Materials best-suited for direct application exhibit a high thermoelectric figure of merit (ZT) close to unity, from room temperature to ∼ 400 K. In this work, we investigated the thermoelectric behavior of Ag2Se, and a sulfur substituted Ag2Se system, i.e. Ag2S0.2Se0.8 and Ag2S0.4Se0.6 , in the temperature range of 300 K to 500 K. With strong anharmonic lattice vibration and semiconducting electronic structure, these materials showed thermal conductivity less than 1 W m−1 K−1, electrical resistivity ∼ 1 mΩ cm, together with moderate Seebeck coefficient values of ∼ − 140 μV K−1 in the low-temperature phase. We were able to achieve ZT equal to unity in a wide temperature range of 350–400 K, with a maximum value of ZT = 1.08 at 350 K for the Ag2S0.4Se0.6 material.

Investigation of Thermoelectric Properties of Ag 2 S x Se 1-x (x = 0.0, 0.2 and 0.4)

Thermal diodes, in which the magnitude of the heat flow |J| changes with its direction, are a key technology for constructing heat management systems. Composite thermal diodes are composed of two materials possessing opposite temperature dependence of thermal conductivity. Silver chalcogenides Ag2(S,Se,Te) were employed in this work because of their drastic change in thermal conductivity, with a phase transition in the temperature range of 300 K < T < 450 K. A jump of 200% to 500% in their thermal conductivity is observed at the phase-transition temperature, most likely due to the change in electron thermal conductivity. A thermal diode consisting of Ag2S0.6Se0.4 and Ag2S0.1Te0.9, both of which were selected by considering the measured thermal conductivity, has been prepared, and its performance evaluated. The developed thermal diode was confirmed to possess a thermal rectification ratio (TRR) = |Jlarge|/|Jsmall| = 2.7 ± 0.1 when installed between two heat reservoirs kept at TH = 413 K and TL = 300 K. Notably, this TRR is the highest ever reported for a solid-state thermal diode.

High-Performance Solid-State Thermal Diode Consisting of Ag 2 (S,Se,Te)

—Gold and silver are capable of forming stable natural compounds with different elements. For gold 36 minerals are known: 10 in the class “Native metals, intermetallic compounds” and 26 in the class “Sulfides (selenides, tellurides, arsenides, antimonides, bismuthides).” For silver, 194 minerals are registered in the MMA, presented in 8 classes. Compared to gold in the class “Native metals, intermetallic compounds” there are far fewer silver minerals. They are widely represented in the classes “Sulfides” and “Sulfosalts” (124). Among silver minerals “halides” are also known (fluorides, chlorides, bromides, iodides), seven; simple and complex oxides, two; there is one mineral each in the classes “sulfates,” “phosphates,” “arsenates,” and “vanadates.” Both metals are in the compositions of 12 minerals: native gold, native silver, weishanite, uytenbogaardtite, petrovskaite, fishesserite, penzhinite, petzite, mutmannite, sylvanite, krennerite, and bezsmertnovite. Every year, the MMA commission registers four to six gold and silver minerals, so there are prospects for the discovery of new minerals of these precious metals in the near future.
The indicator elements (fixers) that form natural compounds with gold and indicate the possible presence of gold minerals in sulfide ores include 13 elements: 7 metals (Ag, Cu, Pd, Hg, Sn, Tl, Fe), 3 chalcogens (Te, S, Se), and 3 metalloids (As, Sb, Bi). For silver, this range is wider and also includes such elements as Mn, Zn, Ge, Cd, V, O, H, F, Cl, Br, I, In, and N.
This review presents and summarizes the results of studies on the composition of native gold with copper, mercury, and palladium impurities in different types of gold deposits. The article describes the gold and silver minerals established by the author together with colleagues in a study of productive mineral assemblages in some gold deposits in Russia: the Ulakhan, Yunoe, Krutoe, Dzhulietta, Dorozhnoe (Magadan region), Konechnoe (Taimyr Peninsula), Kupol, Valunistoe (Chukotka), Maletoyvayam (Kamchatka), etc.
Data are presented on gold and silver minerals found in sulfide ores and of economic importance. Some of their technological properties are characterized, including solubility and behavior under the influence of various reagents. The presence of refractory minerals (calaverite, sylvanite, krennerite, petzite, aurostibite, maldonite, fishesserite, uytenbogaardtite, petrovskaite, penzhinite, weishanite, yuanjianite, hunchunite, anyuite, krynovite, nadjagite, and bogdanovite, bilibinskite, criddleite, etc.), as well as the chemical composition gold and silver, are important technological factors and should be taken into account when developing a technology for extracting precious metals from ores. The presence of increased concentrations of indicator elements (fixers) of gold and silver minerals in sulfide ores is an important indicator and argument for detailed mineralogical and geochemical studies of the source raw material. Identification of mineral (micro-, nano-) forms of gold and silver in sulfide ores is necessary for developing and improving rational dressing schemes.

Gold and Silver Minerals in Sulfide Ore

Gold and silver minerals in low-sulfidaton ores of the Julietta deposit (northeastern Russia)

XPS and Ag L 3 -edge XANES characterization of silver and silver-gold sulfoselenides

Thermodynamic properties of solid solutions in the system Ag2S–Ag2Se

Physicochemical models of formation of gold–silver mineralization at the Rogovik deposit (Northeastern Russia)

The chemical compositions of acanthite, naumannite, and associated ore minerals have been studied from the samples of polychronous Au–Ag ores at the Rogovik deposit. The following admixtures have been detected: S in naumannite (0–2.9 wt %), Se in acanthite (0–7.45 wt %), argyrodite (~4.8 wt %), and galena (~3.1 wt %), and Fe in sphalerite (~1.2 wt %). The physicochemical parameters of ore formation have been reconstructed on the basis of mineralogical and geochemical data and thermodynamic calculations. Eh–pH (25°C, 1 bar), logfO2–pH, logfS2–T, logfSe2–T, and logfS2–logfSe2 (100–300°C, 1–300 bars) diagrams for the Ag–S–Se–H2O system with the stability fields of Ag sulfoselenides Ag2S1–x Se x of various composition (step x = 0.25, where 0 ≤ x ≤ 1) have been calculated for the first time. It has been established that Ag sulfoselenides of the naumannite series from polychronous ores of the Rogovik deposit precipitated below 70–133°C under reductive conditions (logfO2 =–65…–50) from near-neutral solutions containing elevated Se and relatively lowered S. It has been established that Ag sulfoselenides of acanthite series were formed later then naumannite but in the same range of logfO2 values at temperatures below 110–177°C from solutions with high S concentration and relatively lowered concentration of Se. The complex composition of the studied Au–Ag ores, whose characteristic feature is extremely variable mineralogy, is confirmed.

T.V. Zhuravkova

Papers

Thermodynamic properties of solid solutions in the system Ag2S–Ag2Se

Physicochemical models of formation of gold–silver mineralization at the Rogovik deposit (Northeastern Russia)

Physicochemical formation conditions of silver sulfoselenides at the Rogovik deposit, Northeastern Russia