Physical and chemical constants
CASNo.: 13 14- 13-2 chemical formula: ZnO molecular weight: 8 1.37 appearance: white solid relative density: 5.606 melting point:1975 c (decomposition) boiling point: 230. 100 ml (30 c) band gap: 3.3eV standard molar enthalpy of formation: -348.0 kJ/mol standard molar entropy: 43.9 J/(k mol) MSDS number: ICSC 0208 EU classification: harmful to the environment (n) warning properties standard word: R50/53 (highly toxic to aquatic organisms.
[Edit this paragraph] Chemical properties
Zinc oxide mainly exists in the form of white powder or sphalerite. A small amount of impurities such as manganese in sphalerite make the ore appear yellow or red. When zinc oxide crystal is heated, it will overflow a small amount of oxygen atoms (0.007% of the total oxygen atoms at 800°C), making the substance yellow. When the temperature drops, the crystal turns white again. When the temperature reaches1975 C, zinc oxide will decompose to produce zinc vapor and oxygen. Elemental carbon can be used to reduce zinc in zinc oxide, and zinc oxide reacts at high temperature: ZnO+C → Zn+Co Zinc oxide is an amphoteric oxide, which is insoluble in water or ethanol, but soluble in most acids, such as hydrochloric acid: ZnO+2HCl → ZnCl 2+H2O, and can react with strong base to generate soluble zincate. For example, when it reacts with sodium hydroxide: ZnO+2NaOH+H2O → Na2 [Zn (OH) 4] zinc oxide can react slowly in fatty acids to generate corresponding carboxylates, such as oleate and stearate. Zinc oxide can react with hydrogen sulfide. In industrial production, this reaction is often used to remove hydrogen sulfide from mixed gas: ZnO+H2S→ ZnS+H2O. When zinc oxide is mixed with concentrated zinc chloride aqueous solution, basic zinc chloride is generated, which has hardening properties similar to cement and is often used in dental surgery. Zinc phosphate tetrahydrate (Zn3 (PO4) 2 4H2O) produced by the reaction of zinc oxide and phosphoric acid also has the same properties. Zinc oxide will react violently with magnesium powder, aluminum powder, chlorinated rubber and linseed oil, which may cause fire or explosion. The ointment containing zinc oxide will produce hydrogen peroxide when it is mixed with water and exposed to ultraviolet rays.
[Edit this paragraph] Physical properties
crystalline structure
Two crystal structures of zinc oxide Zinc oxide crystals have three structures: hexagonal wurtzite structure, cubic sphalerite structure and rare sodium chloride octahedral structure. The wurtzite structure is the most common because it has the highest stability among the three structures. Cubic sphalerite structure can be obtained by gradually generating zinc oxide on the surface. In these two crystals, each zinc or oxygen atom forms a regular tetrahedron structure, and its center is together with the adjacent atoms. The octahedral structure can only be observed at a high pressure of 1000 billion Pascal. The wurtzite structure and sphalerite structure have central symmetry, but they are not axisymmetric. The symmetry of crystal makes wurtzite structure have piezoelectric effect and hot spot effect, and sphalerite structure has piezoelectric effect. The point group of wurtzite structure is 6mm (international symbol) and the space group is P63mc. In the lattice constant, a = 3.25 angstrom and c = 5.2 angstrom; The c/a ratio is about 1.60, which is close to the ideal hexagonal ratio 1.633. In semiconductor materials, zinc and oxygen are mostly bonded by ionic bonds, which is one of the reasons why they have high voltage electrical properties.
mechanical character
The hardness of zinc oxide is about 4.5, and it is a relatively soft material. The elastic constant of zinc oxide is smaller than that of group III-V semiconductor materials such as gallium nitride. Zinc oxide has good thermal stability and thermal conductivity, high boiling point and low thermal expansion coefficient, so it is useful in the field of ceramic materials. Among all kinds of tetrahedral semiconductor materials, zinc oxide has the highest piezoelectric tensor. This characteristic makes zinc oxide one of the important materials for electromechanical coupling.
electrical characteristics
At room temperature, the band gap of zinc oxide is about 3.3 eV, so pure zinc oxide is colorless and transparent. High band gap makes zinc oxide have the advantages of high breakdown voltage, strong electric field maintenance ability, low electronic noise and high power margin. When zinc oxide is mixed with a certain proportion of magnesium oxide or cadmium oxide, the band gap will change between 3-4 eV. Even if it is not doped with other substances, zinc oxide has the characteristics of n-type semiconductor. The characteristics of N-type semiconductors were once thought to be related to the non-integer ratio of compound atoms, but the study of pure zinc oxide became a counterexample. The performance of N-type semiconductors can be adjusted by using III main group elements such as aluminum, gallium and indium or halogens such as chlorine and iodine. But zinc oxide is difficult to be made into p-type semiconductor. Useful additives include alkali metal elements such as lithium, sodium and potassium, V main group elements such as nitrogen, phosphorus and arsenic, and metals such as copper and silver, but they all need to be effective under special conditions.
