Ultra-High Pressure Experimental Methods And Its Application In Inorganic Synthesis

The ultrahigh pressure technology is a special investigation tool for physics, chemistry and the synthesis of materials. Its extreme physical conditions can effectively change the atomic distance and electronic states. Therefore, it is often used to modulate the atomic space, probe the properties change with atomic space and etc. in the most frontier research topics.When ultrahigh pressure technology is applied to inorganic synthesis, some physical properties would change with the denisification, such as conductivity, optical absorption and magnetic properties and etc. The elements in inorganic compounds synthesized under ultrahigh pressure usually have large coordination numbers and unusual oxidation states. In modern inorganic synthesis, ultrahigh pressure is widely applied to the reactions that cannot be carried out under normal conditions and many new compounds, new phases and new routes of chemical reaction were obtained.Due to the complexity of the high pressure technique, the ultrahigh pressure research was mainly related to the geology and condensed matter physics for a long time. Recently, high-pressure chemistry became more attractive with the development of ultrahigh pressure technique, the synthesis, characterization, the thermodynamic and kinetic and mechanism of reaction is mainly concerned in inorganic chemistry. Among many different types of high pressure apparatus, the large volume high press (LVP) has a distinct advantage in inorganic synthesis and preparation of sample, because of its uniform pressure and large sample sizes.The State Key laboratory of Inorganic Synthesis and Preparative Chemistry in Jilin University upbuilt the first ultra high pressure chemistry laboratory of China in 2003. The investigations of inorganic synthesis under ultra high pressure experiment method are based on the Walker, Cubic and Quickpress 3.0 Piston-Cylinder high temperature and high pressure (HTHP) apparatus (three LVP apparatuses providing different pressure). The Walker module device is the first equipment that the pressure could reach more than 20GPa. For the success and reliability of experiments, the appropriate sample assembly, accurate pressure and temperature calibrations of the high pressure devices are crucial. Herein, we completed the design of sample assembly, pressure and temperature calibrations of the Walker, Cubic and Piston-Cylinder apparatus. The pressure calibration on LVP above 10GPa is the first time to complete in China. Then these high pressure devices were used to synthesise the multiferroic materials, precious metal nitrides, diamond, etc.Based on the results of many experiments, we designed several sample assemblies of the Walker, Cubic and Piston-Cylinder high pressure devices for different requirements of experiments. The standards for assemblies were chosen include the materials and the sizes, and the preparation methods of some assemblies are also designed.The 1000-tons Walker module high temperature and high pressure apparatus can generate different high pressure using tungsten carbide (WC) anvils with different truncated edge length (12mm, 8mm, 6mm or 4mm). The calibration of pressure was performed by the fixed points of phase transitions method. The resistance change of standard materials for pressure calibration with load was measured to calibrate the pressure in the assemblies at room temperature (Bi and Tl were used for calibration of TEL=12mm assemblies; Bi, Tl, ZnTe for calibration of TEL=8mm assemblies; Bi, Tl, ZnTe, Pb for calibration of TEL=6mm assemblies; Bi, Tl, ZnTe. Pb, ZnS, GaAs for calibration of TEL=4mm assemblies). The pressure calibration at high temperature for TEL = 6mm and 8mm assemblies was based on the coesite-stishovite transition (8.6GPa 1000°C) and the garnet-perovskite transition (6.1GPa 1000°C) of CaGeO3. The calibration curve of the pressure versus load was obtained. We found the sample pressure at high temperature is lower than that at room temperature for the same load. This is because the rigidity of pressure media increases at high temperature and the the hardness of the WC anvils decreases at high temperature. The calibration was theoretically analyzed and its accuracy and logicalness was confirmed include that there is a nearly linear part in the pressure calibrations using the anvils with TEL 4mm, 6mm and 8mm.The pressure calibration for Cubic type device was carried out based on the measurement of resistance transition of Bi and Tl and the melt point of NaCl under high pressure (1.2GPa, 1050℃). The sample assembly was improved to reduce the pressure gradient, contact resistance and etc. The pressure distribution for Cubic type device was also checked.The pressure calibrations for Piston-Cylinder HTHP device were based on the primary scale and modified the results by measuring of salt melting point. According to many experiments, we found that the temperature is