Development history of pure metal preparation technology

Review of the development history of metal materials

Stone Age (5000 years ago) → Bronze Age (1200 years ago) → Iron Age

Sanxingdui Museum is located in the northeast corner of Sanxingdui Site, a national key cultural relics protection unit, on the Duck River in the west of Guanghan City, Sichuan Province, 38 kilometers south of Chengdu and 26 kilometers north of Deyang. It is a large modern theme site museum in China. /kloc-0 laid the foundation stone in August, 1992, and 1997 10 officially opened.

Excavation process

1. Initial period (1929-1934)

1929 A jade pit was found in Yan Jiayuan, Zhenwu Village, Sanxingdui Site, and 300 or 400 jade articles were unearthed.

193 1 year, the British priest Dong once ran around, making most of the jade unearthed in 1929 belong to the Museum of West China University.

1932 Ge, director of the Museum of West China University, put forward the idea of archaeological excavation in Guanghan, which was approved by the Education Department of Sichuan Provincial Government.

Ge and both arrived in Guanghan.

In March, Ge Helin cleaned the jade pit near Zhenwu Village and tried to dig trenches on the east and west sides.

2. Preliminary investigation and excavation (195 1 year-1963)

195 1 year, Wang Jiayou and Jiang Dianchao of Sichuan Provincial Museum investigated Sanxingdui and Moon Bay, and found large ancient sites for the first time.

1958, the Archaeological Teaching and Research Group of the History Department of Sichuan University investigated Sanxingdui site again.

1963 sanxingdui site was jointly excavated by Sichuan provincial museum and history department of Sichuan university. The meeting was presided over by Feng, a famous archaeologist, curator of Sichuan Provincial Museum and professor of history department of Sichuan University.

3. Two Pits Excavation and Ancient City Reproduction (1980 -2005)

1980 ~ 198 1 year, Sanxingdui site was jointly excavated by Sichuan Cultural Relics Management Committee and Guanghan County for the first time, revealing a large area of housing base.

1982165438+10 ~ 83 65438+ 10, the Sanxingdui site was excavated for the second time, and the pottery kiln was discovered at Sanxingdui site for the first time.

1984 The Sanxingdui site was excavated for the third time from March to February, and the cultural accumulation from Longshan period to early Western Zhou Dynasty was excavated in Xiquankan, and the upper and lower limits of Sanxingdui site were determined.

1984 65438+February ~1985 65438+1October, Sanxingdui site was excavated for the fourth time, and it was found that Sanxingdui levee was artificially rammed, which put forward the viewpoint that Sanxingdui site was the capital of Shu for the first time.

1From March to May, 1986, the Sichuan Provincial Cultural Relics Management Committee, the Sichuan Provincial Institute of Cultural Relics and Archaeology, the History Department of Sichuan University and Guanghan County jointly excavated Sanxingdui Site for the fifth time, with an excavation area of1200m2, and found a large number of ash pits and house remains, which pushed the age limit of Sanxingdui Site to 5,000 years ago.

1July, 986 18 The local brick factory found a sacrificial pit and dug up jade and stone tools when borrowing soil from the secondary excavation area. The sixth excavation of sanxingdui site.

1July, 986 18 The Sichuan Provincial Cultural Relics Management Committee, the Sichuan Provincial Institute of Cultural Relics and Archaeology and Guanghan County jointly excavated the sacrificial pit, and the number was No.1 sacrificial pit. A total of 420 bronze, gold, jade, amber, stone tools, pottery and other artifacts were unearthed, including ivory 13.

On August 14, No.2 sacrificial pit was found about 30 meters southeast of No.1 sacrificial pit.

On August 20th, the No.2 Sacrificial Pit was excavated and cleaned up. A total of 302 precious cultural relics such as copper, gold, jade and stone/kloc-0 (including fragments and identifiable individuals), 67 ivory and about 4,600 seashells were unearthed.

