Liquid nitrogen is liquid nitrogen with molecular weight of 28.013, relative density of 0.8081 (-195.8℃), density of 1.2507kg/m3 (at 0℃, l atmospheric pressure), melting point of -209.86℃, boiling point of -195.8℃, critical temperature of -147.05℃, critical pressure of 3.39Mpa (33.5 atmospheres). Critical density 0.31 kg / kg, liquid density 0.8l kg / kg (boiling point), latent heat of evaporation 161.19 kilojoules / kg, specific heat of constant pressure 1.034 kilojoules / kg - ℃; thermal conductivity 2.28 × 10-4 joules / cm - sec - ℃. It is a colorless, transparent, odorless, non-toxic, low-viscosity transparent liquid, which is not thermally conductive and electrically conductive, not spontaneous combustion, chemically stable, and does not act as a chemical compound with any substance. 1 unit volume of liquid nitrogen can produce about 650 times the volume of nitrogen, nitrogen is the main component of air, the content of up to 78% in the air (by volume), liquid nitrogen as the air liquefaction separation of the largest products, industrial oxygen by-products, general purity of 99.99%. Liquid nitrogen at room temperature is easy to gasification, preservation difficulties, transport and carry more trouble, in the absence of liquid nitrogen production areas, the application is limited.
Liquid nitrogen is a more convenient cold source, because of the unique properties of liquid nitrogen, has gradually been valued and recognized in the animal husbandry, medical care, food industry, as well as low-temperature research areas such as more and more common applications. In electronics, metallurgy, aerospace, machinery manufacturing and other aspects of the application continues to broaden and develop.
First, the application of animal husbandry
1, widely used in livestock frozen with improved technology
In a variety of livestock, cattle semen freezing preparation, preservation technology is the most successful, since the 1950s has formed a complete set of stereotypes of the process.
Liquid nitrogen is commonly used as the cold source for freezing bovine semen. Pellets of semen in the liquid nitrogen-cooled fluorine plate (PTFE), copper mesh, aluminum plate drip frozen. It is important to keep the surface of the semen receiving it at a constant distance (1 to 2 centimeters) from the surface of the liquid nitrogen. During the drip-freezing process, a temperature of -80 to -120°C is maintained. The equilibrated semen is well mixed and sperm viability is checked before drip freezing. The drop should be rapid and the particles should be uniform, and about 10 particles per milliliter of diluted semen is appropriate. After the drop freezing is finished, it should stay for 2 to 3 minutes, and put into liquid nitrogen immediately when all the particles have been frozen. After sampling (generally randomly selected 2 particles), the thawing activity rate of 0.3 or more, can be loaded in a gauze bag, labeled in the liquid nitrogen preservation. After freezing the semen of one bull per drop, the fluoride plate and other utensils must be replaced. Currently, fine tubes are available in 0.25 ml and 0.5 ml capacities and are made of non-toxic plastic (polyvinyl chloride). One end of the tube is filled with cotton plugs and polyethylene powder, the powder is cured in the presence of water that automatically seals, insemination and become a piston to push the semen; the other end of the injection of semen, you can polyethylene powder or steel beads (or plastic beads) sealing, pay attention to sealing the seals and semen between the space left between the 10 to 13 millimeters, to prevent the frozen process due to the expansion of the fine tube caused by bursting.
Semen storage The frozen semen of cattle is stored in liquid nitrogen as a cold source, and can be removed at any time when needed. In order to prevent the impact of temperature changes on semen quality, the action should be taken quickly to minimize the time spent in the air. When the frozen semen is extracted from the storage container, the semen should not exceed the base of the neck of the liquid nitrogen container, to avoid the decline of semen thawing viability due to temperature rise. The frozen semen of cattle has a history of more than 40 years. Tests have proved that the frozen semen preserved to date still has the ability to inseminate. However, it is generally believed that with the prolongation of the preservation time, the sperm viability and insemination ability of bovine frozen semen gradually decreases. The exact time frame for the long-term preservation of bovine frozen semen requires continued research and observation.
2, livestock and a variety of animals in the embryo transfer, preparation and preservation of embryos
At present, most of the embryo freezer, is an intelligent freezer. The instrument adopts microcomputer control technology, special software, can accurately control the amount of liquid nitrogen application, so as to ensure that the cryopreserved biological products are frozen at a suitable rate of freezing temperature reduction and freezing.