almost repeatable using the same power output, the sample assemblies and pressure for the same sample. The error is less than 5% when the pressure changed. Therefore the relation between the temperature and the output power can be used to estimate of temperature when the thermocouple suddenly breaks or no thermocouple is used to lower the cost. The temperature gradient was measured for Piston-Cylinder equipment. The results show that the temperature is the highest in the center of the graphite heater. There is a constant temperature region around the center of the graphite heater, which shrink with the temperature increasing. The constant temperature region is about 3mm in length along the axes of the graphite heater at 1200°C. The sample should be placed in the constant temperature region during the HTHP experiments.Many inorganic synthesis and the material properties were studied using the Walker, Cubic and Piston-Cylinder type devices include:1. BiMnO₃ is one of the few multiferroic materials that has the ferromagnetism and ferroelectricity simultaneously, which only can be synthesized under HTHP. The powder sample of BiMnO₃ was obtained at 5GPa and 800°C using the Walker Module HTHP apparatus. The metal Bi can be found from the decompostion at the temperature higher than 1400°C and the pressure of 5GPa, or the temperature higher than 1000°C and pressure of 10GPa.2. Diamond is the superhard materials which can be synthesized at 5.5GPa and 1300°C with the catalyst such as Fe, Co or Ni. Many efforts have been done to lower its synthesis pressure and temperature. A new route was adopted to reduce Na₂CO₃ with metallic K at 2GPa and 500°C. The starting materials are sealed in the platinum capsule and react for 18h in Cubic type HTHP apparatus. The result of XRD and Raman spectra reveal that there is diamond in the final products. The results of depth analysis of Raman spectra show that the peaks of carbonate became weaker and the peak of diamond became stronger in the deep part of our sample, which indicated that the carbonate covered the diamond.3. B6O is the superhard materials that can be the replacement of diamond or cubic BN. Icosahedral B6O single crystals with diameters ranging from 100 nm to 1.3μm were obtained using the reaction between boron and milled boron oxide for 6 hours at 2 GPa and 1400°C. The starting materials were encapsulated in an h-BN capsule and put into the piston-cylinder device for the reaction. The well-crystallized icosahedral B6O single crystals with diameters ranging from 20 nm to 300 nm were obtained using the reaction between boron and boron acid at pressures between 1 and 5 GPa and at temperatures between 1300 and 1400 oC. These synthetic temperature and pressure is much lower in comparison to previous work (P>5GPa, T>1700 oC).4. Noble metal nitrides is the super-hard materials due to its the large bulk modulus. IrNx nano-material has been synthesized using the mixture of Ir and h-BN ball milled for 40h as starting materials. The reaction was fulfilled at 10GPa and 1600°C for 1h using the Walker type HTHP apparatus. The synthesis condition is much lower than previously reported (50GPa, 2000K). We also tried to synthesize PtN. The mixture of Pt and h-BN (the molar ratio of 1/20) were ball-milled for 40h, then put into the Walker type HTHP apparatus and reacted at 10GPa, 1600 oC for 1h. The Pt-N compound maybe obtained because the Raman spectra indicated the presence of Pt-N and N-N vibration modes.5. If porous material can be stable at extreme conditions of HTHP, it can be used in space exploration and etc. due to the low density and high strength. The stability of porous materials such asα-ZrP, Y-type zeolite and MCM-41 were studied under high pressure. Theα-ZrP still keeps stable under 13.5GPa. The Y-type zeolite presents some amorphism under high pressure, while it still maintains some certain framework structure under 9GPa. MCM-41 with template still keeps stable under 10GPa and totally collapses under 1.5GPa without template. The result indicates that the template has deeply effect on the stability of MCM-41. Piston-Cylinder HTHP device is employed to study the HTHP phase graph of MCM-41. 6. The nano-materials TiS₃ is obtained under 2GPa and 600°C and TiS2 is synthesized when the temperature increased to 1000°C using Piston-Cylinder device. The TiS₃ can change to TiS2 at 632°C and ambient pressure, which indicates the high pressure could restrain the materials decompose. The formation of nano-materials is because the pressure can suppress the long-range atomic diffusion and crystal growth rate.7. Experiment on reaction of Am-Gt-Py (Amphibole garnet pyroxenite) with spinel-lherzolite was carried out at 1500°C and 3.5GPa for 24 hours in a Cubic type HTHP apparatus. The results show that the reason of low Mg peridotite formation is not the sediment of magma but the melts-peridotite reaction. The melts-peridotite reaction is also one of the reasons of the rich Mg adakitic melt formation.