1988 10 sanxingdui site was excavated for the seventh time, and the soil ridge of sanxingdui was tried to be excavated, and it was determined that the soil ridge was the south wall of the inner city wall. ~ 1989 1 month

1990 65438+ 10 From October to May, the Sanxingdui site was jointly excavated for the eighth time, and adobe was found in the East City Wall, and the structure, tamping method and age of Sanxingdui ancient city wall were known for the first time.

In March, a demonstration meeting on the restoration scheme of copper trees unearthed in the sacrificial pit of Sanxingdui site was held to pre-assemble the copper trees.

199 1 year 65438+February 9th, Sichuan Provincial Cultural Relics Management Committee and Sichuan Provincial Institute of Cultural Relics and Archaeology jointly issued.

~1May, 1992 Sanxingdui site was excavated, and the test excavation of Xicheng wall was confirmed.

1994165438+10 In October, Sichuan Provincial Cultural Relics Management Committee and Sichuan Provincial Institute of Cultural Relics and Archaeology excavated Sanxingdui site for the tenth time. Through investigation, the south wall of Sanxingdui site was found and a trial excavation was carried out.

1996 10 Sino-Japanese cooperation in environmental archaeology of Sanxingdui site, with magnetic field as the main project.

~ 165438+ 10 monthly radar detection, infrared remote sensing detection and photography, satellite image analysis, micro-landform investigation, carbon dating, pollen analysis, siliceous body analysis, diatom analysis, etc.

1997165438+10 In October, the Sichuan Provincial Cultural Relics Management Committee and the Sichuan Provincial Institute of Cultural Relics and Archaeology excavated the Sanxingdui site for the eleventh time, and excavated the cemetery of Rensheng Brick Factory at Sanxingdui site. A total of 28 tombs were found and a large number of jade tools were found. Among them, the discovery of "Jade Cone" with Liangzhu cultural style has caused researchers to rethink the cultural origin of Sanxingdui jade.

1999 65438+ 10 ~ Sichuan Provincial Cultural Relics Management Committee and Sichuan Provincial Institute of Cultural Relics and Archaeology excavated the Moon Bay Wall of Sanxingdui Site for the second time 12. A large number of cultural accumulations from Longshan to the early Shang Dynasty were found under the wall, and the wall was overlapped by the accumulation in the Yin Ruins, so the age of the Moon Bay Wall can be determined as the early Yin Ruins.

From June 5438+February, 2000 to July 5438+0, 2006, Sanxingdui site was excavated for the 13th time by Sichuan Provincial Cultural Relics Management Committee and Sichuan Provincial Institute of Cultural Relics and Archaeology. A large number of cultural accumulations of Sanxingdui Phase IV site were discovered in Yan Jiayuan, which made people have a clear understanding of the cultural outlook and the lower limit of the times of Sanxingdui Phase IV site.

In March 2005, the Sichuan Provincial Cultural Relics Management Committee and the Sichuan Provincial Institute of Cultural Relics and Archaeology conducted the 14th excavation of Samsung Site. The foundation of a large rammed earth building was found in Guanqingshan.

Follow-up work (2005-present)

At present, the archaeological workstation of Sanxingdui Site is fully sorting out the comprehensive report of Sanxingdui Site.

This work is expected to be completed in early 2008.

Question: Can "base metal" be changed to "precious metal"?

Gold and silver are called "precious metals" because of their beautiful and rare colors, while other metals are called "base metals" accordingly.

Alchemy hopes to transform base metals into precious metals through a certain process, which objectively promotes the development of materials science. In the following 1000 years, people accumulated some experience in material preparation, which laid the foundation for the formation and development of material science after 19 century.

Several famous "alchemists" Moore, Boyle and Newton.

17 1 1 year, a blast furnace with a height of 6 meters and a side length of 2.5 meters appeared in Britain, with a daily output of 6 tons of iron. 1856 Englishman Henry? Bessel first extracted steel from iron.