3, liquid nitrogen ultra-low temperature preservation of microorganisms technology
The strains of bacteria stored in -196 ℃ liquid nitrogen long-term preservation method, which is the principle of using microorganisms in -130 ℃ below the metabolism tends to stop and effectively preserve microorganisms. Macrofungi is an important group of mycorrhizal organisms (a class of fungi forming large substrates in mycorrhizal organisms, referring to mushrooms or mycorrhizal organisms in a broad sense), and many kinds of species have high nutritional and medicinal values, and they are a class of mycorrhizal organisms that have the most promising prospects for development and application at present; moreover, some macrofungi are able to decompose withered and dead plants, and they have important roles to play in the maintenance of the material cycle of the natural world and the ecological balance. Paper industry and environmental purification; some macrofungi can cause tree diseases or damage to a variety of wood products, the strengthening of the understanding of such pathogenic fungi, is conducive to the prevention and reduction of the occurrence of harm. Large fungi normative preservation of microbial strain resources for the safety, efficient preservation and, on the long-term effective preservation of genetic resources, off-site preservation of biodiversity **** enjoy has a heavy significance
4, agricultural biogenerics preservation
Shanghai invested more than 41 million yuan to begin construction of China's first domestic agricultural biogenerics integrated library. Agricultural circles say that the seed industry, which has the most potential for national market development, will use this gene bank as a source of breeding materials. A total floor space of more than 3300 square meters of Shanghai agricultural biological gene bank, set up in the Shanghai Academy of Agricultural Sciences, will be preserved plant seeds, plant isolated material, animal germ cells, microbial strains and plant genetic engineering materials of five categories of agricultural biological genetic resources.
5, the preservation of liquid nitrogen vaccine
Liquid nitrogen vaccine is the mid-80s gradually popularized the application of liquid nitrogen vaccine, the first use of Ⅲ-FCl26 inflammation of chicken herpesvirus liquid nitrogen vaccine, and later domestic and foreign and developed SBl, 301/B, Z4, 814 and other Ⅱ liquid nitrogen vaccine, and the combination of bivalent seedling II + III and I + II + III trivalent seedling, the application of the application of the once had an effective control of some areas of the chicken. Effective control of some areas of chicken Malick's disease, II + III bivalent vaccine is not a long time, in 1993 Malick's disease and in North America and the severe state of the region appeared in a large-scale epidemic, in order to effectively control Malick's disease, some countries began to learn from the Netherlands since the use of the I type of liquid nitrogen vaccine in 1927, the use of liquid nitrogen vaccine, the effective control of the experience of Malick's disease, the United States, Canada, Brazil, Japan, Europe have successively introduced from the Netherlands Ministry of Agriculture and Fisheries CVl988 / Rispens seedling seed poison, in their respective countries using different process technology production, preserved in liquid nitrogen tanks.
6, the use of liquid nitrogen treatment of dairy cows nipple milk holes outside the mouth closure, livestock skin tumors and so on is also more common.
Second, the application of medical care
1, cryogenic medicine
China's rapid development of clinical cryogenics, promoting the development of transplantation medicine, especially in the bone marrow, hematopoietic stem cells, skin, cornea, endocrine glands, as well as blood vessels and valves and other cryopreservation and transplantation applications have made remarkable achievements. Successful hematopoietic stem cell transplantation depends on the preservation of hematopoietic stem cell viability. During the cooling and freezing process of biological samples, a certain amount of heat is released during the phase change from liquid to solid, causing their temperature to rise back up, and the freezing process without controlling the rate of cooling will result in the death of tissue cells. Accurately determine the phase transition point of biological samples, using a microcomputer to prepare the cooling program, in order to increase the amount of liquid nitrogen input in the sample phase transition, to overcome the temperature of the phase transition samples back up, so that the cells are safe and quickly through the phase transition period, which is to improve the frozen samples of the key links to the survival rate.
