Preparation method of titanium-doped and high-hydrated magnesiochromite single crystal under high temperature and high pressure

Abstract

The application discloses a preparation method of titanium-doped and high-hydrated magnesio-chromian spinel monocrystal under high temperature and high pressure, and the method comprises the following steps: preparing a cylindrical magnesio-chromian spinel sample by taking solid basic magnesium carbonate powder, solid basic chromium acetate crystal powder, liquid tetraisopropyl titanate, solid oxalic acid powder, solid brucite powder, solid chromium hydroxide powder and liquid dilute nitric acid as starting raw materials; preparing water source sheets by taking brucite powder and chromium hydroxide powder with a weight ratio of 4:1 as water sources; placing two water source sheets at two ends of the cylindrical magnesio-chromian spinel sample and putting them into a double-capsule structure sample bin together to carry out high temperature and high pressure reaction, so as to obtain titanium-doped and high-hydrated magnesio-chromian spinel monocrystal; and the method solves the blank of the preparation technology of the titanium-doped and high-hydrated magnesio-chromian spinel large-grain monocrystal under the current high temperature and high pressure conditions, and obtains experimental samples of the titanium-doped and high-hydrated magnesio-chromian spinel large-grain monocrystal.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of synthesis of mineral single crystal samples, and particularly relates to a preparation method of titanium-doped and high-hydrated magnesio-chromite single crystals under high temperature and high pressure. BACKGROUND

[0002] As an important end member component of the chromite subfamily of the spinel family of minerals, magnesio-chromite has a chemical composition formula of MgCr2O4 and is an important oxide mineral rich in magnesium and chromium. The percentage of the chemical composition of magnesio-chromite mineralogy oxides can be expressed as: MgO / (MgO+Cr2O3) = 20.96% and Cr2O3 / (MgO+Cr2O3) = 79.04%. Due to the very high content of chromium elements in magnesio-chromite, magnesio-chromite is an important raw material for industrial production of chromium. Generally, magnesio-chromite is a typical oxide mineral with a normal spinel structure in nature, and in the corresponding unit cell, the cubic closest packing ratio of magnesio-chromite with an inverse spinel structure is 0, which shows obvious physical and chemical properties of the normal spinel structure.

[0003] Generally, the natural magnesio-chromite exposed in nature presents a grayish yellow and blackish gray streak granular aggregate, accompanied by opaque metallic luster, high hardness, large specific gravity and other characteristics, and no obvious cleavage optical mineralogical properties. The existing meteorite planetary geological data research results show that as far back as 1864, in the southwest of France, in the area of Orgeuil, 20 meteorites fell in succession within a few square kilometers, which is known as the famous Orgeuil meteorite in the history of the development of planetary science in the world. The Orgeuil meteorite is a very important carbonaceous chondrite meteorite rich in water and organic compounds, and one of the largest is as high as 14 kg. Astronomers all over the world have done a lot of research work on the Orgeuil meteorite produced in France. Recently, astronomers from the world-renowned lunar and planetary research institutions have selected three fragments of the Orgeuil meteorite in France to carry out high-resolution transmission electron microscopy, selected area transmission electron diffraction, high-resolution scanning electron microscopy backscattered electron image and mineral crystal composition chemical analysis. For the first time, it is found that the diameter of the magnesio-chromite single crystal particle is 0.5mm-2.0mm, and the magnesio-chromite single crystal is perfectly inlaid in the rare gem sodium-chromite single crystal (NaCrSi2O6) and diopside single crystal (CaMgSi2O6), and the three minerals of magnesio-chromite single crystal, sodium-chromite single crystal and diopside single crystal are arranged in the form of solid solution; Based on the results of high-resolution transmission electron microscopy and selected area transmission electron diffraction, the equiaxed crystal system magnesio-chromite single crystal diffraction ring can be clearly observed, so it can be seen that the magnesio-chromite single crystal has perfect spinel structure crystal form; Based on the results of high-resolution scanning electron microscopy backscattered electron image and mineral crystal composition chemical analysis, it is found that in addition to the magnesio-chromite single crystal, there are also chromite single crystal (FeCr2O4), magnetite single crystal (Fe3O4), chromium green (Cr2O3) and other spinel group and other oxide minerals; sodium-chromite single crystal, diopside single crystal, serpentine single crystal, olivine single crystal and other silicate minerals; and talc single crystal and other water-containing hydroxide minerals, which belong to different groups of minerals, and the magnesio-chromite single crystal mineral is associated with each other. All these discovered magnesio-chromite single crystal, chromite single crystal, sodium-chromite single crystal and chromium green, the four chromium-containing mineral phases are the most important potential carriers of the abnormally high isotropic chromium element in the Orgeuil meteorite.

[0004] In the crystal structure of magnesiochromite, the transition metal titanium element in the fourth period and IVB group can easily occupy the octahedral position, and then form the isomorphous substitution of B-site trivalent cation. Because the valence of the metal chromium element in the lattice position of magnesiochromite (its valence: +3) and the doped transition metal titanium element (its valence: +4) are completely different, this isomorphous substitution belongs to the nonequivalent isomorphous substitution. The main valence of titanium element is -1, 0, +2, +3 and +4, which is a typical light weight, small density, high mechanical strength, strong resistance to wet chlorine corrosion and high melting point refractory rare metal element. In nature, the content of titanium element is relatively rich, ranking tenth, but the titanium ore resources are scattered and difficult to extract. The titanium-rich ores mainly include rutile (TiO2) of tetragonal system and ilmenite (FeTiO3) of trigonal system, which have a very wide distribution in the crust and lithosphere. In addition, a high content of rare element titanium is found in living bodies, water bodies, soil and rocks. Existing geological data research shows that as a titanium-containing rutile group oxide mineral of tetragonal system-rutile, the rutile rock ore deposits in China are widely distributed in Dafulshan of Zaoyang in Hubei Province, Nianzigou of Dai County in Shanxi Province, Yangchong of Xinxian in Henan Province and Liujiazhuang of Laixi County in Shandong Province. Among them, Hubei Province is the province with the richest internal reserves of rutile mineral resources in China, reaching 5.3443 million tons, accounting for more than 70% of the total internal reserves (7.5086 million tons) of rutile mineral resources in China. As a titanium-containing corundum group oxide mineral of trigonal system-ilmenite, the most famous large or super large ilmenite mineral resources in the world mainly include Irmen Mountains in Russia, Krager in Norway, Iron Mountains in Wyoming, USA, Lake Elard in Quebec, Canada, and Sichuan Panzhihua Iron Mine in China. The results of optical microscope structure observation show that the natural ilmenite single crystal particles collected in the field present a granular or flaky microcrystalline structure, and are widely distributed between or in the cracks of magnetite single crystal particles.