Alchemy focuses on practical operation, and this technology is really beneficial to future generations. Many equipment and technologies used in modern chemistry are developed from this, and some oil refining technology, water purification technology, synthetic rubber and the manufacture of some modern materials in pharmaceutical technology are closely related to it.

From the end of 19 to the middle of the 20th century.

Low alloy high strength steel → ultra-high strength steel → alloy tool steel → high speed steel.

Stainless steel → heat-resistant steel → wear-resistant steel → electrical steel.

Aluminum alloy → copper alloy → titanium alloy → tungsten alloy → molybdenum alloy

Metal materials still occupy a dominant position in the material family.

Main advantages:

1, the metal material has comprehensive mechanical properties, high reliability and safe use;

2. It has a good temperature range; Good process performance;

3. Rich reserves, suitable for large-scale application.

Steel material

Since the industrial revolution, steel has been the most important material used by human beings and the foundation of national industrialization. The production capacity of steel is an important symbol of a country's comprehensive strength. At present, the world steel output is still increasing year by year.

Qi Xiangdong, secretary-general of China Iron and Steel Industry Association, said that in 2005, the iron and steel industry should take strict control of investment in fixed assets as the primary task, and at the same time further improve the quality and efficiency of the iron and steel industry.

Development trend of iron and steel industry

The product structure is changing: the proportion of high value-added products such as plates, pipes and strips has increased greatly.

Industrial concentration has been further improved: the number of enterprises with a steel output of more than 5 million tons has increased from 13 to 15, accounting for 45% of the national steel output.

Main application fields: As the most important material in industry, the leading position of steel material will not be shaken for a long time to come.

Power system: industrial boilers, heat exchange tubes, large rotors and impellers, etc.

Automobile industry: main structural parts, lathes and machinery industries.

Railways and bridges, ships and offshore drilling platforms, weapons industry: tanks, cannons, firearms.

Petroleum mining machinery and pipelines, chemical pressure vessels, building steel bars and frames,

nonferrous metal

Nonferrous metal materials are an important part of metal materials. Although its output is only 6% of steel, it sometimes plays an irreplaceable role with its unique performance.

Aluminum alloy: the most important light metal alloy with low density (2.7g/cm3), atmospheric corrosion resistance, good electrical conductivity, high specific strength and good processability. It is an important structural material in aviation industry and various industrial fields.

Titanium alloy: It has low density (4.5g/cm3), high strength, high temperature resistance and corrosion resistance, and has important applications in aerospace and other industrial fields.

Magnesium alloy: The density is only 1.7g/cm3, with high specific strength and strong vibration damping capacity, which plays an important role in the aerospace field.

Beryllium alloy: density 1.8g/cm3, high specific stiffness, stable size, small inertia and high specific heat, used for inertial navigation and aerospace low-weight rigid parts, and can be used for heat sinks and airplane heads; Neutron reflection cross section is high, which is used in the reflection layer of atomic energy reactor.

Copper alloy: used in machinery, instruments, motors, bearings, automobiles and other industries.

Zinc alloy: used for battery zinc plate, photographic and offset plate making, molds and instrument parts.

Nickel alloy: the working temperature can reach 1050℃, and it is used for high-temperature components in aviation, rocket engines and reactors.

Manganese alloy: good vibration damping, used for submarine propellers, drill pipes, etc.

Lead alloy, tin alloy: used for fuse, fuse, solder, etc.

Tungsten alloy: melting point 3407℃, high density (19.3g/cm3), which can be used for high-power armor-piercing projectiles.

Molybdenum alloy: melting point is 26 10℃, and it has high specific strength at1100-1650℃.

Niobium alloy: melting point 2477℃, used as a high temperature material for aircraft and spacecraft propulsion system.

Gold, silver, platinum, palladium, rhodium, iridium, etc. Good chemical inertness, bright color, long-term colorfastness, and can be used as decorations, electronic circuit leads, precision resistors, thermocouples, etc.