[Edit this paragraph] Preparation method
Zinc oxide exists in natural sphalerite, but its purity is not high. Zinc oxide used in industrial production is usually obtained by burning zinc or roasting sphalerite. The global annual output of zinc oxide is about100000 tons. 【 654.38+0 】 There are the following production methods.
Indirect method
The raw material of indirect method is zinc ingot or zinc slag obtained by smelting. Zinc is converted into zinc vapor in a graphite crucible at a high temperature of 1000℃, and then oxidized by blown air to generate zinc oxide, and zinc oxide particles are collected after cooling. Indirect method was popularized by French scientist leclerc in 1844, so it is also called French law. The indirect method for producing zinc oxide is simple and the cost is greatly affected by raw materials. The diameter of zinc oxide particles produced by indirect method is about 0. 1- 10 micron, and the purity is between 99.5% and 99.7%. In terms of total output, indirect method is the most important zinc oxide production method. Zinc oxide produced by indirect method can be used in rubber, varistor and coating industry. The heavy metal content of zinc ingot or zinc slag directly affects the heavy metal impurity content of the product. Products with low heavy metal content can also be used in livestock feed, medicine, medical care and other industries.
Direct teaching method
The direct method takes various zinc-containing minerals or impurities as raw materials. Zinc oxide reacts with coke by heating, which is reduced to metallic zinc, and then oxidized to zinc oxide by oxygen in the air to remove most impurities. The zinc oxide particles obtained by direct method are coarse, and the product purity is between 75% and 95%, which is generally used in rubber and ceramic industries with low requirements.
Chemical wet separation method
Wet chemical methods can be roughly divided into two categories: acid method and ammonia method. They react with raw materials by acid or alkali respectively, and then prepare zinc carbonate or zinc hydroxide precipitate. After filtration, washing, drying and calcination at 800℃, high-purity light zinc oxide with particle size of 1~ 100 nm was finally obtained. Acid method is usually to react zinc-containing raw materials with sulfuric acid to obtain impure zinc sulfate solution containing heavy metal ions. Then, a large number of iron, manganese, copper, lead, cadmium and arsenic plasmas were removed by multiple redox precipitation to obtain pure zinc sulfate solution. Neutralizing the solution with soda ash to obtain solid basic zinc carbonate. The basic zinc carbonate was washed, dried and calcined to obtain light zinc oxide. The products produced by acid method are of high quality. Ammonia method usually uses ammonia water and ammonium bicarbonate to react with zinc-containing raw materials to obtain zinc-ammonia complex, then removes impurities to obtain qualified zinc-ammonia complex solution, and then evaporates ammonia to convert the zinc-ammonia complex into basic zinc carbonate. Finally, the light zinc oxide is obtained by drying and calcining. The cost of ammonia process is relatively low.
Hydrothermal synthesis
Hydrothermal synthesis refers to a production method of inorganic synthesis by heating the aqueous solution of the reaction system to the critical temperature in a closed reactor (autoclave), thus creating a high-pressure environment. The zinc oxide obtained by this method has small particle size and good crystallization. Nano-ZnO powders with different shapes and sizes can be obtained by combining hydrothermal method with template technology. At present, this method is only in the experimental stage, and there are still problems such as complex process equipment and high cost, but it is also considered as a method with great industrialization potential.
Spray pyrolysis
Spray pyrolysis method is to spray metal salt solution into high temperature atmosphere in fog, and directly obtain nano-oxide powder through solvent evaporation and subsequent thermal decomposition of metal salt; Or spraying the solution into a high-temperature atmosphere for drying and then heat-treating to form powder. The nano-powder prepared by this method has high purity, good dispersibility, uniform particle size distribution, good chemical activity, simple process operation, easy control and low equipment cost, and it is one of the preparation methods of nano-zinc oxide powder with the most industrialization potential.