[0005] The molecular structure of magnesium-chromian spinel does not contain water molecules or hydroxyl groups, and shows obvious nominal anhydrous mineral properties. However, previous water solubility experiments of spinel infrared spectrum under high temperature and high pressure conditions show that the amount of water dissolved by spinel can be as high as hundreds of ppm. Water is one of the most important volatile components in the main layers of the earth's interior, especially in the mantle transition zone from 410 km to 660 km (corresponding to pressure and temperature: 16.0-23.0 GPa and 1450-1800 DEG C). The existing experimental research results of physical properties and spectroscopy of minerals and rocks under high temperature and high pressure conditions, such as conductivity, Brillouin scattering elastic wave velocity, thermal diffusivity, thermoelectric conductivity, and vacuum Fourier transform infrared spectroscopy, show that the trace water in the nominal anhydrous mineral can improve the physical and spectral properties of the mineral and rock by several orders of magnitude, and has a very important influence on the mineral physics properties. Looking at the artificial synthesis of magnesium-chromian spinel in the field of laboratory materials science at home and abroad, the main methods include: microwave activation solid phase reaction method, high alkali wet method alcoholysis method, ammonia co-precipitation method, high polymer chemical gel method, high temperature solid phase sintering method, freeze drying method, etc. Due to the existing synthesis technology, most of which adopts simple solution chemical reaction or direct physical grinding of sample powder, it is more suitable for preparing nanoscale magnesium-chromian spinel crystals. Since the high temperature and high pressure experimental geoscience field usually needs micron-sized or larger particle mineral single crystal experimental samples, it is obvious that the nanoscale magnesium-chromian spinel samples obtained by the previous material synthesis cannot meet the minimum particle size requirement. So far, there is no effective synthesis method. More geoscience researchers usually use natural magnesium-chromian spinel samples to replace artificial synthesis samples to meet the needs of high temperature and high pressure experimental geoscience research, but these natural samples have the obvious disadvantage of uneven distribution of rare element titanium. Therefore, it is particularly urgent to effectively synthesize a large particle titanium-doped and high-water-content magnesium-chromian spinel single crystal to meet the needs of various high temperature and high pressure laboratory simulation of geoscience research, especially the research on the preferred orientation and anisotropy of the crystal lattice of magnesium-chromian spinel single crystal under high pressure. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a preparation method of titanium-doped and high-water-content magnesium-chromian spinel single crystal under high temperature and high pressure, to solve the current technical blank of preparation of large particle titanium-doped and high-water-content magnesium-chromian spinel single crystal under high temperature and high pressure, and to obtain experimental samples of large particle titanium-doped and high-water-content magnesium-chromian spinel single crystal.

[0007] The technical scheme of the present application is:

[0008] A method for preparing titanium-doped and high-hydrated magnesiochromite single crystals under high temperature and high pressure, the method comprising: preparing a cylindrical magnesiochromite sample using solid basic magnesium carbonate powder, solid basic chromium acetate crystalline powder, liquid tetraisopropyl titanate, solid oxalic acid powder, solid brucite powder, solid chromium hydroxide powder, and liquid dilute nitric acid as starting materials; preparing water source sheets using brucite powder and chromium hydroxide powder in a weight ratio of 4:1 as water sources; placing two water source sheets at both ends of the cylindrical magnesiochromite sample and together into a double capsule structure sample bin for high temperature and high pressure reaction to obtain titanium-doped and high-hydrated magnesiochromite single crystals.

[0009] The purity of the solid basic magnesium carbonate powder is >99.99%, the purity of the solid basic chromium acetate crystalline powder is >99.99%, the purity of the liquid tetraisopropyl titanate is >99.99%, the purity of the solid oxalic acid powder is >99.99%, the purity of the solid brucite powder is >99%, the purity of the solid chromium hydroxide powder is >99%, and the concentration of the liquid dilute nitric acid is 10%.

[0010] The method for preparing the cylindrical magnesiochromite sample comprises:

[0011] Step 1, weigh 60 milliliters of dilute nitric acid with a concentration of 10% into a notch beaker;

[0012] Step 2, weigh 5.0 grams of solid basic magnesium carbonate powder into the notch beaker, and place a magnetic stirring rotor into the notch beaker;

[0013] Step 3, cover the notch beaker with a glass surface dish, and place it on a high-temperature magnetic stirring hot plate in a fume hood, and stir at a speed of 700 revolutions per minute for 72 hours at room temperature;

[0014] Step 4, according to the stoichiometric ratio of magnesiochromite Mg(Cr, Ti)2O4, weigh 22.02 grams of solid basic chromium acetate crystalline powder and 250 microliters of liquid tetraisopropyl titanate into the notch beaker;

[0015] Step 5, cover the notch beaker with a glass surface dish;

[0016] Step 6, place the notch beaker on a high-temperature magnetic stirring hot plate in a fume hood, and stir at a speed of 800 revolutions per minute for 48 hours at room temperature;

[0017] Step 7, weigh 2 grams of solid oxalic acid powder into the notch beaker;

[0018] Step 8, place the notch beaker on a high-temperature magnetic stirring hot plate in a fume hood, cover it with a glass surface dish, and set the condition parameters of the high-temperature magnetic stirring hot plate to 80°C and a stirring speed of 1000 revolutions per minute for 36 hours;

[0019] Step 9, remove the glass surface dish of the beaker, increase the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the entire notch beaker is completely evaporated;

[0020] Step 10, take out the magnetic stirring rotor in the notch beaker, clean all the surface-bonded powder samples into the beaker, and take out all the mixed powder in the notch beaker into a graphite crucible;

[0021] Step 11, place the graphite crucible into the muffle furnace, increase the temperature to 1100°C at a rate of 300°C / hour, and keep the temperature constant for 5 hours;

[0022] Step 12, reduce the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a rate of 200°C / hour;

[0023] Step 13, grind the sample powder in a corundum mortar for 1 hour;

[0024] Step 14, cold-press the mixed sample powder into 3 pieces of Φ10.0mm×3.0mm sample discs; vertically stack the 3 cold-pressed sample mixtures together and place them at the bottom of the graphite crucible;

[0025] Step 15, hang the graphite crucible containing the 3 stacked samples in the center of the high-temperature oxygen atmosphere furnace;

[0026] Step 16, place a stainless steel container containing secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace;

[0027] Step 17, connect the topmost end of the high-temperature oxygen atmosphere furnace body with the argon inert gas cylinder, the proportionally adjustable carbon monoxide and carbon dioxide cylinders;

[0028] Step 18, open the argon inert gas valve for 30 minutes of continuous gas filling, and under the protection of argon inert gas, increase the temperature of the sample to 800°C at a rate of 400°C / hour;

[0029] Step 19, after the temperature in the furnace body reaches 800°C, switch the carbon monoxide and carbon dioxide cylinder control valves to make the volume ratio of carbon monoxide and carbon dioxide in the sample oxygen atmosphere furnace reach 4:1;

[0030] Step 20, again increase the temperature of the sample chamber in the furnace body to 1440°C at a rate of 200°C / hour, and keep the temperature constant for 15 minutes;

[0031] Step 21, after the sample is kept at a temperature of 1440°C for 15 minutes, pull out the graphite crucible containing the sample, the four-hole alumina tube, and the upper round cover of the furnace body together from the furnace body and directly immerse them in the stainless steel container for quenching into magnesiochromite glass;

[0032] Step 22, the quenched glassy magnetoplumbite sample was taken out from the graphite crucible and grinded in a corundum mortar to obtain magnetoplumbite glass powder, which was dried in a vacuum drying oven at 200℃ for 12 hours;

[0033] Step 23, the magnetoplumbite glass powder was cold-pressed into a cylindrical magnetoplumbite sample with a diameter of 4.0mm and a height of 4.0mm on a cold isostatic pressing machine using a tungsten carbide die.

[0034] The preparation method of the water source sheet is as follows:

[0035] Step 24, the brucite powder and the chromium hydroxide powder were cold-pressed into two water source sheets with a diameter of 4.0mm and a height of 0.1mm on a cold isostatic pressing machine using a tungsten carbide die, with a weight ratio of 4:1.

