J. Tafto

THE DEFECT STRUCTUFE OF SrTi1−xFexO3−y (x = 0–0.8) INVESTIGATED BY ELECTRICAL CONDUCTIVITY MEASUREMENTS AND ELECTRON ENERGY LOSS SPECTROSCOPY (EELS)

Abstract The electrical properties and defect structure of selected compositions in the SrTi1−xFexO3−y system (x = 0–0.8) have been studied using van der Pauw 4-point conductivity measurements and electron energy loss spectroscopy (EELS). Using X-ray powder diffraction and selected area electron diffraction (SAD), the basic crystal structure was determined to be cubic perovskite for all the investigated compositions. A superstructure cell 2·2·1 times the ordinary perovskite cell was found in materials with x = 0.6 and x = 0.8. The conductivity was measured on sintered tablets as a function of the partial pressure of oxygen (pO2 = 10−25 to 1 atm) at 600–1100°C. The materials investigated are predominantly p-type electronic conductors at high, n-type conductors at low, and ionic conductors at intermediate oxygen partial pressures. All conductivity contributions increase with increasing iron content. This can be attributed to the acceptor role of the iron, decreased band gap and decreased activation energy for oxygen vacancy migration. The EELS spectra show a shoulder below the oxygen K-edge, increasing in magnitude with increasing iron content and oxygen partial pressure. This shoulder is assigned to empty electron energy states at some of the oxygen atoms, indicating that electron holes are associated with lattice oxygen in the structure. Spectra from the Fe L-edge showed small changes, suggesting that there are few or no empty states at the iron atoms.

Defect structure of the high-dielectric-constant perovskite CaCu{sub 3}Ti{sub 4}O{sub 12}

Using transmission electron microscopy (TEM) we studied CaCu{sub 3}Ti{sub 4}O{sub 12}, an intriguing material that exhibits a huge dielectric response, up to kilohertz frequencies, over a wide range of temperature. Neither in single crystals, nor in polycrystalline samples, including sintered bulk and thin films, did we observe the twin domains suggested in the literature. Nevertheless, in the single crystals, we saw a very high density of dislocations with a Burger vector of [110], as well as regions with cation disorder and planar defects with a displacement vector (1/4)[110]. In the polycrystalline samples, we observed many grain boundaries with oxygen deficiency, in comparison with the grain interior. The defect-related structural disorders and inhomogeneity, serving as an internal barrier layer capacitance in a semiconducting matrix, might explain the very large dielectric response of the material. Our TEM study of the structure defects in CaCu{sub 3}Ti{sub 4}O{sub 12} supports a recently proposed morphological model with percolating conducting regions and blocking regions.

The passive behaviour of Mg in alkaline fluoride solutions. Electrochemical and electron microscopical investigations

Abstract The passive behaviour of pure Mg at 25°C was studied by electrochemical methods in solutions of pH 7–14 containing 0–4 M KF. The dependences of the quasi-steady corrosion current density on pH and [KF] were assessed. Solutions of NaF and NH 4 F were also tested. The corrosion films were examined by SEM, TEM and AES. Porous, gel-like Mg(OH) 2 films were formed in solutions with small, or no, additions of KF. At high [KF], very protective films were formed (corrosion current densities 10 −9 −10 −8 A cm −2 ). The composition of these films was not identified. MgF 2 and KMgF 3 were identified by electron diffraction in some other corrosion films.

Composition and the thermoelectric performance of β-Zn4Sb3

β-Zn4Sb3 is a promising thermoelectric material due to the abundance of zinc and antimony and reports of high efficiency in bulk samples. This work establishes the high temperature properties of β-Zn4Sb3 across the phase stability window. By controlling the stoichiometry, the Hall carrier concentration can be tuned from 6–9 × 1019 cm−3 without requiring extrinsic dopants. The trend in Seebeck coefficient on carrier concentration is rationalized with a single, parabolic band model. Extremely low lattice thermal conductivity (0.4–0.6 W m−1 K−1) coupled with a moderate effective mass (1.2 me) and mobility leads to a large figure of merit (zT of 0.8 by 550 K). The single parabolic band model is used to obtain the carrier concentration dependence of the figure of merit and an optimum carrier concentration near 5 × 1019 cm−3 is predicted.

