| In nature,there is an abundance of various minerals in the form of sulfur-iron compounds,such as iron sulfide,ferrous sulfide,pyrrhotite,pyrite,and marcasite.With continued developments in science and technology,Fe-S compound materials have shown great prospects and values in various fields such as thermoelectricity,solar energy,catalysis,and electrodes.In addition,with the exploitation and use of these minerals,a large number of associated minerals rich in sulfur and iron are abandoned as tailings.After oxidation,these abandoned tailings will release acidic substances and heavy metal elements,which are prominent sources of pollution in acidic mine sewage.Exploiting their potential resource value can make full use of these resources,avoid environmental pollution,and develop new material development ideas.Presently,the two main factors restricting the use of waste materials from Fe-S systems are the effective separation and acquisition of these materials and the improved material properties.Therefore,for the sustainable development of energy,information,materials,and the environment,synthetic Fe-S system compound materials and an effective way to improve the properties of these natural Fe-S system compound materials must be developed urgently.The high pressure and high temperature(HPHT)method is an important technique used in material preparation and performance control,which not only helps to realize the rapid preparation of materials,but also endows materials with new electrical,optical,and magnetic properties.It is particularly important to study the effective preparation and performance improvement of Fe-S system compound materials using this technique.In this paper,the preparation and performance control of Fe-S system compound materials represented by pyrite are studied using HPHT methods,and the application prospects of these kinds of materials in thermoelectricity are discussed.The following are the main research contents:(1)A series of experiments were carried out to study the HPHT Fe-S reaction under different pressure and temperature conditions,using iron powder and sulfur powder as raw materials.High temperature accelerates the full reaction of iron and sulfur elements,where the pressure change can affect the micro-morphology of the products of the sulfur-iron reaction and the relative content of each phase.In the pressure range of 1–2GPa,as the pressure increases,the content of pyrite in the product decreases and the content of pyrrhotite increases.In the pressure range of 2–5 GPa,with increasing pressure,the contentions of both pyrite and pyrrhotite decrease.(2)Bulk pyrite samples were prepared at 1 GPa and 870 K under HPHT.During this study,we found that the thickness of the pyrite and pyrrhotite layer formed in the sample under high pressure was much greater than that under normal pressure.The sample is a lump pyrite sample with a natural pyrite metering ratio(1:2)and a P-type semiconductor at room temperature with a forbidden band width of 1.75 e V.The concentration of magnetic impurities was relatively small.The following properties were measured:remanence Br=3.4×10-3emu/g,coercivity Hc=99.7(Oe),saturation magnetization(3T):Ms=20.3×10-3emu/g,which has soft magnetism.Nano-pyrite particles were successfully prepared at 1 GPa by means of HPHT.When the reaction temperature is 870 K,the Fe3O4:S12system shows the best preparation effects.The nano-pyrite is fully dispersed in the sample with particles showing?100 nm in diameter.No impure phases of pyrite compounds,such as pyrrhotite,ferrous sulfide and white iron ore,were found in the products.(3)Regulation mechanism of metal additives on the thermoelectric properties of natural pyrite materials under HPHT.At 3 GPa,Alx-Fe S2 samples(x=0,0.05,0.2,0.4,0.6)with different aluminum additions were prepared.Under high pressure,aluminum can fully reduce pyrite into pyrrhotite with high conductivity(Fe7S8).Another product,Al2S3,is dissolved in the sample,providing more N-type carriers for the sample,effectively improving the electrical,thermal,and thermoelectric properties of pyrite.Sample Al 0.6-Fe S2has the lowest thermal conductivity(2.5 WK-1m-1@570 K),highest conductivity(1.8×104S/cm@570 K)and the largest z T value(1.36×10-3@570 K),which is much higher z T value(3.63×10-5@570 K)than natural pyrite samples without the addition of aluminum at the same temperature.With increasing aluminum content,the hardness and density of natural pyrite samples decrease and the fracture toughness increases.At 3 GPa,Bix-Fe S2(x=0.2,0.4,0.6)samples with different bismuth additions were prepared.Bismuth can also reduce natural pyrite at high temperature and high pressure,and the products obtained after reduction are a ferrous sulfide phase and bismuth sulfide phase.The Bi0.6-Fe S2sample has the largest Seebeck coefficient(–254.4?V/k@520 K),the lowest thermal conductivity(1.53 WK-1m-1@520 K)and highest z T value(1.36×10-2@520 K).Compared with bismuth-free pyrite,this z T value at the same temperature(1.18×10-5@520 K)is three orders of magnitude higher.(4)The effects of pressure conditions(1–5 GPa)on the phase composition,microstructure,and thermoelectric properties of Cu-Pyrite system samples were studied by means of HPHT.Cu-Pyrite samples were prepared from copper and natural pyrite at an atmospheric pressure of 5 GPa and 870 K.The phase composition of the sample changes with changing synthesis pressure.Among them,chalcopyrite2is the main product of the atmospheric reaction.By increasing the pressure to 1 GPa,the brown pyrite phase(Mooihoekite,Cu9Fe9S16)appears in the product,while the bornite phase(bornite,Cu5Fe S4)appears in the sample when the reaction pressure exceeds 2 GPa.The increased synthesis pressure can effectively reduce the thermal conductivity of the system.The samples synthesized at 5 GPa have the lowest thermal conductivity(0.71WK-1m-1@570 K)and highest z T value(0.038@570 K).Compared with the z T value of pure pyrite samples at the same temperature,this z T value is three orders of magnitude higher.(5)The effects of synthesis pressure(1–5 GPa)on the phase composition,microstructure,and thermoelectric properties of Cu5-S2-Pyrite samples were studied by means of HPHT.Cu5-S2-Pyrite samples were prepared from copper,sulfur,and natural pyrite in the pressure range of 1–5 GPa.The phase composition of the sample changes with changing synthesis pressures.Copper Sulfide(Cu2S)and Bornite(Cu5Fe S4)were formed in the reaction products at 1 GPa.Nukundamite((Cu,Fe)4S4)and Digenite(Cu2-XS)were formed from 3 GPa reaction products.Nukundamite((Cu,Fe)4S4)and Copper Sulfide(beta-copper Sulfide,Cu2S)with different structures were mainly formed in the products under 5 GPa.After the HPHT reaction,all samples are P-type semiconductors with positive Seebeck coefficients.With increasing synthesis pressure,the conductivity of the sample increases and the Seebeck coefficient decreases.The 1 GPa sample has the largest Seebeck coefficient(231?V/k@670 K)and the lowest thermal conductivity(0.71 W/Km@670 K).The 3-GPa sample has the highest conductivity(824 S/m@420 K),and the 5-GPa sample has the highest power factor(838.8?W/km@670 K)and z T value(0.52@670 K),which is four orders of magnitude higher than the z T value of the pure pyrite sample.HPHT technology is helpful for the rapid preparation of Fe-S system compound materials represented by pyrite.Moreover,the properties of the modified materials can be effectively improved by adjusting factors such as additive materials and synthetic pressure. |
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