the temperature of a (hidden) transition between the melt and a mesomorphic phase which mediates the crystal growth. The rate-determining step in our model of crystal growth is the attachment of chain sequences from the melt onto the lateral face of a mesomorphic layer at the growth front. The necessary straightening of the sequence prior to an attachment is the cause of the activation barrier. A theory based on this view describes correctly the observations. With a knowledge of T _zg it is possible to fully establish the nanophase diagram describing the stability ranges of crystalline and mesomorphic layers in a melt. An evaluation of data from small-angle X-ray scattering, calorimetry and optical growth rate measurements yields heats of transition and surface free energies of crystals and mesophase layers, as well as the activation barrier per monomer associated with the chain stretching. According to the theory, the temperature dependence of the crystallization rate is determined by both the activation energy per monomer and the surface free energy of the preceding mesomorphic layer. Data indicate that the easiness of crystallization in polyethylene is first of all due to a particularly low surface free energy of the mesomorphic layer.     

Growth of high-homogeneity GaSb:Te crystals for thermophotovoltaic energy converters

Problems concerning the technology of growing highly homogeneous semiconductor crystals are discussed. The dependence between the inhomogeneity of GaSb:Te substrates and the efficiency of thermophotovoltaic converters (TPVCs) fabricated on their base via the diffusion technique is examined. The macro- and microhomogeneities of grown crystals can be substantially increased on account of the theoretically substantiated and experimentally implemented approximation to diffusive mass transfer conditions in the melt. The characteristics of TPVCs on substrates made of crystals with the most homogeneous properties exceed the analogous characteristics of other samples.     

Influence of ultrasonic vibrations on the growth of semiconductor single crystals

The influence of ultrasonic vibrations on striations in InSb, GaAs and Bi-Sb alloy single crystals grown by a modified Czochralski method was investigated. Ultrasonic vibrations at frequencies of 0.15, 0.25, 0.6, 1.2, 2.5, 5 or 10 MHz were introduced into the melt parallel to the pulling axis. The introduction of ultrasonic vibrations at a frequency up to 5 MHz eliminates the striations in GaAs and Bi-Sb alloy single crystals growing with constant diameter. It was found that for Bi-Sb alloy single crystals of constant diameter growth, after ‘processing’ of the melt with ultrasonic vibrations, the striations do not reappear until after 2 h. The effectiveness of the influence of the ultrasonic vibrations on the decrease of growth striations in InSb, GaAs and Bi-Sb alloy growing crystals was estimated with the help of the calculation of the sound absorption coefficient.