[Edit this paragraph] Application field
Rubber manufacturing
50% of zinc oxide produced in industry flows to rubber industry. As important reactants of rubber vulcanization, zinc oxide and stearic acid are one of the raw materials for rubber manufacturing. The mixture of zinc oxide and stearic acid enhances the hardness of rubber. Zinc oxide is also an important additive for automobile tires. Zinc oxide can not only vulcanize, but also greatly improve the thermal conductivity of rubber, thus contributing to the heat dissipation of tires and ensuring driving safety. Zinc oxide additive can also prevent the rubber from being corroded by mold or ultraviolet rays.
silicate industry
Zinc oxide is an additive of cement, which can shorten the hardening time of cement and improve the waterproof performance of cement. In the production of glass and ceramics, zinc oxide can be used as a flux to reduce the sintering temperature of glass and ceramics. The transparency of zinc oxide added with aluminum, gallium and nitrogen reaches 90%, which can be used as glass coating, allowing visible light to pass through and reflecting infrared light at the same time. Paint can be painted on the inside or outside of the window glass to achieve the effect of heat preservation or insulation.
Medicine and health
Zinc oxide has deodorizing and antibacterial functions, so it is often added to cotton fabrics, rubber, food packaging and so on. Adding zinc oxide to food not only has a certain antiseptic effect, but also can be used as a zinc source to supplement zinc necessary for human body. Zinc oxide can be used to improve skin health, such as baby powder, diaper rash ointment, zinc ointment, dandruff shampoo and antiseptic. Zinc oxide mixed with about 0.5% iron oxide is called calamine, which is made into calamine lotion for treating acute pruritic dermatosis. Some sports bandages are also doped with zinc oxide to prevent athletes from soft tissue injury during sports. Zinc oxide has a particularly strong absorption ability to ultraviolet rays with a wavelength of 280-400nm, so it is often used in various sunscreen products to prevent sunburn and other skin diseases caused by ultraviolet rays. The clove oil zinc oxide cement powder with zinc oxide as raw material can be used as filling material or hole sealing material, and zinc oxide is usually the abbreviation of clove oil zinc oxide cement powder. Zinc oxide is an additive of cigarette filter. The charcoal filter mixed with zinc oxide and iron oxide can remove a lot of hydrogen cyanide and hydrogen sulfide from flue gas without affecting its flavor.
Colored material
Zinc white pigment zinc oxide is called zinc white in pigments, and its transparency is between lithopone and titanium dioxide. China white is a special kind of zinc white, a kind of pigment painted by artists. Compared with the traditional white lead, zinc white can be permanently preserved in the sun, will not be polluted by sulfur-containing air, and is non-toxic and cheap. The coating containing zinc oxide is a traditional metal anticorrosion coating, which is especially effective for galvanized iron. Compared with organic coatings, zinc oxide has strong coloring power and hiding power, and can prevent mildew and ultraviolet radiation, and has better anti-corrosion effect.
Electronic field
Zinc oxide has a high band gap at room temperature, so it is often used to manufacture laser diodes and light-emitting diodes. Compared with gallium nitride with the same high band gap, zinc oxide has a larger exciton binding energy (about 60meV at room temperature), so its luminous brightness is higher. In addition, the stability of zinc oxide under high energy radiation and wet chemical corrosion is also an important reason for its wide application. Using aluminum-doped zinc oxide as transparent electrode, the cost and toxicity of this composite material are much smaller than that of traditional indium tin oxide. Zinc oxide has been applied to solar cells and liquid crystal displays. Zinc oxide can also be used to make transparent thin film transistors (TTFT). Because it belongs to a field effect transistor, the element does not need a PN junction, which avoids the problem that zinc oxide is difficult to make a P-type semiconductor.