[0036] The method for placing the two water source sheets at both ends of the cylindrical magnetoplumbite sample and putting them together into a double-capsule structure sample chamber for high-temperature and high-pressure reaction to obtain titanium-doped and high-hydrated magnetoplumbite single crystals includes:

[0037] Step 25, the cylindrical magnetoplumbite sample and the two water source sheets were sealed in a double-capsule structure experimental sample chamber with an inner sleeve of graphite tube and an outer sleeve of gold-palladium alloy tube;

[0038] Step 26, the double-capsule structure sample chamber was placed on a laboratory Kawai-1000t typical 6-8 type multi-faceted top large cavity high-temperature and high-pressure equipment, and the pressure and temperature were set to 3.0GPa and 1100℃, respectively, for hot-pressing sintering at a pressure increasing rate of 0.5GPa / hour and a temperature increasing rate of 10℃ / minute, with a reaction time of 72 hours at constant temperature and pressure;

[0039] Step 27, after 72 hours of constant temperature and pressure at 3.0GPa and 1100℃, the temperature in the sample chamber was reduced from 1100℃ to 800℃ at a cooling rate of 3℃ / minute, and then held at 800℃ for 1 hour; then the temperature in the sample chamber was reduced from 800℃ to room temperature at a cooling rate of 5℃ / minute;

[0040] Step 28, after the temperature in the sample chamber was reduced to room temperature, the pressure in the sample chamber was reduced from 3.0GPa to atmospheric pressure at a pressure reducing rate of 0.5GPa / hour;

[0041] Step 29, the sample was taken out from the Kawai-1000t typical 6-8 type multi-faceted top large cavity high-temperature and high-pressure equipment, and the graphite tube and gold-palladium alloy tube of the double-capsule structure sample chamber were removed; the cylindrical sample was cut in half using a diamond wire cutter; and magnetoplumbite single crystals were selected under an Olympus microscope with a magnification of 20 times.

[0042] In the high-temperature and high-pressure reaction, two groups of tungsten-rhenium thermocouples are used for temperature calibration, each group of tungsten-rhenium thermocouple is composed of two kinds of tungsten-rhenium alloy with different materials, and the chemical composition is W 95％ Re 5％ And W 74％ Re 26％ Each group of tungsten-rhenium thermocouple is symmetrically arranged at the upper and lower ends of the double capsule structure sample bin.

[0043] The present application has the beneficial effects that:

[0044] The present application has the beneficial effects that:

[0045] [Mg4(OH)2(CO3)3]+8HNO3→4Mg(NO3)2+3CO2+5H2O3Mg(NO3)2+2[Cr3(OH)2(OOCCH3)7]→3MgCr2O4+6(NH3·H2O)+4CH4+

[0046] 8CO2+16CO

[0047] MgCr2O4+2C 12 H 28 O4Ti→Mg(Cr,Ti)2O4+10C2H2+4CO2+18H2

[0048] Mg(OH)2→MgO+H2O

[0049] 2Cr(OH)3→Cr2O3+3H2O

[0050] The invention, the selected initial raw material basic magnesium carbonate [chemical formula: Mg4(OH)2(CO3)3] is a white, sparse and crisp powder solid material, which has stable chemical properties, is almost insoluble in water and ethanol, easily soluble in dilute acid solution and foaming. The selection of basic magnesium carbonate powder is due to its stable performance, loose texture and the superior characteristics of easy solubility in dilute acid, so it is an excellent raw material for providing magnesium element in artificial synthesized magnesium chromium iron ore. The initial raw material basic chromium acetate [also known as: chromium (III) acetate hydroxide or chromium (III) acetate hydroxide, chemical formula: Cr3(OH)2(OOCCH3)7] is a light gray green to blue solid crystalline powder, which is stable in chemical properties at room temperature, does not decompose, non-toxic and soluble in water. Basic chromium acetate can be used as high-purity metal chromium, some glaze, colored glass and other industrial production. The selection of basic chromium acetate crystalline powder is due to its superior characteristics of easy decomposition and strong chemical reaction activity in dilute acid solution, so it is an excellent raw material for providing chromium element in artificial synthesized magnesium chromium iron ore. The initial raw material tetraisopropyl titanate [also known as: titanium tetraisopropyl titanate, titanium tetrapropyl titanate, titanium (IV) propoxide; titanium (IV) propoxy, chemical formula: C 12 H 28O4Ti], an organic compound in liquid state, easy to absorb moisture and hydrolysis in air, has high water chemical activity, and is easy to dissolve in mineral oil, isopropyl alcohol, hexane, cyclohexane and benzene, toluene and other organic solutions. Tetraisopropyl titanate is mainly used in the synthesis of ester exchange reaction and condensation reaction catalyst, and the preparation and production of metal or rubber products adhesive, adhesion promoter, Ziegler catalyst and other fields, which has extremely wide application. Because of the high water chemical activity of tetraisopropyl titanate and the dissolution in dilute nitric acid solution, tetraisopropyl titanate is an excellent raw material for providing rare trace metal elements titanium in artificial synthesis of magnesium chromium iron ore. The initial raw material of solid brucite [molecular formula: Mg(OH)2] belongs to a typical white translucent glass luster water-containing mineral, and the brucite will undergo dehydration reaction at a temperature below 700 DEG C to produce periclase (molecular formula: MgO) and release a large amount of water. The initial raw material of solid chromium hydroxide [molecular formula: Cr(OH)3] belongs to a typical gray-green chromium-containing water-containing powder, which can react with acid and strong base to produce corresponding chromium salt and water, showing a clear physical and chemical property of amphoteric hydroxide, which can be used for industrial production and raw material processing of trivalent chromium salt, chromium trioxide, chromium pigment, etc. Generally, the chromium hydroxide will undergo dehydration reaction at a temperature of 500 DEG C to produce green chromium oxide and release a large amount of water. In the high-pressure sample chamber, the brucite and the chromium hydroxide containing water in a certain ratio are placed, and the dehydration reaction will occur under high temperature and high pressure conditions to produce a large amount of water, which provides a good water source for synthesizing titanium-doped and high-water-content magnesiochromite single crystal. In the chemical reaction product involved in the present application, NH3·H2O, CH4, C2H2, CO2, CO and H2 obtained are all high-temperature volatile substances.

[0051] The present application needs to synthesize titanium-doped and high-water-content magnesiochromite single crystal, and the synthesized sample contains titanium-doped magnesiochromite single crystal matching with titanium ore resource development and comprehensive utilization, which is widely used in the experimental simulation research of mineral and rock physical and chemical properties under high temperature and high pressure conditions. Compared with the natural magnesiochromite sample exposed in nature, the present application may exist impurity ions such as zinc ions, magnesium ions and aluminum ions. In the preparation process of the titanium-doped and high-water-content magnesiochromite single crystal, the laboratory environment is pure, and the sample is in a sealed environment and does not contact with impurities. The titanium-doped and high-water-content magnesiochromite single crystal obtained is a pure substance with good chemical stability, which provides important experimental sample guarantee for the measurement of physical property parameters of titanium-doped and high-water-content magnesiochromite single crystal, especially the research of crystal axis anisotropy and lattice optimization direction of mineral physical and chemical properties of magnesiochromite single crystal under high pressure.

[0052] Compared with the artificial synthesized magnesiochromite single crystal visible to the prior art, the synthetic method of the present application has obvious advantages such as simple operation process, short reaction time, and the obtained magnesiochromite single crystal has high purity, large size, stable chemical properties and other superior physical and chemical properties. Especially important is that the titanium content (7000-8000 ppm wt%) and water content of the synthetic product of magnesiochromite are high, and the titanium content and water content can be completely controlled. The magnesiochromite single crystal particle size is large, which can completely meet the sample demand of conductivity, synchrotron X-ray diffraction, confocal Raman spectroscopy, vacuum Fourier transform infrared spectroscopy and other high-temperature and high-pressure single crystal mineral physical and spectral experimental simulation under high-temperature and high-pressure conditions on the diamond pressure chamber high-pressure equipment. The method provides important experimental sample guarantee for the measurement of physical property parameters of titanium-doped and high-water-content magnesiochromite single crystal, especially the research on the lattice preferred orientation and crystal axis anisotropy of single crystal minerals under high pressure, and breaks through the technical bottleneck of the existing synthesis of magnesiochromite single crystal. DETAILED DESCRIPTION

[0053] The specific preparation method of the present application comprises:

[0054] Solid basic magnesium carbonate powder (purity: >99.99%), solid light gray green to blue alkaline chromium acetate crystal powder (purity: >99.99%), liquid tetraisopropyl titanate (purity: >99.99%), solid oxalic acid powder (purity: >99.99%), solid brucite powder (purity: >99%), solid chromium hydroxide powder (purity: >99%) and liquid dilute nitric acid (concentration: 10%) are used as starting materials.

