Study on the structure and properties of tantalum metal
Tantalum is an indispensable strategic raw material for the development of electronics industry and space technology. With its unique structure and properties, tantalum has great applications in orthopedics, electronics, chemical and metallurgical industries. This article mainly introduces the resources of tantalum metal, the preparation of tantalum metal and the structure and properties of tantalum metal and its applications.
What is tantalum?
Table of Contents
Tantalum was discovered by the Swedish chemist Ekberg in 1802 and named tantalum after the Greek mythological figure Tantalus. In 1903, German chemist W. von Bolton prepared the plastic metal tantalum for the first time as a filament material. In 1940, large-capacity tantalum capacitors appeared and were widely used in military communications. During the Second World War, the demand for tantalum increased dramatically. After the 1950s, due to the continuous expansion of tantalum applications in capacitors, high-temperature alloys, chemical and atomic energy industries, the demand has increased year by year, which promoted the research and production of tantalum extraction technology. China established a tantalum metallurgical industry in the early 1960s. The United States is the country with the largest tantalum consumption. In 1997, the consumption reached 500 tons, of which 60% was used to produce tantalum capacitors. Japan is the second largest country in tantalum consumption, with a consumption of 334 tons. At the beginning of the 21st century, with the rapid development of capacitor production, the market was in short supply. It is estimated that the world’s production of tantalum capacitors will reach 250 million pieces, and 1,000 tons of tantalum will be consumed. According to statistics from the US Geological Survey, the world’s proven reserves of tantalum are 14,000 tons. Among them, 4,500 tons in Australia, 3,200 tons in Nigeria, 1,800 tons in the Democratic Republic of Congo, 1,800 tons in Canada, and 1,400 tons in Brazil. China’s resources are mainly distributed in Jiangxi, Fujian, Xinjiang, Guangxi, Hunan and other provinces. From the perspective of future development needs, capacitors are still the main application areas of tantalum. If calculated on the basis of reserves of 24,000 tons, it can only guarantee 24 years of demand. Nevertheless, the prospects for tantalum resources are still promising. First of all, in the world’s very rich niobium deposits, a large amount of tantalum resources are associated. Among them, the tantalum resources of the Gadal niobium and tantalum mines in southern Greenland amount to 1 million tons. Secondly, the West has begun to use a large amount of tin slag containing less than 53% Ta2O. In addition, the research and utilization of substitutes have also made rapid development. For example, aluminum and ceramics can replace tantalum in the field of capacitors; silicon, germanium, and cesium can be used in electronic instruments to replace tantalum to make rectifiers.
Li Qingkui’s team in the Advanced Target Materials Laboratory of Zhengzhou University mainly made high-purity tantalum metal targets. The metal targets made by magnetron sputtering and other processes formed thin films are widely used in the electronic information industry. In order to discuss metals in a deeper level, I explored the resources, preparation, structure, performance and application of tantalum metal.
The physicochemical properties of tantalum and niobium are similar, so they coexist in minerals in nature. The classification of tantalum or niobium ore is mainly based on the content of tantalum and niobium in the mineral. The occurrence form and chemical composition of tantalum-niobium minerals are complex. In addition to tantalum and niobium, they often contain rare earth metals, titanium, zirconium, tungsten, uranium, thorium and tin. The main minerals of tantalum are: Tantalite [(Fe,Mn)(Ta,Nb)2O6], heavy tantalite, fine spar and black rare gold ore. Tantalum is contained in tin smelting waste, which is also an important resource of tantalum. The world’s tantalum reserves (in terms of tantalum) have been ascertained at approximately 134,000 short tons, with Zaire taking the first place. In 1979, the world production of tantalum minerals (in terms of tantalum) was 788 short tons (1 short ton = 907.2 kg). China has made great achievements in the process of extracting tantalum from minerals with relatively low tantalum content.