Application of nanometer zinc oxide
Nano-zinc oxide has many new physical and chemical properties different from ordinary zinc oxide materials because of its small particle radius and large specific surface area, so it has incomparable special properties and new uses. Nano-zinc oxide shows a very broad and attractive application prospect in aerospace, electronics, metallurgy, chemistry, biology and environmental protection. For example, nano-zinc oxide powder can absorb ultraviolet rays and transmit more than 85% visible light. Therefore, nano-zinc oxide can be used as an additive for automobile glass and building glass to shield potential harmful ultraviolet rays. Nano-zinc oxide anti-ultraviolet glasses have been sold on the market. As nano-zinc oxide is a good photocatalyst, it can automatically decompose negatively charged electrons that move freely in water and air under illumination, leaving positive charge holes at the same time, thus activating oxygen in the air to become active oxygen, which can kill most germs and viruses. Therefore, glass and ceramic products added with nano-zinc oxide can have self-cleaning performance. When firing ceramics, the dosage of nano-zinc oxide can be reduced to 1/2 ~ 1/3 of that of ordinary zinc oxide, but the strength and hardness are higher, the sintering temperature is lower and the surface is brighter. Nano-zinc oxide powder also has the optical characteristic of "angular discoloration effect", that is, the color of the painted object can change with the change of the observer's line of sight angle. Nano-zinc oxide powder can be used as fabric in automobile coatings to make "chameleon" cars, which can bring different bright colors to observers during the movement. The film made of nano-zinc oxide has pressure sensitivity (mainly nonlinear volt-ampere characteristics). When the zinc oxide varistor material is subjected to an applied voltage higher than its own varistor voltage, it enters the breakdown voltage region, and the small change of voltage will cause the rapid increase of current. This characteristic makes zinc oxide varistor materials widely used in overcurrent protection of various circuits. With the rapid development of integrated circuits, people are more and more inclined to low voltage and low power consumption, and the varistor with voltage less than 5V becomes more and more important. The pressure-sensitive characteristics of zinc oxide varistor come from its grain boundary effect (related to the number of interfaces, the more interfaces, the greater the pressure-sensitive voltage, and vice versa). Increasing the particle size of zinc oxide crystal or reducing the thickness of zinc oxide material is an effective way to reduce its varistor voltage. Nano-sized zinc oxide rod-shaped particles are used to manufacture sensors for measuring air components. Specific components in the air contact with various nano-materials on the sensor to generate corresponding electrical signals. Nano-zinc oxide doped with some elements is highly sensitive to organic vapors and harmful gases such as alcohol and acetone. It is often used for health testing, monitoring the alcohol concentration in human blood and monitoring the alcohol concentration in the atmosphere.
[Edit this paragraph] Use history
Humans have learned to use zinc oxide as paint or external medicine for a long time, but the history of human discovery of zinc oxide is difficult to trace back. In Cha Carabin, an ancient Indian medical work, a medicine which was later identified as zinc oxide was recorded for treating eye diseases and trauma. /kloc-in the 20th century, the Greek doctor Dior Scoori Des also mentioned using zinc oxide as ointment. Avicenna's Hui Hui prescription, completed at 1025, describes zinc oxide as the first choice for treating various skin diseases, including skin cancer. Nowadays, people no longer use zinc oxide to treat skin cancer, but it is still widely used in other common skin diseases. As early as 200 BC, the Romans learned to make brass by reacting copper with zinc oxide. Zinc oxide is converted into zinc vapor in the shaft furnace and rolled into the flue for reaction. Dior Scoori Des also introduced this point. Starting from the12nd century, Indians learned about zinc and zinc ores and began to smelt zinc in primitive ways. Zinc smelting technology was introduced to China in17th century. 1743, the first zinc smelting plant in Europe was established in Bristol, England. [3] Another main use of zinc oxide in ancient and modern times is paint, which is called zinc white. In 1834, zinc white became a watercolor pigment for the first time, but it was insoluble in oil. But this problem was soon solved by the new zinc oxide production process. 1845, leclerc began to produce large-scale zinc-white oil painting pigments in Paris, and by 1850, zinc-white was popular all over Europe. The purity of zinc white is so high that at the end of 19, an artist painted zinc white as the background color. However, a hundred years later, cracks appeared in these paintings. In the second half of the 20th century, zinc oxide was mainly used in rubber industry. In the 1970s, zinc oxide was used as an additive for copy paper, but in the 20th century, zinc oxide was eliminated as an additive for copy paper. At the same time, zinc oxide with fine grains began to expand its application in the field of nano-materials.
[Edit this paragraph] Security issues
Zinc oxide can be added to food for preservation or zinc supplementation, but the product requirements are strict, especially to control the content of harmful heavy metals. Zinc oxide itself is nontoxic, but inhalable zinc oxide particles are harmful. Zinc oxide dust may be produced in zinc smelting industry, brass preparation and zinc plating. In order to prevent smog, galvanized sheet cannot be directly welded, and the zinc film on the surface needs to be removed first.