The anodization process generates alumina by electrolysis using the oxidation principle. Alumina, which spontaneously generates colored anodized film, has the function of anti-corrosion and anti-oxidation. Aluminum anodizing film can be divided into blocking type and multi-pass type. A compact barrier oxide film can be obtained by anodizing in nearly neutral electrolyte. This film insulates well and can be used to make capacitors and other devices.
Aluminum is an active metal, which spontaneously forms an oxide film of 0.01 ~ 0.10Lm in the air. This natural oxide film is amorphous, thin and porous, with low mechanical strength. Although it has a certain protection capability for aluminum, it is far from enough to meet people's requirements on the decoration, protection and functional application of aluminum and its alloy. Therefore, the process of anodizing aluminum in electrolyte has been developed continuously. Since the 1920s, the use value of aluminum anodized film has been increasing. Some recent developments will bear fruit in the 21st century.
Aluminum anodizing film
Aluminum anodizing film can be divided into blocking type and multi-pass type. A compact barrier oxide film can be obtained by anodizing in nearly neutral electrolyte. This film insulates well and can be used to make capacitors and other devices. When anodizing in acidic or weakly alkaline electrolytes, they can form multi-pass oxide film due to their ability to dissolve alumina. The membrane has a unique structure. Next to the surface of the metal aluminum is a thin and dense barrier layer on which a thick and loose porous layer is formed. The membrane cells of the porous layer are hexagonal closely packed, with a nano-sized micropore in each center. These holes are uniform in size and perpendicular to the surface of the matrix, and they are equal to each other.
Advantages of pore oxidation film
For a long time, people have paid more attention to the multi-pass oxide film with larger application and rapid development. Its advantages are as follows:
high hardness of the barrier layer can exceed corundum;
good abrasion resistance, corrosion resistance and chemical stability;
the morphology and size of the hole can vary within a larger range with different electrolytic processes, and the thickness of the film can be adjusted;
the preparation process is simple with low requirements on environmental conditions and equipment.
Although there is no unified explanation for the morphological changes of the two types of anodic oxide films: blocking type and multi-pass type. The concept of critical current density related to the morphology of the film was proposed based on the systematic study of ion migration in the formation of oxide film in solutions such as chromic acid, phosphoric acid and oxalic acid. If the anodic oxidation current density is higher than the critical current density, the barrier film will be formed. If it is below the critical current density, a multi-pass film is formed. It breaks the traditional view that the morphology of the membrane is closely related to the type of electrolyte.
Application of aluminum anodizing film
It was initially hoped that the aluminum anodized film would have good corrosion resistance, wear resistance and electrical insulation. By the mid-1930s, people began to be interested in the porous structure of aluminum oxide film and realized the precipitation of colored materials in the porous film. It was not until the 1960s that the electrolytic coloring of aluminum profile was officially used in production, making the color aluminum profile widely used.
In the last 10 years, many new achievements have been made in aluminum anodization technology. For example, some new measures have been taken to accelerate the anodization speed, some of which can increase the speed by 2 ~ 3 times. Another example is the new technology of oxidation at room temperature, which meets the requirement of cooling which consumes a lot of energy. The quality of the oxide film can be greatly improved by pulsed anodic oxidation. In addition, a series of advantages such as high efficiency, low cost and power saving can be obtained by using alternating current oxidation. However, its wide application is affected by the thin film layer (less than 10Lm), yellow color and low hardness. By adding additives recently, the film quality has reached the level of direct current electrooxidation. These new developments have made the process of aluminum anodizing remarkably updated and improved. I believe that in the new century, this work will make new breakthroughs. However, since the late 1980s, the most interesting technological problem of aluminum anodization has been the development and research of various functional membrane materials for the porosity of aluminum oxide film. As the pore size of alumina film is only a dozen or dozens of nanometers, it can play an important role in the demand of various nanomaterials. That is to say, a great work on the nanometer micro-pores of the membrane will make the aluminum anodization technology rejuvenate in the 21st century, and become a promising new thing with the high-tech matching.
At present, the research on making aluminum oxide porous membrane develop towards functionalization mainly starts from two aspects. One is to use its porous structure to develop new ultra-precision separation films; Another is to prepare new functional materials by depositing materials with different properties, such as metals, semiconductors, and polymers, in their nano-sized micropores.
In the first type mentioned above, there are few oxide films. For example, in preparation for membrane separation of aluminum anode oxide film, aluminum is first anodized in acidic electrolyte to form a layer of oxide film on the surface of aluminum, and then the aluminum matrix and barrier layer on the back of the film are removed by electrochemical or chemical methods to obtain a precise separation film.
In the preparation process, the shape, arrangement and size of the holes are required to be uniform, and the size of the holes can be adjusted as required. Compared with various organic separation films, this kind of membrane has better mechanical strength, heat resistance, chemical stability and dimensional stability. It can be used as the separation membrane of gas, liquid and blood at room temperature, and can also be used for the separation of high-temperature gas, such as the deoxidation and desulfurization of flue gas.