2, clinical medicine
Liquid nitrogen is currently the most widely used refrigerant in cryosurgery. It is one of the best refrigerants found so far, and it is injected into the cryomedical apparatus, like a scalpel, to do any surgery. Cryosurgery is a treatment that uses low temperatures to destroy diseased tissue. Cells due to the rapid change in temperature, the formation of crystals inside and outside the organization, so that the cell dehydration, crumpling, so that electrolytes and other changes, freezing can also slow down the local blood flow, microvascular blood stasis or embolism caused by cell hypoxia death.
It is reported that liquid nitrogen cryotherapy facial proliferative and pigmented diseases 2138 cases, of which 1,445 cases were cured, the cure rate of 67.59%; 329 cases of apparent effect, 207 cases of effective ＼ the total effective rate of 82.97%. The cure rates of freckles and seborrheic keratosis were 79.23% and 84.69% respectively; nevus (including naevus) 52.25% and blepharoplasty 41.67%. The patients with flat warts had new rashes in the lesions after freezing, so it is not suitable to use freezing treatment for flat warts in the progressive stage, and the effect of freezing is good in the stationary stage. 2 cases of nevus with chloasma had hyperpigmentation after treatment, and the pigmentation of these 2 cases returned to normal after effective treatment, which suggests that it is not suitable for those with chloasma to use freezing treatment. For the treatment of nevus, according to the part and the patient's requirements to take appropriate treatment, black and smaller nevus (diameter <3mm) can be taken to liquid nitrogen freezing method of treatment, the larger nevus should be surgically resected and pathologically examined for the best.
Liquid nitrogen freezing with traditional Chinese medicine for the treatment of warts, no toxic side effects, little pain, high cure rate, low recurrence rate, no postoperative scars, fewer complications, shorter treatment time, economic, is one of the ideal methods of treatment of warts
Liquid nitrogen cryotherapy for the treatment of nodular itchy diagnosis, mucus cysts, multiple warts, and neurodermatitis adopted by the liquid nitrogen direct injection method of therapy
Liquid nitrogen cryotherapy for the treatment of nodular itchy diagnosis, mucus cysts, multiple warts and neuro-dermatitis. Chronic pharyngitis, liquid nitrogen will be sprayed directly on the surface of the lesion, cooling fast, deep, wide surface, and can eliminate the intermediate zone, to receive satisfactory results in the near future. Each freeze-thaw cycle is mastered at about 1min, and 4 to 5 freeze-thaw cycles are consecutive. Only 1 time per week.
Third, in the food industry
1, liquid nitrogen in the application of food quick-freezing
In many of the preservation method, frozen preservation method is most widely used, the effect is also very significant. As one of the frozen preservation methods of liquid nitrogen quick-freezing has long been adopted by food processing enterprises, because it can achieve low-temperature deep-freezing ultra-quick-freezing, but also conducive to the realization of the frozen food part of the glass, so that the food thawed to maximize the recovery to the original state of freshness and the original nutrients, and greatly improve the quality of frozen food, so in the quick-freezing industry has shown a unique vitality. Compared with other freezing methods, liquid nitrogen quick-freezing has the following obvious advantages:
① fast freezing speed (freezing speed than the general freezing method is about 30-40 times faster): the use of liquid nitrogen quick-freezing, so that the food quickly through the 0 ℃ ~ 5 ℃ the largest ice crystal growth zone, food researchers have made a useful attempt in this regard.
② Maintain the quality of food: due to liquid nitrogen quick-freezing time is short, by liquid nitrogen quick-freezing of food can be maximized to maintain the color, aroma, taste and nutritional value before processing. Duan Zhenhua et al. used liquid nitrogen to quick-freeze betel nut, and the results show that the betel nut treated with liquid nitrogen maintains a high chlorophyll content and good flavor.
③ Material dry loss is small: general freezing dry loss rate of 3-6%, while liquid nitrogen quick-freezing can be reduced to 0.25-0.5%.
④ Low cost of equipment and power, easy to realize mechanization and automation line, improve productivity.
At present, liquid nitrogen quick-freezing mainly spray freezing, impregnation freezing and cold air freezing in three ways, of which the spray freezing is the most widely used.
