Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

Band Engineering of Thermoelectric Materials

Several materials have been identified over the past few years as promising candidates for the development of new generations of magnetoresistive devices. These range from artificially engineered magnetic multilayers' and granular alloys, in which the magnetic-field response of interfacial spins modulates electron transport to give rise to 'giant' magnetoresistance, to the manganite peravskites, in which metal-insulator transitions driven by a magnetic field give rise to a `colossal' magnetoresistive response (albeit at very high fields). Here we describe a hitherto unexplored class of magnetoresistive compounds, the silver chalcogenides. At high temperatures, the compounds Ag_2S, Ag_2Se and Ag_2Te are superionic conductors; below similar to 400 K, ion migration is effectively frozen and the compounds are non-magnetic semiconductors that exhibit no appreciable magnetoresistance. We show that slightly altering the stoichiometry can lead to a marked increase in the magnetic response. At room temperature and in a magnetic field of similar to 55 kOe, Ag_(2+δ)Se and Ag_(2+δ)Te show resistance increases of up to 200%, which are comparable with the colossal-magnetoresistance materials. Moreover, the resistance of our most responsive samples exhibits an unusual linear dependence on magnetic field, indicating both a potentially useful response down to fields of practical importance and a peculiarly long length scale associated with the underlying mechanism.

Large magnetoresistance in non-magnetic silver chalcogenides

The resistance of a homogeneous semiconductor increases quadratically with magnetic field at low fields and, except in very special cases, saturates at fields much larger than the inverse of the carrier mobility, a number typically of the order of 1 T (refs 1, 2). A surprising exception to this behaviour has recently been observed in doped silver chalcogenides3,4,5, which exhibit an anomalously large, quasi-linear magnetoresistive response that extends down to low fields and survives, even at extreme fields of 55 T and beyond. Here we present a simple model of a macroscopically disordered and strongly inhomogeneous semiconductor that exhibits a similar non-saturating magnetoresistance. In addition to providing a possible explanation for the behaviour of doped silver chalcogenides, our model suggests potential routes for the construction of magnetic field sensors with a large, controllable and linear response.

Non-saturating magnetoresistance in heavily disordered semiconductors

Methods of forming metal-doped chalcogenide layers and devices containing such doped chalcogenide layers include using a plasma to induce diffusion of metal into a chalcogenide layer concurrently with metal deposition. The plasma contains at least one noble gas of low atomic weight, such as neon or helium. The plasma has a sputter yield sufficient to sputter a metal target and a UV component of its emitted spectrum sufficient to induce diffusion of the sputtered metal into the chalcogenide layer. Using such methods, a conductive layer can be formed on the doped chalcogenide layer in situ. In integrated circuit devices, such as non-volatile chalcogenide memory devices, doping of the chalcogenide layer concurrently with metal deposition and formation of a conductive layer in situ with the doping of the chalcogenide layer reduces contamination concerns and physical damage resulting from moving the device substrate from tool to tool, thus facilitating improved device reliability.

Integrated circuit device and fabrication using metal-doped chalcogenide materials

An inverted PCRAM cell is formed by plating the bottom electrode, made of copper for example, with a conductive material, such as silver. Chalcogenide material is disposed over the plated electrode and subjected to a conversion process so that ions from the plated material diffuse into the chalcogenide material.

PCRAM memory cell and method of making same

The study of electrical properties of β‐Ag2Te and β‐Ag2Se has been extended to 4.2°K. The former compound was zone refined without decomposition. Both n‐ and p‐type samples of β‐Ag2Te were studied; all samples of β‐Ag2Se prepared were n type to 4.2°K. Neither semiconductor showed any indication of extrinsic carrier freeze‐out or of impurity banding. Study of oxygen as a possible acceptor in β‐Ag2Te showed no significant effect; a selenium‐doped sample of β‐Ag2Se was n‐type to 4.2°K.

Electrical Properties of β‐Ag2Te and β‐Ag2Se from 4.2° to 300°K

The energy gap of the modification of silver selenide stable below 133\ifmmode^\circ\else\textdegree\fi{}C has been investigated. Studies of the optical-absorption spectrum at 80\ifmmode^\circ\else\textdegree\fi{}K and at 5\ifmmode^\circ\else\textdegree\fi{}K and of the temperature variation of the Hall coefficient and electrical conductivity have been made on polycrystalline samples. Analysis of the data on the electrical properties in the intrinsic region suggests the existence of two phases, not distinguishable by x-ray diffraction, with energy gaps ${E}_{0}$ at 0\ifmmode^\circ\else\textdegree\fi{}K of 0.07\ifmmode\pm\else\textpm\fi{}0.01 eV and 0.18\ifmmode\pm\else\textpm\fi{}0.01 eV, respectively. The latter value agrees with that proposed for the ${\ensuremath{\beta}}_{2}$ phase of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ag}}_{2}\mathrm{Se}$ reported by Baer. The optical-absorption spectra suggest that the band-to-band transitions observed are direct. The extrapolated threshold of absorption is at 0.13 eV at 5\ifmmode^\circ\else\textdegree\fi{}K and correlates best, because of the probable Burstein shift of the absorption edge, with the proposed value of ${E}_{0}=0.07$ eV. It is also concluded that the energy gap increases as the temperature decreases.

Energy Gap in β − Ag 2 Se

Total ion concentration profiles were measured in laminar, premixed, low pressure, 2.7 kPa, acetylene-oxygen flames at equivalence ratios, , from 1.5 to 4.0. Peak total ion concentrations decrease with increasingup to and beyond the soot threshold (c=2.4). Near the soot threshold, a second peak appears in the ion profiles. This second peak contains some of the same ions, e.g., C13H9+, C15H11+, C18H11+, and C19H11+, as the first peak which is ascribed to chemi-ionization. The second peak reaches a minimum, at =2.9, well on the fuel rich side of the soot threshold, and then rises again with increase in equivalence ratio. These results refute the arguments against an ionic mechanism based on reported observations that the total ion concentration increases at the soot threshold. In a sooting, =3.0, flame the ion concentration profiles measured are in excellent agreement with measurements by others using molecular beam sampling techniques. The decay in the total ion concentration profile occurs simultaneously with an increase in soot concentration, consistent with the viewpoint that neutral soot particles are produced by ion recombination. The results support the chemi-ionization, ion-molecule mechanism of soot formation in flames.

Ionization and soot formation in premixed flames

Presentation et discussion de resultats d'etude experimentale realisee sur des flammes laminaires, a basse pression, de premelange acetylene-oxygene

Ion concentrations in premixed acetylene-oxygen flames near the soot threshold

: Boron is an attractive fuel because of its potential energy release with oxygen and because of its lightweight. It is, however, difficult to incorporate into a fuel system because an oxide layer forms on the surface and suppresses combustion. A new synthesis process has been evaluated for the preparation of boron fuel particles coated with titanium. The titanium coating prevents oxide from forming and aids in the combustion. Pure boron particles are produced by reaction of sodium with boron trichloride, the products expanded through a supersonic nozzle, in which titanium tetrachloride is added to produce titanium. The titanium coats the boron particle. The coated particles are separated from the gaseous sodium chloride by a new type supersonic virtual impact collector. The process has been analyzed by computer modeling and an apparatus has been designed to test the concept and prepare coated boron fuel particles for evaluation. (aw)

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Robert J Gill

Papers

Electrical Properties of β‐Ag2Te and β‐Ag2Se from 4.2° to 300°K

Energy Gap in β − Ag 2 Se

Ionization and soot formation in premixed flames

Ion concentrations in premixed acetylene-oxygen flames near the soot threshold

Production and Coating of Pure Boron Powders