[0055] The high-purity solid basic magnesium carbonate powder selected as the initial material of the present application is a white, sparse and crisp material, which has stable chemical properties, is almost insoluble in water and ethanol, is easily dissolved in dilute acid solution and foams. The basic magnesium carbonate powder is selected because of its stable performance, loose texture and excellent characteristics of being easily dissolved in dilute acid, so it is an excellent raw material for providing magnesium element in the artificial synthesized magnesiochromite.

[0056] The high-purity alkaline chromium acetate selected as the initial material of the present application is a light gray green to blue solid crystalline powder, which is stable in chemical properties at room temperature, does not decompose, is non-toxic and soluble in water. Alkaline chromium acetate can be used in the industrial production of high-purity metal chromium, some glazes, colored glass, etc. The alkaline chromium acetate crystal powder is selected because of its excellent characteristics of being easily decomposed and chemically reactive in dilute acid solution, so it is an excellent raw material for providing chromium element in the artificial synthesized magnesiochromite.

[0057] The high-purity titanium tetraisopropoxide selected as the initial substance is a colorless liquid organic compound, which is easy to absorb moisture and hydrolyze in air, has high hydrochemical activity, and is easily soluble in mineral oil, isopropyl alcohol, hexane, cyclohexane, benzene, toluene and other organic solutions. The titanium tetraisopropoxide is mainly used for the catalysts of ester exchange reaction and condensation reaction in synthesis, and is widely used in the fields of adhesives, adhesion promoters, Ziegler catalysts and the like for preparing and producing metal or rubber products. Since the titanium tetraisopropoxide has high hydrochemical activity and is soluble in dilute nitric acid solution, the titanium tetraisopropoxide is an excellent raw material for providing rare trace metal element titanium in artificial synthetic magnoferrite.

[0058] The high-purity solid oxalic acid selected as the initial substance is a chelating agent of metal substances, and the purpose is that the oxalic acid powder has a great influence on the bioavailability of minerals and has a strong coordination function. When the oxalic acid is combined with divalent magnesium ions, the solubility of the divalent magnesium ions can be greatly reduced, and a complex sol of the divalent magnesium ions is formed in the dilute nitric acid solution. At the same time, when the oxalic acid is combined with the rare trace metal element titanium cations, due to the coordination function, a soluble complex of the rare trace metal element cations is formed. The solubility of the tetravalent titanium metal cations in the acid solution is significantly enhanced, so that the tetravalent titanium metal cations are fully dissolved in the dilute nitric acid solution. The high-purity solid brucite selected as the initial substance is a typical white translucent glass luster water-containing mineral. When the brucite is dehydrated at a temperature lower than 700 DEG C, periclase (molecular formula: MgO) is generated, and a large amount of water is released.

[0059] The high-purity solid chromium hydroxide selected as the initial substance is a typical gray-green chromium-containing water-containing powder substance, which can chemically react with acid and strong base, and the products are corresponding chromium salts and water, showing a clear physical and chemical property of amphoteric hydroxide, and can be used for industrial production and raw material processing preparation of trivalent chromium salt, chromium sesquioxide and chromium pigment. Generally, the chromium hydroxide is dehydrated at a temperature of 500 DEG C, and the product is green chromium oxide, and a large amount of water is released.

[0060] The dilute nitric acid (concentration: 10%) selected as the initial substance. If the concentration of the nitric acid is too low, the solubility is limited, which may cause residues of the solid basic magnesium carbonate powder, the solid basic chromium acetate crystal powder, the liquid titanium tetraisopropoxide and the oxalic acid powder. If the concentration of the nitric acid is too high, the oxidation property is enhanced, which may cause the rapid oxidation reaction or direct decomposition of the basic magnesium carbonate in the sample, and produce dense smoke, which may bring certain danger to the preparation.

[0061] Step 1, open the chemical fume hood, select a standard volume of 100 milliliters of volumetric flask, accurately weigh 60 milliliters of 10% concentrated nitric acid, place the glass transfer pipette in a 500 milliliter beaker, along the transfer pipette, carefully move the liquid dilute nitric acid to the beaker, choose a notch beaker as the reaction container mainly considering that after the beaker is covered on the glass surface dish, it is not completely sealed, and the generated gas can be easily volatilized in the fume hood.

[0062] Step 2, accurately weigh 5.0 grams of high-purity solid basic magnesium carbonate powder on a 10-microgram high-precision analytical balance, and carefully add it to the 10% concentration dilute nitric acid solution in the notch beaker, and place the magnetic stirring rotor.

[0063] Step 3, cover the notch beaker containing the dilute nitric acid solution with solid basic magnesium carbonate powder with a glass surface dish, and place it on the high-temperature magnetic stirring hot plate in the fume hood. In order to make the initial solid basic magnesium carbonate powder fully dissolved in the dilute nitric acid solution, and at the same time make it hydrolysis and acidification reaction, the reaction conditions are room temperature, 700 rpm and reaction time 72 hours.

[0064] Step 4, according to the stoichiometric ratio of magnesium-chromite Mg(Cr, Ti)2O4, accurately weigh 22.02 grams of high-purity solid basic chromium acetate crystal powder and 250 microliters of high-purity liquid tetraisopropyl titanate on a high-precision analytical balance, and carefully add them to the dilute nitric acid solution containing basic magnesium carbonate.

[0065] Step 5, cover the dilute nitric acid solution containing solid basic magnesium carbonate powder, solid basic chromium acetate crystal powder and liquid tetraisopropyl titanate with a glass surface dish to ensure that the gas generated during the reaction is volatilized from the notch of the beaker, and at the same time avoid the dilute nitric acid solution of the initial material in the beaker from splashing out during high-speed stirring, thereby causing danger and affecting the precision of the synthesis of magnesium-chromite single crystal.

[0066] Step 6, place the beaker containing the sealed initial dilute nitric acid mixture and the magnetic stirring rotor in the high-temperature magnetic stirring hot plate in the fume hood, and under the conditions of room temperature, 800 rpm and stirring time 48 hours, make the initial solid basic magnesium carbonate powder, solid basic chromium acetate crystal powder and liquid tetraisopropyl titanate fully dissolved in the dilute nitric acid solution mixture without any residue. At the same time, NH3·H2O, CH4, C2H2, CO2, CO and H2, etc. Volatile substances are more easily volatilized in the fume hood.

[0067] Step 7, accurately weigh 2 grams of high purity solid oxalic acid powder on a high precision analytical balance, add high purity oxalic acid powder as an important metal chelating agent in the mixture of solid basic magnesium carbonate powder, solid basic chromium acetate crystal powder and liquid dilute nitric acid solution of titanium tetraisopropoxide, the purpose of which is that the oxalic acid powder has a great influence on the bioavailability of minerals, has a strong coordination effect, when oxalic acid combines with divalent magnesium ions, it can greatly reduce its solubility, and then form a complex sol of divalent magnesium ions in dilute nitric acid solution; at the same time, when oxalic acid combines with rare trace metal element titanium cations, due to its coordination effect, it forms a soluble complex of rare trace metal element cations, and the solubility of titanium metal cations with a valence of +4 in acid solution will be significantly enhanced, so that it can be fully dissolved in dilute nitric acid solution.

[0068] Step 8, place the gap beaker of the mixed solution on the high temperature magnetic stirring hot plate in the fume hood again, cover it with a glass surface dish, and set the condition parameters of the high temperature magnetic stirring hot plate at 80℃, 1000 rpm and stirring time of 36 hours, so that all the initial reagents can form a uniform sol under the combined action of dilute nitric acid and oxalic acid mixed solution.

