S. M. A. Taher

Thermoelectric efficiency of rare earth sesquisulfides

Abstract Thermo-electric efficiency of several rare earth sesquisulfides is presented and compared with those of commonly known thermo-electric materials. The available data show that at high temperatures dysprosium sesquisulfide has the best thermo-electric efficiency when compared with other well-known materials.

A sample preparation technique to study the organic phase of tooth enamel under scanning electron microscopy

The study of the organic phase of heavily mineralized tooth enamel under scanning electron microscope is difficult primarily because of its partial dissolution in demineralizing agents and its instability after demineralization. In this study, enamel from human and rat teeth were treated first with hydrazine to remove the organic phase. A suitable epoxy resin was prepared and then embedded into the voids of deorganified enamel by vacuum impregnation as well as diffusion techniques. After polymerization of the resin samples, the enamel was treated with demineralizing agents such as EDTA, or Cr3SO4 or HCI for different times, which left a resin replica of the organic phase of enamel in each case. Slow and preferential etching of resin replica under a demineralizing agent enabled SEM examination of the larger structures in the deep layers of the enamel, which, otherwise, would be obscured by the dense network of finer structures.

Low temperature calorimetric, magnetic, and optical studies of dysprosium sesquisulfide

Heat capacity, magnetic susceptibility, and optical measurements have been made on γ‐Dy2S3 having the Th3P4 bcc defect‐type structure. The compound remains paramagnetic down to 2 K, the lowest temperature of measurements. A heat capacity anomaly with its peak at 3.3 K can be interpreted using the Stark levels of the ground state manifold 6H15/2(4f9) of Dy+3 obtained from optical spectra. By subtracting the corresponding Schottky term from the total heat capacity, the lattice contribution is estimated. By further analyzing the temperature dependence of this contribution, in comparison with those for diamagnetic Y2S3, La2S3, and Lu2S3, the limiting Debye temperature value of Dy2S3 is found to be 290 K.

Electrical Resistivity of (NdxGd1−x)3−yS4: Magnetic Effects

We wish to report the electrical resistivity p of single crystal (NdxGd1−x)3−yS4 where (0 ≤ × ≤ 1, 0 < y < 0. 33) between 2 and 300 K. X-ray diffraction analyses reveal that all samples possess the high temperature γ-phase structure which is represented as the bcc Th3P4 defect structure. The electrical resistivity was measured by a four point DC-technique with pressure contacts at the gold-coated bars of samples with dimensions of 1 × 2 × 8 mm. The measured resistivity values without and with a magnetic field of 7. 7 kG as a function of 1/T are shown in Fig. 1. As shown in the figure, the resistivity in each sample decreases linearly with decreasing temperature and goes through a minimum as the Curie temperature is reached. The variations in ρmin. and temperatures of ρmin. are attributed to different rare earth concentrations as well as the total metal to sulfur ratios in the samples. Application of a magnetic field reduces the resistivity below about 60 K in all samples. The ρ vs T curves for all samples strongly suggest that the increase in resistivity is directly related to the state of magnetic ordering through temperature or applied magnetic field. Since the carrier concentration is greater than 8 × 1019/cm3 in all samples, the model of Cutler and Mott (1) suggests that EF(Fermi energy) lies near but above Ec (conduction band energy).

Study of electron conduction in LaSx by electrical resistivity and thermo-EMF measurements

Combined electrical resistivity and thermo-electric power measurements have been made on La3-xVxS4 (0 ≤ x ≤ 13, V = Lanthanum Vacancy) of -phase between 2 K and 300 K. Samples with low sulfur concentrations show metallic conduction and superconductivity at low temperatures. Sulfur rich samples toward stoichiometric compositions La2S3 show electron localization and motion of the electrons is activated with an activation energy similar to that of the narrow band gap materials. Analysis of the experimental data lends support to the hopping type conduction amongst localized states.

Electrical Resistivity and Magnetic Field Effects of NdS3−xVxS4

We wish to report electrical resistivity,ρ, of single crystal (Nd3−xVxS4) where 0 <x<0.33 between 2 and 300K. Samples were prepared either by direct reaction of the elements in closed quartz tubes (between 600 and 800C) or by passing hydrogen sulfide over rare earth oxides (R 0) and grown as ingots from the melt between 1800 and 2100C using Bridgeman techniques. X-ray diffraction measurements reveal all samples possess the high temperature γ - phase or Th3P4 bcc structure. The density of conduction electrons in these solid solution samples range from 1016 to 1021 carriers/cm3. The electrical resistivity was measured by a four point DC-technique with pressure contacts at the gold-coated bars of samples having dimensions of 1 × 2 × 8mm. Overall accuracy of the resistivity measurements is typically 2% from 2 to 40K and 1% from 40 to 300K. The resistivity of all samples was also measured under different magnetic fields up to 7.7kG. For magnetic measurements a magnetometer was used with magnetic fields up to 30kG at temperatures between 4 and 300K.

Magnetic and Thermal Properties of Ce3−xS4

The high temperature phase of cerium sesquisulfide is characterized by the Th3P4 bcc defect structure and usually expressed as Ce3−xVxS4 (0 ≤ × ≤ 0. 33) to indicate the vacancies Vx in the rare earth sublattice. (1) Between the extremes of Ce3S4 (x = 0) and Ce2S3 (x = 0. 33) various ratios between Ce and S are possible and when prepared and melted into ingots or single crystals all samples show the same Th3P4 defect structure. (2) This note reports the magnetic susceptibility (X) and the heat capacity (Cp) for two such samples, namely CeS1.39 and CeS1.46.

Abstract: Magnetic and thermal properties of YbS1.387

Magnetic and thermal properties of Ybs1.387 are presented between 4 and 300 K for the magnetic susceptibility of (χ) and between 2 and 20 K for the heat capacity (Cp). The sample appears to be paramagnetic over the temperature range studied. A plausible explanation for both the Cp and the χ data obtained can be made by considering the Yb3+(4f) ions to be found in a crystal line environment where the ground state level 2F7/2 is split by the crystalline electric field into four Kramers doublets found at 0, 6, 120, and 300 cm−1.