The development history of metallography

Although the use history of metal materials in human society is very long, for a long time, the related technology of metal materials only stayed in the manual stage, and those who master the related technology can only be called craftsmen. The reason is that they only have experience and don't understand the nature of metal materials.

186 1 year, an Englishman, Shoby, first studied the microstructure of metals with an optical microscope and got a preliminary understanding of the microstructure of metals, thus establishing a new discipline-metallography.

1905 x-rays were used in the study of metals, and the regularity of metal atom arrangement was found.

Metallurgy was born.

Humans have further understood the internal microstructure of metals, discovered many scientific laws and explained many phenomena that were not understood in the past.

The appearance of electron microscope enables people to understand the internal structure of metal in more detail, and the understanding of its microscopic world has taken a big step forward.

In recent 20 years, various electron microscopic analysis equipment has been successfully developed, and people can already see the arrangement of atoms in materials, which makes the research of metal materials enter a brand-new stage.

Constantly explore new functions: superalloys, titanium alloys, intermetallic compounds, damping alloys, superconducting alloys, shape memory alloys, hydrogen storage alloys, nano-metallic materials and amorphous metallic materials.

Amorphous metal

1960, the Duwez group of the University of California obtained the amorphous alloy (Au70Si30) for the first time by rapid cooling technology, and found that the amorphous alloy has many incomparable advantages over conventional alloys.

It has the highest strength, the best toughness, the most corrosion resistance and the easiest magnetization.

Amorphous structure: both crystal and amorphous are real solids. The crystal is long-range ordered, and the equilibrium position of atoms in the crystal is a translational periodic array. Amorphous crystals are long-range disordered and short-range ordered, and the atomic arrangement is aperiodic, also known as metallic glass.

The kinetic properties of glass transition are related to the cooling rate. With the increase of cooling rate, the glass transition temperature decreases.

In order to freeze atoms to keep the displacement of amorphous solids, it must be satisfied that the atomic relaxation time (T) is longer than the experimental cooling time.

Compared with the crystalline phase of thermodynamic equilibrium state with the lowest energy, amorphous solid is in metastable state.

Once metallic glass is formed, it can last almost indefinitely.

The basic process of crystallization: nucleation and growth.

The time of starting crystallization in the C curve determines the state of the product.

Two directions: reducing critical cooling speed and developing rapid cooling technology.

Amorphous structural features:

(1) Amorphous state is a metastable state, which is formed under certain conditions, so it will be transformed into crystalline state under certain conditions, and the nucleation rate is high in the process of transforming into crystalline state, so very fine crystals can be obtained, and some excessive structures can be formed under many conditions.

(2) There are no dislocations, phase boundaries and crystal boundaries in the amorphous alloy, and there is no second phase, which can be said to be a solid without crystal defects.

(3) In principle, a uniform alloy phase with any composition can be obtained, so the range of alloy materials is greatly broadened, and superior properties that crystalline alloys cannot obtain can be obtained.

Properties of amorphous alloys;

(1) Special physical properties: Excellent magnetism is a prominent feature of many amorphous alloys. Alloys with soft magnetic properties are easy to magnetize, and the properties of some amorphous permanent magnet alloys have been greatly improved after partial crystallization. Amorphous alloys also have higher resistivity, the density is 1-2% lower than that of crystalline alloys, the diffusion coefficient of atoms is one order of magnitude higher, and the thermal expansion coefficient is about half that of crystals.

(2) Excellent corrosion resistance: Because its structure is more uniform, it is not easy to form a micro battery when corroded, so it has stronger corrosion resistance. For example, in FeCl3 _ 3 solution, steel is completely corrosion-free, while Fe-Cr amorphous alloy is basically corrosion-free. In sulfuric acid, the corrosion rate of Fe-Cr amorphous is one thousandth of that of stainless steel. The main function of chromium is to form a chromium-rich passivation film.