[0069] Step 9, remove the glass surface dish of the beaker, and adjust the temperature of the high temperature magnetic stirring hot plate to 110℃ until the mixed solution in the entire gap beaker is completely evaporated.

[0070] Step 10, take out the magnetic stirring rotor in the gap beaker on the high temperature magnetic stirring hot plate, and clean all the powder samples adhered to its surface into the beaker. Carefully take out all the mixed powder in the gap beaker with a medicine spoon and place it in a graphite crucible. The purpose of using a graphite crucible is that the carbon that constitutes the graphite crucible inevitably produces a certain concentration of carbon monoxide and carbon dioxide during high temperature calcination, thereby controlling the oxygen fugacity of the magnesio-chromite sample in the graphite crucible, and finally realizing the valence control of the valence metal cations chromium and titanium in the magnesio-chromite sample.

[0071] Step 11, place the graphite crucible containing the mixed powder into a muffle furnace under atmospheric pressure and high temperature conditions, and raise the temperature to 1100℃ at a relatively low heating rate of 300℃ / hour, and keep the temperature constant for 5 hours. The relatively slow high temperature calcination rate and the longer constant temperature time are to more favorably control the oxygen atmosphere in the graphite sample bin and more favorably remove the residual nitric acid, oxalic acid and other organic matter in the mixed powder.

[0072] Step 12, reduce the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a cooling rate of 200℃ / hour, which is slower than the heating rate, so as to more easily form a honeycomb-like loose sample powder, and carefully take out the mixed sample powder.

[0073] Step 13, the sample powder of loose and honeycomb-like magnesiochromite is placed in a super-hard thick corundum mortar, and is ground for 1 hour to obtain a fine-grained and homogenized powder sample.

[0074] Step 14, the homogenized and fine-grained magnesiochromite powder sample mixture is cold-pressed into 3 sample discs with a diameter of 10.0 mm and a thickness of 3.0 mm by means of a high-precision tungsten carbide die of a stainless steel tablet press. The 3 cold-pressed sample mixtures are stacked vertically and carefully placed at the bottom of a graphite crucible.

[0075] Step 15, two symmetrical circular holes with a diameter of 1.0 mm are drilled on the wall of the graphite crucible containing the 3 stacked sample discs using a high-speed electric drill. A 0.5 mm platinum-rhodium alloy wire is carefully threaded through the two 1.0 mm symmetrical circular holes in the wall of the graphite crucible, and is suspended in the center of the high-temperature oxygen atmosphere furnace. The platinum-rhodium wire connected to the two ends of the graphite crucible is fixed to a vertical 0.6 mm diameter four-hole alumina tube with an outer diameter of 5.0 mm and a length of 40 cm. The upper end of the four-hole alumina tube is fixed to the center of a circular cover that can be easily inserted and removed from the furnace body.

[0076] Step 16, a 3 liter stainless steel container containing secondary deionized pure cold water is placed on the side of the high-temperature oxygen atmosphere furnace in advance, which is used to quickly cool the sample by directly pulling it out of the high-temperature oxygen atmosphere furnace and immersing it in the 3 liter secondary deionized water in the cold water stainless steel container at a very high temperature. The main purpose is to avoid the oxidation / reduction of the variable valence elements chromium and titanium during the slow cooling process of the furnace, to achieve rapid quenching of the sample, and to completely retain the glassy magnesiochromite sample.

[0077] Step 17, at the top of the furnace body of the high-temperature oxygen atmosphere furnace, the argon inert gas cylinder, the proportionally adjustable carbon monoxide and carbon dioxide cylinders are connected to each other, the amount of gas introduced into the sample chamber is controlled by a manometer, and each gas can be switched and adjusted at any time during the high-temperature calcination process of the sample. In this invention, argon inert gas is used to provide an absolutely reducing oxygen atmosphere environment when the furnace temperature is below 800°C.

[0078] In this invention, proportionally adjustable carbon monoxide and carbon dioxide are used to control the oxygen fugacity of the sample during high-temperature calcination when the furnace temperature is above 800°C. If argon inert gas is continuously introduced when the furnace temperature is above 800°C, it will cause over-reduction in the sample chamber, which may cause the variable valence elements chromium and titanium to be reduced to metallic chromium and titanium in turn. Therefore, when the temperature is above 800°C, we use proportionally adjustable carbon monoxide and carbon dioxide mixed gas to control the oxygen fugacity of the sample in the high-temperature oxygen atmosphere furnace cavity, and the reaction principle is The partial pressure of oxygen in the sample chamber can be well adjusted, thereby controlling the valence of the variable valence metal elements chromium and titanium in the titanium-doped and high-hydrated magnesiochromite single crystal.

[0079] The maximum rated temperature of the high-temperature oxygen atmosphere furnace body is 1800°C. The circulating cooling water of the high-temperature oxygen atmosphere furnace is opened to reduce the temperature of the upper and lower furnace body, avoiding the temperature of the entire furnace body being too high, which may cause carbon monoxide and carbon dioxide leakage, thereby causing danger.

[0080] The high-sensitivity argon, carbon monoxide, and carbon dioxide concentration monitoring alarm is opened to avoid gas leakage during high-temperature calcination in the oxygen atmosphere furnace, ensuring the safety of the operator.

[0081] Step 18, open the argon inert gas valve, rotate the pointer button controlled by the gas pressure gauge, and continue to fill for 30 minutes, which aims to properly expel excess air in the sample chamber. Under the protection of argon inert gas, the sample is subjected to high-temperature calcination at a temperature increasing rate of 400°C / hour to 800°C.

[0082] Step 19, after the temperature in the furnace body reaches 800°C, quickly switch the carbon monoxide gas cylinder and carbon dioxide gas control valve, rotate the pointer button controlled by the gas pressure gauge, and make the volume ratio of carbon monoxide and carbon dioxide in the sample oxygen atmosphere furnace reach 4:1, which aims to well adjust the oxygen fugacity in the sample chamber during high-temperature calcination.

[0083] Step 20, after the mixed gas flow of carbon monoxide and carbon dioxide with a volume ratio of 4:1 controls the oxygen fugacity in the sample chamber reaches stability, which requires about 3-5 minutes, then increase the temperature of the sample chamber in the furnace body to 1440°C at a temperature increasing rate of 200°C / hour, and constant temperature calcination for 15 minutes to make it melt into glassy magnesiochromite. During the temperature increasing process of the high-temperature oxygen atmosphere furnace, the sample chamber is subjected to two completely different temperature increasing rates of 400°C / hour and 200°C / hour in different temperature interval ranges of room temperature-800°C and 800°C-1440°C, respectively. In this invention, a slower temperature increasing rate is applied as the temperature of the sample chamber in the high-temperature oxygen atmosphere furnace increases, which will be more conducive to the formation of strong ionic bonds such as Mg-O, Cr-O, and Ti-O in titanium-doped magnesiochromite; will more accurately achieve temperature control of the sample chamber in the high-temperature oxygen atmosphere furnace; and will completely avoid the temperature of the local area in the furnace body being too high due to unbalanced heat transfer of the sample chamber, thereby easily damaging the heating element of the oxygen atmosphere furnace and achieving multiple purposes.

[0084] The high-temperature calcination process of the carbon monoxide and carbon dioxide mixed gas controlling oxygen atmosphere aims to provide purer magnesio-chromite glassy substance for realizing the synthesis of large-particle titanium-doped and high-hydrated magnesio-chromite single crystal, and the high-temperature calcination under the oxygen atmosphere condition can better control the valence state of the variable valence metal elements chromium and titanium in the product.