2, the application of liquid nitrogen in beverage processing
At present, there are many beverage manufacturers use nitrogen or nitrogen and C02 mixture to replace the traditional C02, the beverage inflatable packaging. Nitrogen-filled, highly carbonated beverages cause fewer problems than those filled only with carbon dioxide; nitrogen is also ideal for filling non-sparkling beverages such as wine and fruit juices. The advantage of filling liquid nitrogen in non-aerated beverage cans is that the small amount of liquid nitrogen injected can exclude the oxygen in the headspace of each can, so that the gas in the upper space of the storage tank is inert, thus extending the storage period of perishable products.
3, liquid nitrogen in the fruit and vegetable storage and preservation of the application
Liquid nitrogen for fruit and vegetable storage and preservation has the advantages of gas conditioning, can regulate the agricultural and sideline products in the peak season and off-season supply and demand contradictions, reduce the loss of storage. The function of gas conditioning is to increase the concentration of nitrogen, while controlling the proportion of nitrogen, oxygen and C02 gas, and keep it in a stable state, reduce the respiratory strength of fruits and vegetables, delaying the maturation process, so that fruits and vegetables to maintain the freshness of the picking state and the original nutritional value, to prolong the freshness of fruits and vegetables.
4, the application of liquid nitrogen in meat processing
Liquid nitrogen in the raw meat grinding, chopping or mixing and other processes to join, can effectively improve product quality. For example, in salami processing, the use of liquid nitrogen can improve the water retention of meat, prevent the oxidation of fat, and improve the quality of slicing and appearance; used in meat snacks, preserved meat and other reprocessed meat processing, not only to accelerate the protein solubility and enhance the water retention of the minced meat mix, and to maintain the unique shape of the product is particularly effective. In addition, the raw material meat by liquid nitrogen rapid cooling, not only in a longer period of time to maintain the characteristics of hot meat, gas and ensure the health and safety of meat; in the processing process, no longer have to worry about the impact of rising temperatures on the quality of meat, and processing can not be affected by the raw material temperature, processing time, the impact of seasonal factors, and can also be made to process at a low partial pressure of oxygen in a certain range of products to extend the shelf life.
5, liquid nitrogen in the application of low-temperature crushing of food
Low-temperature crushing, that is, cooled to the embrittlement point temperature of the material, under the action of the external force broken into powder process. Low-temperature crushing of food is a new food processing technology developed in recent years, the technology is particularly suitable for the processing of food containing more aromatic components, high fat content, sugar content and more gelatinous substances. With liquid nitrogen for low-temperature crushing processing, even raw materials such as bones, skin, meat, shells, etc., all crushed at once, so that the finished product particles are small and maintain their effective nutrition. Such as Japan will be frozen by liquid nitrogen seaweed, chitin, vegetables, spices, etc., into the pulverizer crushing, can make the finished product fine particle size up to 100μm below, and basically maintain the original nutritional value. In addition, the use of liquid nitrogen for low-temperature crushing, can also be crushed at room temperature is difficult to crush the material, heat-sensitive and heat-induced deterioration, easy to decompose substances. In addition, liquid nitrogen can be crushed such as fat meat, water, vegetables and other food materials at room temperature is difficult to crush, and can be manufactured so far there has been no new processed food.
6, liquid nitrogen in the application of food packaging
A company in London, England, developed a simple and practical food preservation and packaging methods, that is, in the packaging of food, to the bag to add a few drops of liquid nitrogen. When the liquid nitrogen evaporates into gas, its volume rapidly expanding, in the bag to quickly replace most of the original gas, reducing the deterioration of food caused by oxidation, thus greatly extending the freshness of food.
7, the application of liquid nitrogen in the refrigerated transportation of food
Refrigerated transportation is a very important part of the food industry. The development of liquid nitrogen refrigeration technology, the development of liquid nitrogen refrigerated trains, refrigerated cars and refrigerated containers is now the international **** with the development trend. The application of economically developed countries for many years has shown that the liquid nitrogen refrigeration system is the only commercially competitive with the mechanical refrigeration system of cold storage and preservation of technology, but also the direction of development of refrigerated food transportation.
8, liquid nitrogen in the food industry in other applications
As a result of liquid nitrogen refrigeration, egg juice, liquid condiments, soy sauce can be processed into a free-flowing and can be poured out of the granular frozen food, these foods can be used at any time and is very easy to prepare. When grinding spices and water-absorbent food additives such as sugar substitutes and lecithin, liquid nitrogen is injected into the mill to protect the valuable ingredients and also to increase the grinding yield. Zhou Shunhua et al. demonstrated that the use of liquid nitrogen quenching and cooling in concert with high-temperature thawing for pollen wall-breaking is characterized by good results, high wall-breaking rate, high speed, and the physiological activity of the pollen remains stable and uncontaminated.