Preparation of tantalum metal
Smelting method: Tantalum-niobium ore is often accompanied by a variety of metals. The main steps of tantalum smelting are to decompose the concentrate, purify and separate tantalum and niobium to prepare pure compounds of tantalum and niobium, and finally to prepare the metal. The decomposition of ore can adopt hydrofluoric acid decomposition method, sodium hydroxide melting method and chlorination method. The separation of tantalum and niobium can be carried out by solvent extraction (commonly used extractants are methyl isobutyl copper (MIBK), tributyl phosphate (TBP), sec-octanol and acetamide, etc.), fractional crystallization and ion exchange. Separation: First, decompose tantalum and niobium with hydrofluoric acid and sulfuric acid for the concentrate of tantalum-niobium iron ore, and dissolve tantalum fluoride and niobium fluoride in the leaching solution. At the same time, the associated elements such as iron, manganese, titanium, tungsten, silicon are also dissolved In the leaching solution, a strong acidic solution with a complex composition is formed. The tantalum-niobium leachate is extracted with methyl isobutyl ketone and the tantalum-niobium is simultaneously extracted into the organic phase, and the trace impurities in the organic phase are washed with sulfuric acid solution to obtain a pure organic phase washing solution containing tantalum-niobium and the raffinate combined, which contains The trace amounts of tantalum, niobium and impurity elements are strong acid solutions and can be recovered comprehensively. The pure organic phase containing tantalum and niobium is stripped with dilute sulfuric acid solution to obtain the organic phase containing tantalum. Niobium and a small amount of tantalum enter the aqueous phase, and then the tantalum is extracted with methyl isobutyl ketone to obtain a pure niobium-containing solution. The pure tantalum-containing organic phase is back-extracted with water to obtain a pure tantalum-containing solution. The organic phase after stripping tantalum is returned to the extraction cycle. Pure fluorotantalic acid solution or pure fluoroniobate solution reacts with potassium fluoride or potassium chloride to produce potassium fluorotantalate (K2TaF7) and potassium fluoroniobate (K2NbF7) crystals, and can also react with ammonium hydroxide to generate hydrogen Tantalum oxide or niobium hydroxide precipitates. The hydroxide of tantalum or niobium is calcined at 900-1000°C to generate oxide of tantalum or niobium.
Preparation of tantalum:
① Metal tantalum powder can be prepared by metal thermal reduction (sodium thermal reduction) method.
Reduce potassium fluorotantalate with metallic sodium under an inert atmosphere: K2TaF7+5Na─→Ta+5NaF+2KF. The reaction was carried out in a stainless steel tank. When the temperature was heated to 900°C, the reduction reaction was completed quickly. The tantalum powder prepared by this method has irregular grain shape and fine grain size, which is suitable for making tantalum capacitors. Metal tantalum powder can also be prepared by molten salt electrolysis: use the molten salt of potassium fluorotantalate, potassium fluoride and potassium chloride as the electrolyte, dissolve tantalum pentoxide (Ta2O5) in it, and electrolyze it at 750℃ to obtain Tantalum powder with a purity of 99.8 to 99.9%.
② Ta2O5 can be reduced by carbothermal to get metallic tantalum. The reduction is generally carried out in two steps: first, a mixture of a certain ratio of Ta2O5 and carbon is made into tantalum carbide (TaC) in a hydrogen atmosphere at 1800～2000℃, and then TaC and Ta2O5 are made into a mixture vacuum according to a certain proportion. Reduced to metal tantalum. Tantalum metal can also be obtained by thermal decomposition or hydrogen reduction of tantalum chloride. The dense metal tantalum can be prepared by vacuum arc, electron beam, plasma beam melting or powder metallurgy. High-purity tantalum single crystal is prepared by crucible-free electron beam zone melting method.
The structure and properties of tantalum metal
Crystal structure: the unit cell is a body-centered cubic unit cell, and each unit cell contains 2 metal atoms.
Cell parameters: a=330.13pm, b=330.13pm, c=330.13pm, α=90°, β=90°, γ=90°, Mohs hardness: 6.5, melting point: 2996°C. The texture of tantalum is very hard, the hardness can reach 6-6.5. Its melting point is as high as 2996℃, second only to carbon, tungsten, rhenium and osmium, ranking fifth. Tantalum is malleable and can be drawn into thin filaments. Its coefficient of thermal expansion is very small, only 6.6 parts per million per degree Celsius rises. In addition, its toughness is very strong, even better than copper.
Physical properties: The texture is very hard and malleable.