The second type of oxide film described above has a number of variations, especially in optical and photoelectric devices. When the light is irradiated to the aluminum anodized film in the direction of the parallel membrane surface, due to the single directivity of the porous structure of the film, H polarization and V polarization will be attenuated to different degrees, resulting in the anisotropy of the electromagnetic field of the light and affecting the polarization characteristics of the light. A variety of materials with different optical properties were precipitated in the nano-sized micropores of the porous membrane, and various kinds of polarizing photons, optical phase plates and optical elements for optical communication were developed according to their different effects on the polarization characteristics of light. For example, if the three elements, A u, A l, N I are deposited into the micropores of A porous membrane, 1 Lm of membrane thickness can meet the requirement that the sold edge crystal biophotes are over 1 mm.
Fluorescence materials, photosensitizers and so on are filled in the nano-sized pores of the oxide film of aluminum. For example, by combining soaking with heat treatment, Tb3+ can be introduced into the micropores of porous membrane, and then green light can be generated under the action of external electric field. This kind of functional porous membrane will be a new way to develop the photoelectric element. As the hole of the membrane is at the nanometer level, it can be further developed into ultra-fine luminous element.
Secondly, aluminum oxide film can be made into magnetic film. Magnetic materials (such as Fe, Co, N I and magnetic alloy) can be filled in the holes of alumina film by vacuum deposition and electrodeposition, and then the film with magnetic function can be made. It has a broad application prospect. For example, it can be used in making various magnetic cards, magnetic tape, disk and so on. The results show that the shape of magnetic metals deposited into the nano-sized pores of porous membrane can be elongated by the special structure of aluminum anodized film. In addition, the preferred orientation of magnetic metal crystallization is generally consistent with that of its magnetic axis. The magnetic film formed in this case shows high magnetic protection and typical vertical magnetization characteristics. So it can be used as the vertical magnetic recording medium. The results of the study on Fe composite magnetic film show that the thinner the composite magnetic film is, the higher the overwriting characteristics and the density of the magnetic recording medium is. Therefore, it is possible to obtain high vertical magnetic recording density by using the special structure of nano-sized micro-pores of aluminum anodized film.
Third, the aluminum oxide film used in the solar energy selective absorption film, is also distinctive. Solar energy is one of the most important sources of energy in the future. All energy problems on earth can be solved by using 1/10000 of the solar energy received on earth. Therefore, the study on the comprehensive utilization of solar energy has attracted more and more attention in the world. The study on the preparation of solar energy absorber by functional treatment of alumina porous membrane has shown good application prospect.
In order to use solar energy effectively, the material of solar absorber film is required to have a higher absorption rate in the solar radiation spectrum, while the emission rate in the thermal radiation spectrum should be as small as possible. For example, in the nano-sized micro-pores of alumina porous membrane made by phosphoric acid solution, Ni was electrodeposited to make functional membrane with selective absorption of solar energy. By measuring the reflectance, it is found that this kind of film has ideal selective absorption characteristics.
Electrodeposition of Fe, Ni and other metals into the pores of the film can make the film heat-resistant obviously stronger than the selective absorption film prepared by other materials. However, the corrosion resistance of the coating is not good enough. It is expected to be improved by sealing the hole or coating the film surface with corrosion resistant coating and changing the surrounding environmental conditions.
Because of its high signal-to-noise ratio (SNR), the beam microelectrode has attracted much attention in recent years. There are many methods to prepare beam microelectrodes, and the minimum diameter of a single microelectrode is required to reach 0.1Lm. Obviously, the smaller the area of the active electrode is, the higher the signal-to-noise ratio is. Therefore, how to minimize the area of the active electrode has become the key to the preparation of high-performance beam microelectrodes. Alumina porous membrane has nanoscale microporous structure, which provides favorable conditions for preparation of high-performance beam electrode. During the preparation, the aluminum sheet can be anodized to form A porous membrane, and then the porous membrane can be separated from the aluminum matrix. The metal (such as A u, P t, etc.) can be deposited in the nanoscale micropores by vacuum deposition and other methods, and its surface can be connected with the conductor to remove the barrier layer in the oxide film. Then the beam microelectrode can be obtained.
By using the excellent thermal conductivity of matrix aluminum and the maximum internal surface area of the micro pores in the anodized aluminum film on the surface, a thin film with both high thermal conductivity and good value properties can be developed. For example, Pt is a good catalyst for many chemical reactions. There is an experiment. The aluminum anodized film was impregnated in the hot H2PtCl6 solution, and after being air-dried, it was then cooked and burned to form a P/Al2O3/Al catalytic film. The experimental results show that the film has good thermal conductivity and catalysis.
There are, of course, other areas where it is possible to use porous membranes that are anodized with aluminum. For example, after anodizing aluminum, MoS2 is deposited in the film hole, forming a golden oxide film with good self-lubrication. The liquid crystal oxide composite film can be made by filling the hole of aluminum oxide film. The liquid crystal can be used to separate and concentrate oxygen by its selectivity and arrangement control. In addition, the porous membrane of alumina as a core membrane can be duplicated with the same structure and different materials (such as metal, semiconductor, polymer, etc.) by means of vacuum deposition, electrodeposition and impregnation. These porous membranes with different materials have broad application prospects in many fields.