Fourth, in the electronics industry
1, superconducting technology
Superconductors unique characteristics, so that it may be widely used in a variety of fields. Liquid nitrogen instead of liquid helium as a superconducting refrigerant to obtain superconductors, so that superconductivity technology towards large-scale development and application, considered one of the greatest discoveries in science in the 2 0th century.
Superconducting magnetic levitation technology is based on superconducting ceramics consisting of yttrium barium copper oxygen (YBCO), which transforms from a normal state to a superconducting state when the superconducting material is cooled to the temperature of liquid nitrogen (78 K, equivalent to -196~C). The magnetic field generated by the shielding current and the track magnetic field repel each other, and if the repulsive force is greater than the weight of the train, the car can be suspended in the air. At the same time, during the cooling process, a portion of the magnetic field is captured within the superconductor due to the flux pinning effect of the superconductor. This captured magnetic field and the track magnetic field are attracted to each other, and due to the simultaneous existence of repulsive and attractive forces, the car body can be stably suspended above the track. With conventional magnets between the same sex repulsion, the role of opposites attract different, superconductor and the external magnetic field between the both repulsive and attractive interactions, so that the superconductor or permanent magnets can overcome their own gravity, levitation or upside down in the other side of the following.
2, electronic components manufacturing and testing
Environmental stress screening is to select a number of typical environmental factors, the right amount of environmental stress on the component or the whole machine, the component process defects, that is, the production and assembly process defects inspired, given the correction or replacement. Environmental stress screening using temperature cycling and random vibration is more effective. Temperature cycle test is to use a high rate of temperature change, large thermal stress, so that those different materials composed of components, due to the combination of bad, the material itself is not homogeneous, defects in the process of the hidden problems caused by the rapid failure, the use of the temperature change rate of 5 ℃ / min; Limit temperature of -40 ℃, +60 ℃; the cycle number of times for 8 times. Such a combination of environmental parameters makes the exposure of defects of false soldering, pinching parts, and the components themselves more obvious. For large quantities of temperature cycling tests, the two-box method can be considered. In this case, screening should be done at the level.
Liquid nitrogen is a faster and more effective way to shield and test electronic components, circuit boards.
3, cryogenic ball milling technology
Cryogenic planetary ball mill is a constant flow of liquid nitrogen gas into the planetary ball mill fitted with a heat preservation cover, these cold air will be high-speed rotation of the ball mill jar heat generated in time to be absorbed, so that the ball mill jar equipped with materials, grinding balls are always in a certain low-temperature environment. In the low-temperature environment, mixing, fine grinding, new product development and small batch production of high-tech materials necessary devices. The product is small in size, full-featured, high in efficiency, low in noise, widely used in medicine, chemical industry, environmental protection, light industry, building materials, metallurgy, ceramics, minerals and other departments. The
4, green cutting and processing technology
Cryogenic cutting is the use of cryogenic fluids such as liquid nitrogen, liquid carbon dioxide and cold air, etc. sprayed to the cutting area of the processing system, resulting in a localized cryogenic or ultra-low-temperature state of the cutting zone, the use of the workpiece in the low-temperature conditions produced by the low-temperature brittleness, to improve the workpiece's cutting and processing performance, tool life and the quality of the workpiece surface. According to the different cooling media, cryogenic cutting can be divided into cold air cutting and liquid nitrogen cooling cutting. Low-temperature cold air cutting method is through the tip of the tool to the processing part of the spray -20 ℃ ~ -30 ℃ (or even lower) of the low-temperature airflow, and mixed with a small amount of plant-based lubricants (10 ~ 20m 1 per hour), so as to play a cooling, chip removal, lubrication effect. Low-temperature cold air cutting and traditional cutting compared to improve processing efficiency, improve the surface quality of the workpiece, and almost no pollution to the environment. Japan's Yasuda Industrial Company's machining centers in the motor shaft, the center of the cutter shaft inserted into the adiabatic duct structure, the use of -30 ℃ low-temperature cold air directly to the cutting edge. This structure greatly improves the cutting conditions and facilitates the implementation of low-temperature cold air cutting process. Kazuhiko Yokogawa studied cold air cooling in turning and milling. In milling tests, three conditions were compared, namely, water-based cutting fluid, ambient air (+10°C) and cold air (-30°C), and the results showed that tool durability was significantly improved with cold air cutting. In the turning test, the tool wear rate in cold air (-20 ℃ ) cutting than room temperature wind (+20 ℃ ) cutting significantly decreased.