(3) Excellent mechanical properties: The bonds between atoms in amorphous alloys are stronger than those in ordinary crystals, and there are no crystal defects such as dislocations, so they have extremely high strength. For example, the fracture strength of 4340 super-strength steel is 1.6GPa, while that of amorphous Fe80B20 alloy is 3.63GPa and that of Fe 60 Cr 6 Mo 6 Ba 28 is 4.5GPa. In addition to high strength, amorphous alloy has good toughness and ductility, high hardness and good wear resistance.

Amorphous application

The new generation transformer core is not only easy to magnetize, but also has high resistance, which can greatly reduce eddy current. For example, the magnetic loss of Fe-based soft magnetic materials such as Fe8 1B 13.5Si3.5C2, Fe82B 10Si8 is13-15 that of ordinary silicon steel sheets, and the energy consumption is high.

Because it is difficult to make the bulk amorphous, its application is also limited, but it can be used as a reinforcement of composite materials. Copper-based amorphous alloys with high strength and seawater corrosion resistance can be used as materials for manufacturing submarines, and some iron-based amorphous alloys can be used as chemical filters for fast neutron reactors.

High purity metal is a comprehensive product of many modern high technologies. Although the name "high-purity substance" appeared in 1930s, the research and production of high-purity metal was put on the important agenda only after World War II. First of all, atomic energy research needs a series of high-purity metals. Then, with the development of semiconductor technology, aerospace, radio electronics, etc., the requirements for metal purity are getting higher and higher, which greatly promotes the development of high-purity metal production.

Purity has three meanings for metals. First of all, some properties of metals are closely related to their purity. Pure iron is soft, and cast iron containing impurities is hard. On the other hand, impurities are very harmful. Most metals are brittle because of impurities. For semiconductors, a very small amount of impurities will cause very obvious changes in material properties. Germanium and silicon armour contain trace amounts of harmful impurities such as M and V elements, heavy metals and alkali metals, which will seriously affect the electrical properties of semiconductor devices. Secondly, the study of purity is helpful to clarify the structural sensitivity of metal materials, the influence of impurities on defects and other factors, thus creating conditions for developing new material designs with predetermined material properties. Third, with the continuous improvement of metal purity, the potential properties of metals will be further revealed. For example, ordinary metals are considered to be the most brittle of all metals. However, at high purity, the quilt will appear low temperature plasticity, and at ultra-high purity, it will appear high temperature superplasticity. The discovery of the potential properties of ultra-high purity metals may open up new application fields, open up new breakthroughs in material science and pave the way for the popularization of high technology.

The purity of metal is relative to impurities, which broadly include chemical impurities (elements) and physical impurities (crystal defects). However, the concept of physical impurities only makes sense when the metal purity is extremely high, so the content of chemical impurities is still used as the standard to evaluate the metal purity in production, that is, it is expressed by the percentage of the main metal MINUS the total impurity content, usually expressed by n (the first letter of nine). For example, 99.9999% is written as 6n and 99.99999% as 7N. In addition, the purity of semiconductor materials also shows carrier concentration and low temperature mobility. Metal purity is expressed by residual resistivity RRR and purity grade R, and there is no uniform standard for the definition of purity in the world. Generally speaking, theoretically pure metal should be pure, completely free of impurities, and have a constant melting point and crystal structure. But technically, it is impossible for any metal to achieve absolute purity without impurities, so pure metal has only a relative meaning, which only shows the standards that can be achieved in technology at present. With the improvement of purification level, the purity of metal is also increasing. For example, in the past, the impurity of high-purity metal was 10-6 (one in a million), while the impurity of ultra-pure semiconductor material reached 10-9 (one in a billion), and gradually developed to 10- 12 (one in a trillion). At the same time, the purification difficulty of each metal is different. For example, in semiconductor materials, more than 9N is called high purity, while refractory metal tungsten, such as 6N, is ultra-high purity.