Chemical properties: Tantalum also has excellent chemical properties and high corrosion resistance. It does not react to hydrochloric acid, concentrated nitric acid and “aqua regia” regardless of whether it is under cold or hot conditions. But tantalum can be corroded in hot concentrated sulfuric acid. Below 150°C, tantalum will not be corroded by concentrated sulfuric acid, and only reacts at a temperature higher than this temperature. In 175°C concentrated sulfuric acid for 1 year, the thickness will be corroded. When tantalum is soaked in sulfuric acid at 200°C for one year, the surface layer is only 0.006 mm damaged. At 250°C, the corrosion rate increased, and the thickness of the corrosion was SDS mm per year. At 300°C, the corrosion rate was even faster. After immersion for 1 year, the surface was corroded by 1.368 mm. The corrosion rate in fuming sulfuric acid (containing 15% SO3) is more serious than that in concentrated sulfuric acid. After soaking in this solution at 130 degrees for 1 year, the thickness of the surface corroded is 15.6 mm. Tantalum is also corroded by phosphoric acid at high temperatures, but the reaction generally occurs above 150 degrees. When immersed in 85% phosphoric acid at 250 degrees for 1 year, the surface is corroded by 20 mm. In addition, tantalum is corroded by It can be quickly dissolved in the mixed acid of nitric acid and can also be dissolved in hydrofluoric acid. But tantalum is more afraid of strong alkalis. In a caustic soda solution with a concentration of 110 degrees and 40%, tantalum will be quickly dissolved. In a potassium hydroxide solution of the same concentration, it will be quickly dissolved at 100 degrees. Except for the above, general inorganic salts generally cannot corrode tantalum below 150 degrees. Experiments have proved that tantalum has no effect on alkali solutions, chlorine, bromine water, dilute sulfuric acid and many other agents at room temperature, and only reacts under the action of hydrofluoric acid and hot concentrated sulfuric acid. Such a situation is rarer than F in metals.
Tantalum forms a stable anodic oxide film in acid electrolyte. Electrolytic capacitors made of tantalum have the advantages of large capacity, small size and good reliability. Capacitors are the most important use of tantalum. The amount of tantalum in the late 1970s Always use more than 2/3. Tantalum is also a material for making electron emission tubes and high-power electron tube parts. Anti-corrosion equipment made of tantalum is used in the production of strong acid, bromine, ammonia and other chemical industries. Tantalum can be used as a structural material for the combustion chamber of an aircraft engine. Tantalum-tungsten, tantalum-tungsten-hafnium, and tantalum-hafnium alloys are used as heat-resistant and high-strength materials for rockets, missiles and jet engines, as well as parts for control and regulation equipment. Tantalum is easy to process and shape, and can be used as supporting accessories, heat shields, heaters and heat sinks in high-temperature vacuum furnaces. Tantalum can be used as a material for orthopedics and surgery. Tantalum carbide is used to make cemented carbide at 250°C. Tantalum borides, silicides and nitrides and their alloys are used as heat-releasing elements and liquid metal cladding materials in the atomic energy industry. Tantalum oxide is used to make advanced optical glass and catalysts. In 1981, the consumption ratio of tantalum in various sectors in the United States was approximately: 73% for electronic components, 19% for machinery industry, 6% for transportation, and 2% for others.
Nature and use
The linear expansion coefficient of tantalum is 6.5×10-6K-1 between 0～100℃, the superconducting transition critical temperature is 4.38K, and the atomic thermal neutron absorption cross section is 21.3 barn.
Tantalum is one of the most chemically stable metals at temperatures below 150°C. Only fluorine, hydrofluoric acid, fluoride ion-containing acidic solutions and sulfur trioxide can react with tantalum. React with concentrated alkali solution at room temperature and dissolve in molten alkali. The dense tantalum starts to oxidize slightly at 200°C and oxidizes significantly at 280°C. Tantalum has many kinds of oxides, the most stable is tantalum pentoxide (Ta2O5).
Tantalum and hydrogen above generate brittle solid solutions and metal hydrides such as: Ta2H, TaH, TaH2, TaH3. Under a vacuum of 800 to 1200°C, hydrogen precipitates from tantalum and recovers its plasticity. Tantalum and nitrogen begin to react at around 300°C to form solid solutions and nitrogen compounds; at higher than 2000°C and under high vacuum, the absorbed nitrogen will precipitate out of tantalum. Tantalum and carbon exist in three phases above 2800℃: carbon-tantalum solid solution, low-valent carbide and high-valent carbide. Tantalum can react with fluorine at room temperature, and can react with other halogens at temperatures above 250°C to form halides.
Author: Liang Peng
Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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