Liquid nitrogen cooling cutting mainly has two forms of application, one is the use of bottle pressure will be liquid nitrogen like cutting fluid directly sprayed to the cutting zone; the other is the use of liquid nitrogen heat evaporation cycle to indirectly cool the tool or workpiece. At present, low-temperature cutting is mainly used in titanium alloy, high manganese steel, hardened steel and other difficult to machine materials in the processing. kPRaijurkar H13A carbide tools, and liquid nitrogen circulation cooling tool method on titanium alloy low-temperature cutting test, the test results show that, compared with the traditional cutting method, the tool wear is significantly reduced, the cutting temperature is reduced by 30%, and the surface of the workpiece has been greatly improved. The surface quality of the workpiece has been greatly improved. Wan Guangmin used indirect cooling method to carry out low-temperature cutting processing test on high manganese steel, the results show that the indirect cooling method of low-temperature machining of high manganese steel, the tool impact reduces, the tool wear is reduced, work hardening phenomenon has been improved, and the surface quality of the workpiece has also been improved. Wang Lianpeng et al. used liquid nitrogen spray method in the CNC machine tool low temperature cutting processing 45 hardened steel, the test results show that the use of liquid nitrogen spray type low temperature machining 45 hardened steel can improve tool durability, improve the surface quality of the workpiece.
In the liquid nitrogen cooling processing state, the carbide material can maintain its bending strength, fracture toughness and impact strength, and its hardness increases with the reduction of temperature, so the carbide cutting tool material in the liquid nitrogen cooling can maintain its excellent cutting performance, and as at room temperature, its performance is determined by the number of bonded phase. For high-speed steel, the hardness increases and the impact strength decreases with decreasing temperature, but the overall cutting performance is maintained. He conducted a study on some materials at low temperature to improve its cutting and machinability, selected low carbon steel AISll010, high carbon steel AISl070, bearing steel AISIE52100, titanium alloy Ti-6A 1-4V, casting aluminum alloy A390 five kinds of materials, experimental studies show that: because of the low carbon steel shows a good low-temperature brittleness, low-temperature cutting can be obtained ideal processing results; For high-carbon steel and bearing steel, the application of liquid nitrogen cooling can inhibit the temperature rise in the cutting zone and tool wear rate; in cutting casting aluminum alloy, the application of low-temperature cooling can improve the hardness of the tool and the tool's ability to resist silicone-phase abrasive wear, in the processing of titanium alloys, at the same time, low-temperature cooling of the tool and the workpiece, which effectively reduces the cutting temperature and reduces the chemical affinity between the titanium and the tool material.
V. Other fields
Jiuquan Satellite Launch Center special fuel station production of liquid nitrogen, it is the rocket fuel push agent, a large amount of liquid nitrogen with high pressure to push the rocket fuel to the combustion chamber.
Applied to the development of high-temperature superconducting power cables; applied to the emergency maintenance of liquid pipeline freezing; applied to the low-temperature stabilization of substances and low-temperature quenching; liquid nitrogen cold assembly technology (thermal expansion and contraction phenomenon in the industry) is also widely used; liquid nitrogen artificial rain technology; liquid nitrogen drainage technology and timely precipitation of the liquid-induced spraying is being constantly in-depth research.
The use of nitrogen underground fire, the fire was quickly extinguished, while eliminating the risk of gas explosion, etc..
Why choose liquid nitrogen: because it cools faster than other methods, and does not react chemically with other substances, greatly saves space, provides a dry atmosphere, it is environmentally friendly (liquid nitrogen volatilizes directly into a gas after use and returns to the atmosphere, leaving no contamination), and it is simple and convenient to use.