The preparation of high-purity metals is usually carried out in two steps, namely purification (preliminary purification) and ultra-purification (final purification). Production methods can be roughly divided into two categories: chemical purification and physical naming. In order to obtain high-purity metals and effectively remove impurities that are difficult to separate, it is often necessary to combine chemical purification with physical purification, that is, chemical purification is carried out at the same time as physical purification. For example, when silicon melts in a crucible-free zone, hydrogen can be used as a shielding gas. If a small amount of steam is added to hydrogen, water will react with boron in silicon, and boron that cannot be removed by physical purification can be removed. Another example is that when vacuum sintering is used to purify high melting point metals such as tantalum and niobium, in order to decarbonize, it is sometimes necessary to prepare a little excess oxygen or a certain amount of carbon than stoichiometric for deoxidation. This method is also called chemical physical purification.

I. Chemical purification

Chemical purification is the basis of preparing high purity metals. Impurities in metals are mainly removed by chemical methods. In addition to directly obtaining high-purity metals by chemical methods, purified metals are often made into intermediate compounds (oxides, halides, etc.). ), purified to high purity by distillation, rectification, adsorption, complexation, crystallization, disproportionation, oxidation, reduction, etc. And then reduced to metals such as germanium, silicon, germanium tetrachloride, silicon trihydroxide and silane (Si). There are many chemical purification methods, and the common ones are listed in table 1.

Table 1: Common chemical purification methods

Second, physical purification.

Physical purification mainly uses physical processes such as evaporation, solidification, crystallization, diffusion and electromigration to remove impurities. Physical purification methods mainly include vacuum distillation, vacuum degassing, regional melting, single crystal method (see the chapter on semiconductor materials), electromagnetic field purification and so on. In addition, there is a weightless melting purification method in space.

Vacuum condition is very important in physical purification. The refining and purification of high-purity metals are generally carried out under high vacuum and ultra-high vacuum (10-6- 10-8Pa). The important functions of vacuum in metallurgical process are as follows: (1) to create favorable chemical thermodynamic and kinetic conditions for metallurgical reaction with gaseous products, so that the metallurgical process which is difficult to separate impurities from main metals under atmospheric pressure can be realized under vacuum conditions; (2) reducing the solubility of gaseous impurities and volatile impurities in metals, and correspondingly reducing their contents in main metals; (3) reducing the temperature required for metal or impurity volatilization and improving the separation coefficient of metal and impurity; (4) Reduce or avoid the interaction between metal or other reactants and air, and avoid the influence of gaseous impurities on metal or alloy. Pollution. Therefore, many purification methods, such as vacuum melting (vacuum induction melting, vacuum arc melting and vacuum electron beam melting), vacuum distillation and vacuum degassing, must be carried out under vacuum conditions.

1 vacuum distillation

Vacuum distillation is a method to purify metals by using the difference of vapor pressure and evaporation speed between main metals and impurities at the same temperature and controlling the appropriate temperature under vacuum conditions to make some substances selectively volatilize and selectively condense. This method is mainly used to purify some low-boiling metals (or compounds), such as zinc, calcium, magnesium, gallium, silicon, lithium, selenium and tellurium. With the development of vacuum and ultra-high vacuum technology, especially metallurgical high temperature,