[0085] The constant-temperature calcination is for 15 minutes, and a relatively short calcination time is adopted because the magnesio-chromite powder will be rapidly melted at a temperature higher than 1390℃. If the calcination time is too short, there may be some residual initial powder in the magnesio-chromite melted product, which seriously affects the chemical composition of the prepared magnesio-chromite sample. If the calcination time is too short, it is not conducive to the full chemical diffusion of metal cations such as magnesium ions, chromium ions and titanium ions, and it is also not conducive to the formation of stable chemical bonds of strong ion bonds Mg–O, Cr–O and Ti–O in the magnesio-chromite. If the calcination time is too short, the doped titanium element will be unevenly distributed in the magnesio-chromite, such as stratification and differentiation, thereby seriously affecting the preparation effect. If the calcination time is too short, the density of the product is reduced, and it may be difficult to form high-density magnesio-chromite glass. However, if the calcination time is higher than 15 minutes, it may lead to excessive melting, so that the magnesio-chromite sample is firmly attached to the graphite crucible wall, which is difficult to clean, and also increases the sample preparation cost.

[0086] Step 21, after the sample is constant-temperature calcined at a temperature of 1440℃ for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube and the upper round cover of the furnace body are pulled out of the furnace body together and directly immersed in a 3-liter stainless steel container containing secondary deionized pure cold water, so as to rapidly quench the magnesio-chromite glass. The purpose of rapid quenching is to well preserve the glassy magnesio-chromite sample with uniform composition at high temperature.

[0087] Step 22, the quenched glassy magnesio-chromite sample is carefully taken out of the graphite crucible and is fully ground in a corundum mortar to form fine and uniform composition sample powder. The glassy magnesio-chromite powder is placed in a vacuum drying box at a temperature of 200℃ and is dried for 12 hours.

[0088] Step 23, the magnesio-chromite glass powder is cold-pressed into a cylindrical magnesio-chromite sample with a diameter of Φ4.0mm and a height of 10.0mm on a cold isostatic pressing machine by using a high-precision Φ4.0mm(diameter)×10.0mm tungsten carbide grinding tool.

[0089] In order to obtain the high water content magnesio-chromian spinel, we use brucite powder (molecular formula: Mg(OH)2) and chromium hydroxide powder (molecular formula: Cr(OH)3) as the water source. The mixture of brucite and aluminum hydroxide is selected as the water source, mainly based on the following considerations: first, brucite and chromium hydroxide are both typical water-containing substances, and the dehydration temperature is relatively low. Brucite is a typical white translucent glass luster water-containing mineral, and the dehydration reaction of brucite occurs at a temperature below 700°C, producing periclase (molecular formula: MgO) and releasing a large amount of water. Chromium hydroxide is a typical gray-green chromium-containing water-containing solid powder substance, which can react with both acid and strong base to produce the corresponding chromium salt and water, showing the physical and chemical properties of a typical amphoteric hydroxide, which can be used for industrial production and raw material processing of trivalent chromium salt, chromium dioxide, chromium pigment, etc. Generally, chromium hydroxide undergoes a dehydration reaction at a temperature of 500°C, producing green chromium oxide and releasing a large amount of water. Therefore, this dehydration temperature condition is in the lower temperature range of the process of preparing titanium-doped magnesio-chromian spinel single crystals under high temperature and high pressure conditions, which can ensure that the titanium-doped magnesio-chromian spinel single crystals are in a water environment for a long enough time to ensure the full diffusion of the sample lattice water and the formation of lattice occupation. Secondly, brucite and chromium hydroxide are both magnesium-rich and chromium-rich substances, which can well control the magnesium activity and chromium activity in the process of preparing titanium-doped and high water content magnesio-chromian spinel single crystals in the sample chamber under high temperature and high pressure conditions. Finally, the dehydration end products of the water source substance combination of the brucite and chromium hydroxide with a weight ratio of 4:1 placed at both ends of the sample are periclase (MgO) and chromium green (Cr2O3), which will not react with the sample, ensuring the purity of the titanium-doped and high water content magnesio-chromian spinel single crystal sample. In addition, by adjusting the weight ratio of brucite and chromium hydroxide as the water source water-containing substance and the corresponding height of the water source sheet, the water content in the titanium-doped and high water content magnesio-chromian spinel single crystal sample can be adjusted.

[0090] Step 24, in the cold isostatic press, the brucite powder and the chromium hydroxide powder are cold-pressed into two pieces of water source sheet with a weight ratio of 4:1 and a high-precision Φ4.0mm (diameter) ×10.0mm tungsten carbide grinding tool, and the cold-pressed water source sheet is Φ4.0mm (diameter) ×0.1mm (height).

[0091] Step 25: Seal the cylindrical magnesia-chromium iron ore sample (size: Φ4.0mm (diameter) × 4.0mm (height)) and two water source plates (size: Φ4.0mm (diameter) × 0.1mm (height)) sequentially within a double-capsule experimental sample chamber consisting of an inner sleeve – a graphite tube (size: Φ4.4mm (outer diameter) × 4.4mm (height), wall thickness 0.2mm) and an outer sleeve – a gold-palladium alloy tube (size: Φ4.6mm (outer diameter) × 4.6mm (height), wall thickness 0.1mm). In this invention, the titanium-doped magnesia-chromium iron ore sample is placed in the exact center of the inner graphite sleeve; while the two water source plates, containing brucite and chromium hydroxide in a 4:1 weight ratio, are placed at the symmetrical ends of the inner graphite sleeve close to the sample.

[0092] This invention employs an inner sleeve of a double-capsule sample chamber, using graphite as the sealing material. The main purpose is to control the oxygen fugacity values ​​of carbon monoxide and carbon dioxide within the sample chamber, thereby ultimately constraining the valence states of variable-valence metals chromium and titanium in the magnesium chromite sample.

[0093] The outer sleeve of the double-capsule sample chamber of this invention uses a gold-palladium alloy as the sealing material. The main purposes are: firstly, the gold-palladium alloy seal isolates the sample from the exchange of substances or elements with other pressure-transmitting materials, effectively preventing sample contamination during the preparation of magnesia-chromium iron ore samples under high temperature and high pressure; secondly, the gold-palladium alloy seal effectively prevents water from escaping from the sample tube during the preparation of magnesia-chromium iron ore samples under high temperature and high pressure; finally, the double-capsule sample chamber, composed of a graphite tube and a gold-palladium alloy tube, creates a more sealed oxygen atmosphere environment, better controlling the oxygen fugacity within the sample chamber, thereby more effectively constraining the valence states of the variable-valence metals chromium and titanium in the magnesia-chromium iron ore sample.

[0094] Step 26: Magnesium chromite is one of the important magnesium- and chromium-rich oxide minerals in the lower crust and upper mantle regions of Earth and other terrestrial planets. To realistically simulate the growth environment of magnesium chromite in the lower crust of Earth and other terrestrial planets, and to invert the temperature and pressure conditions for the stable existence of magnesium chromite mineral phases, a double-capsule sample chamber composed of graphite tubes and gold-palladium alloy tubes was placed on a typical 6–8 type multi-faceted top large-cavity high-temperature and high-pressure equipment of Kawai-1000t in the laboratory. The pressure and temperature were set to 0.5 GPa / hour and 10℃ / minute, respectively. Hot pressing sintering was carried out under the conditions of 3.0 GPa and 1100℃, respectively, and the reaction time was constant temperature and pressure for 72 hours.