Thermoelectric properties of Cu doped ZnSb containing Zn3P2 particles

We prepared ZnSb containing Zn3P2 particles of size ranging from a few tens to several hundred nanometers by melting powders of Zn, Sb, and P. Materials with Zn3P2 content up to 3.75% were made and subsequently ball-milled and hot pressed. A reduction in the thermal conductivity of 15% was achieved. By adding 0.2% Cu the carrier concentration increased an order of magnitude, to 2.0 × 1019 cm−3, while the mobility remained unaffected. The resulting increase in electrical conductivity together with the reduced thermal conductivity, led to a significant increase in the dimensionless figure of merit, in excess of 0.9 around 550 K.

Structure refinement of Al3Zr using single-crystal X-ray diffraction, powder neutron diffraction and CBED

The structure of the intermetallic compound A13Zr has been studied at 293 K by single-crystal X-ray diffraction (Mo Ka radiation, ,~ -- 0.71069 A), powder neutron diffraction {A[Ge(711)] = 1.0867 A} and convergent-beam electron diffraction (CBED) (200 keV, A = 0.0251/k). The structure of AI3Zr comprises four close-packed metal sub-lattices and has the tetragonal space group 14/mmm with a = 3.9993 (5), c = 17.283 (2) A, V= 276.43 (6) ,~3, Z= 4, Dx=4.136gcm-3, /z=45.11cm-l. The new z coordinates of four A1 and four Zr atoms on the e position [ZA~(e)and Zzr(e)] were determined by singlecrystal X-ray diffraction: Zn~(e)=0.37498(5) and

Nanoscale inclusions in the phonon glass thermoelectric material Zn4Sb3

We have investigated the thermoelectric material Zn4Sb3 using transmission electron microscopy (TEM). Nanoscale inclusions with a diameter of about 10 nm were observed, constituting on the order of 1% by volume of the material. Studies using energy filtered imaging, electron diffraction, and high-angle annular dark-field STEM indicate that the inclusions consist of Zn. These inclusions are expected to scatter the medium and long-wavelength phonons effectively, thus contributing to phonon glass behavior which results in the exceptionally low thermal conductivity for this thermoelectric material.

Impurity band conduction in the thermoelectric material ZnSb

ZnSb is favourable as a thermoelectric material, from both an environmental and a global resources point of view. Its efficiency can possibly be improved by the reduction of the thermal conductivity through nanostructuring and optimization of doping. These tasks require a better understanding of the material and, in particular, of the interplay between preparation techniques and material properties. We have prepared bulk polycrystalline samples and report on low-temperature electrical transport measurements (6 K to room temperature). The data have been interpreted in terms of hole impurity band conduction: intrinsic acceptor defects creating bands that are conducting when there are also compensating donors. Modelling the transport reveals qualitatively good agreement. Quantitative differences are discussed in terms of the structure of the samples, which has been studied by using x-ray diffraction, a scanning electron microscope and an electron microprobe. The systematics of adding different amounts of Mn and Cr to ZnSb has been studied and has the effect of varying the density of states in impurity bands and varying the hole concentrations.

Characterization of planar crystal lattice defects in the high-temperature superconductor YBa2Cu3O7

Transmission electron microscopy shows that sintered YBa2Cu3O7 contains many planar defects in the a‐b plane. They range from stacking faults to 10–100 nm thick sheets of disordered material that extend through the entire crystal grains. The stacking faults may be precursors for the thick layers of disordered material. In this letter we characterize the stacking faults and find that the displacement vector switches from (a/2,0,c/6) to (b/2,0,c/6) as the faults cross the twin boundaries.

Evidence of chemical ordering in amorphous hydrogenated silicon carbide

Amorphous Si0.68C0.32: H prepared by radio frequency glow discharge from a mixture of methane and silane was studied by means of the complementary techniques of electron energy‐loss spectroscopy and electron diffraction. The experimental results are consistent with Si and C forming a tetrahedral network with nearest neighbor distances similar to those in crystalline Si and crystalline SiC. There is evidence that the C atoms tend to be surrounded by four Si atoms rather than a random distribution of C and Si on the tetrahedral network.