The main processes of distillation are evaporation and condensation. At a certain temperature, all substances have a certain saturated vapor pressure. When the partial pressure of a substance at atmospheric pressure is lower than its saturated evaporation at this temperature, the substance will continue to evaporate. The condition of evaporation is to continuously supply heat to the evaporated substance and discharge the generated gas; Condensation is the reverse process of evaporation, and the saturated vapor pressure of gaseous substances decreases with the decrease of temperature. When the partial pressure of a gaseous component is greater than its saturated vapor pressure at the condensation temperature, this substance condenses into a liquid phase (or solid phase). In order to carry out the condensation process to the end, the heat released by condensation must be discharged in time. The main factors affecting the purification effect of vacuum distillation are as follows: ① The greater the partial pressure of steam and the pressure difference of each component, the better the separation effect; (2) Temperature and dynamic conditions of evaporation and condensation. Generally speaking, the decrease of temperature can increase the gap between the vapor pressures of metals and impurities and improve the separation effect; ③ The lower the impurity content in the original metal, the better the separation effect; (4) The interaction between metal and evaporative condensate requires the lowest saturated vapor pressure of evaporative condensate; ⑤ Interaction of metal residual gas; ⑥ Structure of distillation unit; ⑦ There are two kinds of vacuum distillation: pot-adding type and pot-lifting type. Generally speaking, metal melt is suspended by electromagnetic field in non-kettle distillation (see figure 1). For distillation process, please refer to the refining process of the above elements.

Figure 1: Crucible-free distillation unit

1-feeding mechanism; 2- Metal to be purified; 3- baffle; 4- cathode; 5- condenser;

6- heat shield; 7- metal collector; 8- vacuum; 9— Vacuum pump device

2. Vacuum degassing

Vacuum degassing refers to the process of removing gas impurities from metals under vacuum conditions. In fact, it reduces the solubility of gaseous impurities in metals. According to Sieuwerts law, the solubility of diatomic gas in metal at constant temperature is proportional to the square root of gas partial pressure. Therefore, increasing the vacuum degree of the system is equivalent to reducing the partial pressure of gas, that is, reducing the solubility of gas in metal, and some gas impurities exceeding the solubility will escape from the metal and be removed. Take the vacuum heat treatment of powder as an example. Under the condition of high vacuum (2.5-6μPa), the water in powder volatilizes rapidly at 100-200℃, and the hydride decomposes and escapes at 600-700℃. Alkali metals and their compounds volatilize at1100-1600℃, and most of the iron volatilizes. At 2300℃, nitrogen volatilizes and escapes. Hydrogen and oxygen, which have greater affinity for metals, are removed by adding carbon for deoxidation ("C"+"O" = co ↑) and the above impurity metal suboxide MeON. Vacuum degassing is widely used to purify high melting point metals such as tungsten, molybdenum, vanadium, niobium, tantalum and rhenium.

3. Regional melting

Zone smelting is a method of deep purification of metals. Its essence is that a narrow melting zone is formed by local heating of the long and narrow ingot, and the narrow melting zone moves slowly along the ingot in a certain direction through moving heating. Using the same equilibrium concentration difference between solid and liquid, impurities are separated into solid or liquid during repeated melting and solidification, and then removed or redistributed. The melting zone is usually heated by resistance, induction or electron beam. The figure below shows the melting of germanium.

Fig. 2: Schematic diagram of regional smelting and purification of germanium.

Zone melting is widely used for the purification of bright and high-melting metals such as tungsten, molybdenum, tantalum and niobium in semiconductor materials, and also for the purification of high-purity metals such as aluminum, gallium, antimony, copper, iron and silver. For germanium containing about 1x 10-3% impurities, the impurity concentration of high-purity germanium can be reduced to 1x 10-8% after six regional purification. After five-zone smelting, the single crystal of tungsten can be increased from 40 to 2000.

4. Electromigration purification

Electromigration refers to the separation caused by the difference of migration or diffusion speed of metals and impurities in a certain direction under the action of electric field. It is a newly developed method for deep purification of metals, which is characterized by good separation effect on interstitial impurities (especially oxygen, nitrogen, carbon, etc.). ), but it is currently only used for the purification of a small amount of metals. Combined with other purification methods, ultra-high purity metals can be obtained.

When the rod-shaped sample is electrified, the parent metal and impurity ions will move in a certain direction, and the drift speed of ions is: V = UF.

Where v is the ion drift velocity; U is ion mobility; F is an external force acting on ions, which is caused by an electric field. And the force acting on the ions through the scattering of conductive electrons. These forces are related to the effective charge number of ions. According to the different charge numbers of parent ions and impurity ions, the diffusion and drift speeds are different, so as to achieve the purpose of separation.