[0095] The preparation process of the high pressure of 3.0 GPa and the sintering temperature of 1100℃ selected by the application is designed based on the physical and chemical properties of magnesiochromite itself. The specific purposes are as follows: firstly, the preparation process of high temperature and high pressure, relatively slow pressure and temperature rising rate and long constant temperature and constant pressure reaction time can completely ensure the complete mineral phase transformation from the initial magnesiochromite glass phase powder to the magnesiochromite crystal phase, and the final product magnesiochromite mineral phase can exist stably under the temperature and pressure conditions; secondly, the preparation process of high temperature and high pressure, relatively slow pressure and temperature rising rate and long constant temperature and constant pressure reaction time can significantly increase the self-diffusion and chemical diffusion coefficient of metal cations such as magnesium ions, chromium ions and titanium ions, so as to realize the unequal valence isomorphism substitution of titanium ions for metal chromium ions in the magnesiochromite crystal, and the reaction is complete and there is no free titanium element residue, and then a perfect rare trace metal element titanium doped magnesiochromite single crystal sample is formed; thirdly, the preparation process of high temperature and high pressure, relatively slow pressure and temperature rising rate and long constant temperature and constant pressure reaction time can completely ensure the formation of stable chemical bonds such as Mg-O, Cr-O and Ti-O, so as to avoid the uneven distribution of doped titanium elements in the magnesiochromite, and then realize the uniform titanium doped magnesiochromite single crystal of isometric system; fourthly, the preparation process of high temperature and high pressure, relatively slow pressure and temperature rising rate and long constant temperature and constant pressure reaction time can make the water-containing substance combination of brucite and chromium hydroxide with a weight ratio of 4:1 to have a dehydration reaction to produce a large amount of water, and the final dehydration product is a mixed oxide of periclase and chrome green, and at the same time, the water can fully diffuse in the titanium doped magnesiochromite single crystal in the sample bin, and then the magnesium chromite sample has a high enough water content; finally, the preparation process of high temperature and high pressure, relatively slow pressure and temperature rising rate and long constant temperature and constant pressure reaction time can make the titanium element in the final preparation product magnesiochromite more uniformly distributed, and at the same time, the density, strength and particle size of the product are increased, so that the titanium doped and high water content large particle isometric system magnesiochromite single crystal sample with uniform element distribution, high mechanical strength and large density and other superior physical and chemical properties is prepared.

[0096] The temperature is accurately calibrated by two groups of tungsten-rhenium thermocouples during the high temperature and high pressure reaction. The tungsten-rhenium thermocouple has the advantages of good temperature-potential linear relationship, reliable thermal stability and low price, can realize the temperature calibration range of 0-2300℃, and is widely used in the fields of high pressure mineral physics experiment, high new metallurgical industry, high temperature electronic thermoelectric system structure engineering, space carrier, nuclear reactor and other ultra-high temperature temperature calibration. Each group of tungsten-rhenium thermocouples is composed of two kinds of tungsten-rhenium alloys with different materials, and the chemical composition is W 95％ Re 5％ and W 74％ Re 26％The tungsten-rhenium thermocouple metal wires with different materials and with a diameter of 0.1 mm are connected at one end and suspended into a cluster shape by using a bench clamp; the other end of the tungsten-rhenium thermocouple metal wires with different materials and with a diameter of 0.1 mm is connected to the positive and negative poles of a high-power welding stabilized direct-current power source respectively. The output current control knob of the high-power welding stabilized direct-current power source is adjusted so that a larger current is passed through the metal wires, the cluster-shaped tungsten-rhenium high-temperature thermocouple wire is completely immersed in the saturated sodium chloride solution, is melted, and is welded into a spherical shape, and the oxide layer on the surface of the spherical thermocouple wire is removed. Two groups of hot tungsten-rhenium thermocouples are prepared by using the same technical scheme, and each group of the tungsten-rhenium thermocouples is symmetrically arranged at the upper and lower ends of a double-capsule structure sample chamber composed of a graphite tube and a gold-palladium alloy tube. According to the application, the tungsten-rhenium double thermocouples are arranged at the upper and lower ends of the sample chamber, so that the temperature in the sample cavity can be accurately calibrated, and the temperature gradient at the upper and lower ends of the sample chamber can be accurately indicated, thereby ensuring that the magnesiochromite sample is in a stable constant temperature zone during the synthesis process.

[0097] Step 27, after the temperature and pressure are kept constant for 72 hours under the conditions of 3.0 GPa and 1100 DEG C, the temperature in the sample cavity is reduced from 1100 DEG C to 800 DEG C at a cooling rate of 3 DEG C / minute, and is kept constant for 1 hour; then the temperature in the sample cavity is reduced from 800 DEG C to room temperature at a cooling rate of 5 DEG C / minute. By using the stepwise cooling and the heating rate (10 DEG C / minute) relative to the sample preparation, the slow constant-pressure cooling rate can further improve the superior physical and chemical properties of the titanium-doped magnesiochromite single crystal sample, such as uniform titanium element distribution, high mechanical strength and large density, and can completely avoid the non-uniform stress of the sample caused by the too fast cooling rate, thereby causing cracks and damage of the magnesiochromite crystal, and the preparation process is more conducive to the crystal growth of the large-grained magnesiochromite single crystal, thereby realizing the preparation of the large-grained magnesiochromite single crystal sample with a size of 100 microns.

[0098] Step 28, after the temperature in the sample cavity is reduced to room temperature, the pressure in the sample cavity is reduced from 3.0 GPa to normal pressure at a depressurization rate of 0.5 GPa / hour. In addition, the preparation process of the titanium-doped and high-hydrated magnesiochromite single crystal sample obtained by hot-pressing sintering is pure and free of any impurities introduced from the sample itself and high-pressure sample assembly.

[0099] Step 29, after the high-temperature and high-pressure preparation reaction is completed, the sample is taken out from the Kawai-1000t typical 6-8 type multi-surface top large-cavity high-temperature and high-pressure equipment. The graphite tube and the gold-palladium alloy tube of the double-capsule structure sample chamber wrapped around the sample are carefully removed, and the cylindrical sample is cut in half from the center by using a high-precision diamond wire cutting instrument. The magnesiochromite single crystal is selected under a 20-fold high-precision Olympus microscope.

[0100] The obtained magnesio-chromite single crystal is a single phase without any other impurity phase; the electron probe (EPMA) detection result shows that the molecular formula of the obtained magnesio-chromite single crystal is MgCr2O4; the multi-functional ion mass spectrometer (ICP-MS) detection result shows that the titanium content in the obtained magnesio-chromite single crystal is 7712 ppm wt%; and the vacuum Fourier transform infrared spectroscopy (FT-IR) detection result shows that the water content of the obtained magnesio-chromite single crystal sample is 339 ppm wt, and the magnesio-chromite single crystal sample has a relatively high water content.

[0101] The obtained titanium-doped and high-water-content magnesio-chromite single crystal is of a cubic crystal system, the space group is Fd3m (no. 227), the lattice parameter is α = β = γ = 90°, and the unit cell volume is The average particle size is 164 microns, and the maximum particle size is 532 microns.

[0102] The obtained titanium-doped and high-water-content magnesio-chromite single crystal has high purity, large particle size, stable chemical properties, high mechanical strength and other superior properties, and more importantly, the titanium content is high (7712 ppm wt%), and the titanium content in the magnesio-chromite single crystal can be completely controlled. By changing the amount of the added initial liquid-state high-purity titanium tetraisopropoxide from 226.9237 microliters to 259.3414 microliters, the corresponding titanium content in the obtained titanium-doped and high-water-content magnesio-chromite single crystal sample can be finally realized from 7000 ppm wt% to 8000 ppm wt%; by changing the weight ratio of the water-containing substances, such as brucite powder and chromium hydroxide powder, and adjusting the different heights of the corresponding two water sources, the total water amount generated by the dehydration reaction of the water-containing substances in the double-capsule structure sample bin composed of a graphite tube and a gold-palladium alloy tube can be controlled, and the water content in the magnesio-chromite single crystal can be finally adjusted. The obtained titanium-doped and high-water-content magnesio-chromite single crystal sample can completely meet the needs of physical experiment simulation of minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions, and breaks through the technical bottleneck of the existing magnesio-chromite single crystal synthesis, and provides important experimental sample support for the study of the lattice preferred orientation and the anisotropy of the crystal axis of the single crystal minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions.

Claims

1. A method for producing a titanium-doped and high-hydrated magnesiochromite single crystal at high temperature and high pressure, characterized by: The method comprises: preparing a cylindrical magnesiochromite sample with solid basic magnesium carbonate powder, solid basic chromium acetate crystalline powder, liquid tetraisopropyl titanate, solid oxalic acid powder, solid magnesite powder, solid chromium hydroxide powder and liquid dilute nitric acid as starting materials; preparing water source sheets with magnesite powder and chromium hydroxide powder in a weight ratio of 4:1 as water source; placing two water source sheets at both ends of the cylindrical magnesiochromite sample and putting them into a double capsule structure sample bin for high temperature and high pressure reaction to obtain titanium-doped and high water content magnesiochromite single crystal; the preparation method of the cylindrical magnesiochromite sample comprises the following steps: step 1, weighing 60 milliliters of dilute nitric acid with a concentration of 10% into a notch beaker; Step 2, weigh 5.0 grams of solid basic magnesium carbonate powder into the notch beaker, and put the magnetic stirring rotor into the notch beaker; Step 3, cover the notch beaker with a glass surface dish, and place it on a high-temperature magnetic stirring hot plate in a fume hood, and stir at a speed of 700 revolutions / minute at room temperature for 72 hours; Step 4, according to the stoichiometric ratio of magnesiochromite Mg(Cr, Ti)2O4, weigh 22.02 grams of solid basic chromium acetate crystalline powder and 250 microliters of liquid tetraisopropyl titanate into the notch beaker; Step 5, cover the notch beaker with a glass surface dish; Step 6, place the notch beaker on a high-temperature magnetic stirring hot plate in a fume hood, and stir at a speed of 800 revolutions / minute at room temperature for 48 hours; Step 7, weigh 2 grams of solid oxalic acid powder into the notch beaker; Step 8, place the notch beaker on the high-temperature magnetic stirring hot plate in the fume hood and cover it with a glass surface dish, and set the condition parameters of the high-temperature magnetic stirring hot plate to 80 °C and 1000 revolutions / minute stirring speed for 36 hours; Step 9, remove the glass surface dish of the beaker, increase the temperature of the high-temperature magnetic stirring hot plate to 110 °C, and evaporate the mixed solution in the entire notch beaker until it is completely evaporated; Step 10, take out the magnetic stirring rotor in the notch beaker, clean all the powders adhered to the surface into the beaker, and take out all the mixed powders in the notch beaker and put them into a graphite crucible; Step 11, put the graphite crucible into a muffle furnace, increase the temperature to 1100 °C at a rate of 300 °C / hour, and keep the temperature constant for 5 hours; Step 12, reduce the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a rate of 200 °C / hour; Step 13, grind the sample powder in a corundum mortar for 1 hour; Step 14, cold-press the mixed sample powder into 3 pieces of sample round sheets with a diameter of 10.0 mm and a thickness of 3.0 mm; vertically stack the 3 pieces of cold-pressed sample mixture together and place them at the bottom of the graphite crucible; Step 15, hang the graphite crucible containing the 3 pieces of stacked sample in the middle of the high-temperature oxygen atmosphere furnace; Step 16, place a stainless steel container containing secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace; Step 17, connect the top of the high-temperature oxygen atmosphere furnace body with the argon inert gas steel cylinder, the proportionally adjustable carbon monoxide and carbon dioxide steel cylinders. Step 18, open the argon inert gas valve for 30 minutes, under the protection of argon inert gas, the sample is calcined to 800 °C at a temperature rising rate of 400 °C / hour; Step 19, after the temperature in the furnace reaches 800 °C, switch the control valves of the carbon monoxide cylinder and the carbon dioxide cylinder, so that the volume ratio of carbon monoxide and carbon dioxide in the sample oxygen atmosphere furnace reaches 4:1; Step 20, then increase the temperature of the sample chamber in the furnace to 1440 °C at a temperature rising rate of 200 °C / hour, and constant temperature sintering for 15 minutes; Step 21, after the sample is constant temperature sintered at 1440 °C for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube and the upper round cover of the furnace are pulled out of the furnace and directly immersed in a stainless steel container for quenching into magnesiochromite glass; Step 22, the quenched glassy magnesiochromite sample is taken out of the graphite crucible, ground in a corundum mortar to obtain magnesiochromite glass powder, and placed in a vacuum drying box at 200 °C for drying for 12 hours; Step 23, the magnesiochromite glass powder is cold pressed into a cylindrical magnesiochromite sample with a diameter of Φ 4.0 mm and a height of 4.0 mm on a cold isostatic pressing machine using a tungsten carbide mold.

2. A method of producing a titanium-doped and high-hydrated magnesiochromite single crystal at high temperature and high pressure according to claim 1, characterized by: The purity of the solid basic magnesium carbonate powder is >99.99%, the purity of the solid basic chromium acetate crystal powder is >99.99%, the purity of the liquid tetraisopropyl titanate is >99.99%, the purity of the solid oxalic acid powder is >99.99%, the purity of the solid brucite powder is >99%, the purity of the solid chromium hydroxide powder is >99%, and the concentration of the liquid dilute nitric acid is 10%.

3. A method of producing a titanium-doped and high-hydrated magnesiochromite single crystal at high temperature and high pressure according to claim 1, characterized by: The preparation method of the water source sheet is: Step 24, the brucite powder and the chromium hydroxide powder are cold pressed into two pieces of water source sheet with a diameter of Φ 4.0 mm and a height of 0.1 mm on a cold isostatic pressing machine using a tungsten carbide mold according to a weight ratio of 4:

1.

4. The method of claim 1, wherein the method is characterized by: The method for obtaining titanium-doped and high-hydrated magnesiochromite single crystal by placing the two pieces of water source sheet on both ends of the cylindrical magnesiochromite sample and together into a double capsule structure sample chamber for high temperature and high pressure reaction comprises: Step 25, sealing the cylindrical magnesiochromite sample and the two pieces of water source sheet in a double capsule structure experimental sample chamber with an inner sleeve of graphite tube and an outer sleeve of gold-palladium alloy tube; Step 26, placing the double capsule structure sample chamber on a laboratory Kawai-1000t typical 6-8 type multi-face top large cavity high temperature and high pressure equipment, setting the pressure and temperature rising rates to be 0.5 GPa / hour and 10 °C / minute respectively, and increasing the pressure and temperature to 3.0 GPa and 1100 °C respectively for hot pressing sintering, and the reaction time is 72 hours of constant temperature and constant pressure; Step 27, after 72 hours of constant temperature and constant pressure at 3.0 GPa and 1100 °C, reducing the temperature in the sample chamber from 1100 °C to 800 °C at a cooling rate of 3 °C / minute, and keeping the temperature constant for 1 hour; then reducing the temperature in the sample chamber from 800 °C to room temperature at a cooling rate of 5 °C / minute. Step 28, after the temperature in the sample chamber is reduced to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to normal pressure at a pressure reduction rate of 0.5 GPa / hour; Step 29, the sample is taken out of the Kawai-1000t typical 6-8 type multi-face top large chamber high temperature and high pressure equipment, and the graphite tube and gold-palladium alloy tube wrapped around the sample are removed; the cylindrical sample is cut in half from the middle using a diamond wire cutter; and the magnesiochromite single crystal is selected under an Olympus microscope with 20 times magnification.

5. The method of claim 1, wherein the method is characterized by: Two groups of tungsten-rhenium thermocouples were used to calibrate the temperature during the high-temperature and high-pressure reaction. Each group of tungsten-rhenium thermocouples was composed of two tungsten-rhenium alloys with different materials, and the chemical composition was W 95% Re 5% and W 74% Re 26% Each group of tungsten-rhenium thermocouples was symmetrically placed at the upper and lower ends of the double capsule structure sample bin.