5. Electromagnetic field purification

The deep purification technology of high melting point metal under the action of electromagnetic field is adopted more and more. Electromagnetic field is not limited to stirring molten metal, but more importantly, it can make molten metal obtain evenly distributed structural defects and refine grain structure during crystallization. When a semiconductor material is pulled into a single crystal, there is temperature fluctuation in the melt during directional crystallization, which will lead to the layered distribution of impurities. A small constant magnetic field is enough to eliminate this temperature fluctuation. In the crystallization process of multiphase system, the second phase can be directionally precipitated by electromagnetic field, resulting in an anisotropic structure similar to magnetic composite materials. Electromagnetic field is also used in suspension smelting, when electromagnetic field plays the role of energy support and stirring, and the second phase (oxide, carbide, etc.). ) is purified by evaporating impurities. Because there is no pollution problem caused by contact with containers, it is widely used to purify almost all high melting point metals, such as tungsten, molybdenum, tantalum, niobium, vanadium, rhenium, osmium, ruthenium, zirconium and so on.

6. Comprehensive application of purification methods

Each purification method uses the difference of some physical or chemical properties between metals and impurity elements to achieve the purpose of purification. For example, vacuum distillation takes advantage of the difference between the saturated vapor pressure and volatilization speed of metals and impurities. Zone melting is to purify and separate impurities by using the solubility difference between solid phase and liquid phase, so each method has certain advantages (good separation effect for some impurities) and disadvantages (poor separation effect for others). Even with the same purification method, the purification effect varies greatly due to the different properties of metals. For example, regional smelting has a good purification effect on high melting point metals, but it is not ideal for some rare earth metals. In order to achieve the effect of deep purification of metals, it is generally necessary to comprehensively apply a variety of purification methods. In this regard, it is very important to use all methods in a reasonable combination and order. Electron beam melting or distillation is usually combined with zone melting or electromigration, that is, electron beam melting or distillation purification is carried out first, and then zone melting or electromigration purification is used as the final purification means. Taking the quilt as an example, in order to obtain ultra-high purity beryllium, it is best to distill and purify it for many times, then melt it in vacuum, and finally carry out regional melting or electromigration purification. After such purification, the purity of beryllium single crystal can reach 99 .999%. When preparing ultra-pure germanium, impurities such as phosphorus, arsenic, aluminum, silicon and boron are generally removed by chemical method, and then purified by zone melting method to obtain electronic grade pure germanium. Finally, the purity requirement of 13N can only be achieved by multiple crystal pulling and cutting. The following table shows the effect of purifying rhenium by combining various methods.

Table 2: Effects of different purification methods on rhenium purification.

7. Purification of metals under space conditions.

The development of space has created new opportunities for purifying gold scraps. Ultra-high vacuum (about 10- 10pa), ultra-low temperature and basically zero gravity in space provide superior conditions for metal purification. Under this condition, there will be no convection in liquid metal, and the distribution of impurities during crystallization will only have pure diffusion properties. There is no need for crucible to melt metal, and ultra-high vacuum is especially beneficial to the volatilization and degassing of impurities. These are ideal conditions for purifying chemically active metals and semiconductor materials by melting, evaporation and zone melting. Taking purified germanium as an example, the separation coefficient of impurity crops is 0. 1/0. 15 when germanium melts vertically on the earth, and 0.23/0. 17 when it is in space. The integrity of the crystal drawn under the condition of no gravity is much better than that under the condition of gravity. Taking indium antimonide as an example, its dislocation density ratio is only 1/6 under gravity. Because the surface tension coefficient of liquid metal in the universe is very large, it is certain to prepare high-purity and high-integrity single crystals by crucible-free zone melting in the universe. In addition, the ultra-low "cosmic" temperature also has a good application prospect.

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