Electrolytic capacitor anodic oxide film growth technology

☀️It can be seen from formula (4-9) that if E is independent of temperature, then ꞇ is proportional to temperature. However, experiments show that in the range of 0~80℃, the ꞇ value of aluminum oxide film has little relationship with temperature, and the same is true for niobium oxide film in the range of 20~60℃. This is because E, which includes space charge effects, is also temperature-dependent.

⛅️The above analysis shows that the interface barrier between the metal and the oxide film only plays a leading role in the control of the oxide film growth, and the final energization speed of the oxide film mainly depends on the state of the oxide film itself.

❄️At high field strength, the relationship between Igj and E appears nonlinear. This phenomenon is shown in Figure 4-4. Therefore, formula (4-1) needs to be modified as follows to make it more in line with the actual situation.

☃️In the formula: j0, U, a, B and other constants are related to the properties of the anodic oxide film of a certain metal.

1.2 The enabling voltage limit and flash voltage of the anodized film

⛄️When the voltage is continuously increased while the energizing current density remains unchanged, the curve shown in Figure 4-5 can be obtained. In the electrolytic cell, with the increase of the energizing voltage, the thickness of the anodic oxide film will increase (0-1). If you continue to boost the voltage under the premise of keeping the current density unchanged, the energization speed will be slower.

🌊The drop is accompanied by an intensifying flash-fire phenomenon occurring on the anode, accompanied by the bubbling of oxygen. Electrolytic capacitor anodic oxide film growth technology.The flash fire is because the oxygen ion concentration increases to the potential where electrons can be released, and the free electrons are accelerated into the film under the action of the electric field, hit the oxide film structure, and develop into local electrical breakdown.

🍏On the other hand, the more bubbles that emerge, the larger the proportion of oxygen ions supplied by the electrolyte that fail to combine with metal ions, the lower the energizing efficiency, and the lower the energizing speed. When a certain voltage is reached (“2” in the figure), the spark intensity increases, and a “crack” sound can be heard during the flash fire. At this time, the voltage cannot continue to increase, and the energization is terminated. The voltage at this time is called the limit The forming voltage, also known as the breakdown voltage. The forming voltage cannot exceed this value.

☔️The valve metal oxide film flash voltage (U flash) has the following main factors:

(1) The properties of the valve metal itself.

(2) The composition of the energizing liquid. Electrolytic capacitor anodic oxide film growth technology.When chlorine, bromine, iodine and other non-oxygen anions exist in the electrolyte, because of their strong activation ability and lower potential than oxygen ions, they have a strong ability to release electrons, which reduces the flash voltage. And can also destroy the oxide film. The order of ions that deteriorate the dielectric properties is

3) The concentration of the enabling solution. Within a certain concentration range, the more concentrated the electrolyte, the lower the resistivity. Since the higher the resistivity is, the higher the flash voltage is, so the electrolyte with a higher concentration generally has a higher flash voltage. As shown in Figure 4-6.

🍐It should be noted that the flash voltage value is independent of the surface morphology of the foil. The clear foil has the same flashover voltage as the etched deep hole etched foil.

1.3 Thickness measurement of energized oxide layer

🍊Because the oxidation itself is very thin, only tens of nanometers or hundreds of nanometers in thickness, and it is in close contact with the valve metal, and the general valve metal surface is intentionally corroded to cause microscopic unevenness, so it is not feasible to directly mechanically peel off and measure the thickness. There are roughly four kinds of thickness measurement methods that can be used in actual operation: electricity method, weighing method, capacitance measurement method and ellipsometry method. These methods need to know some quantity constants of the anodic oxide film in advance, such as dielectric constant, refractive index, and even the refractive index of the valve metal itself, as well as the molecular formula and molecular weight of the oxide film, in addition to the current efficiency of the film energization process and the amount of oxygen released circumstances must also be considered.

1.3.1 Electricity method

🍋According to Faraday’s law of electrolysis, the volume of oxides produced at a constant current density is proportional to the total charge. Electrolytic capacitor anodic oxide film growth technology.If the electrode surface area A is known, its thickness d is

🍌In the formula: Q is the total amount of electricity; M is the molecular weight of the oxide; n is the current efficiency (if there is no oxygen release, it can be assumed to be 100%); z is the valence of the metal oxide; F is the Faraday constant: p is the oxide’s density. The measurement accuracy of this method depends to a large extent on the accuracy of p and A.

1.3.2 Weighing method

🍉Measure the quality of the electrode before and after energization (used to be called “weight” in engineering), and the incremental part is the quality of the oxide film. If the oxide composition and density are known, it can be calculated as follows

🍇In the formula, w 1 and W 2 are the weight of the anode electrode before and after energization, respectively; A is the surface area of the electrode; p is the density of the oxide. Its measurement accuracy depends on the measurement accuracy of the weight.

1.3.3 Capacitance method

The energized AC capacity can be accurately measured. If the relative permittivity and electrode area of the oxide film are known, its thickness can be calculated as follows:

 

🥝When the energizing electrolyte is selected and the temperature is constant, no matter what current density boost is used (within a reasonable range), the structure of the energized oxide film is basically unchanged, that is, it is constant. Therefore, when the valve metal anode material and its effective surface area are unchanged, the product of capacitance and energization voltage (CU value) for different capacitances is a constant. This conclusion has an important guiding role for the selection of aluminum foils with different corrosion hole sizes for energization.

🍅In order to ensure that the capacitor can work reliably, the general enabling voltage is proportional to the working voltage (or rated voltage U ,). The proportional coefficient of aluminum is generally between 1.3 and 1.5, and the proportional coefficient of tantalum is between 3 and 5. Under the condition that the ratio of the energizing voltage to the working voltage of the electrolytic capacitor remains unchanged, there are

That is, the product of capacitance and working voltage (rated voltage) is also a fixed value.

🍆The CU value can be used to compare the enabling performance of different valve metal materials. From the CU values of valve metal oxide films in Table 4-4, it can be seen that under certain energizing conditions, the product of different valve metals is quite different. This data can be used as an important reference for the development of new anode materials.

🥑In actual situations, different corrosion processes and even different corrosion production lines are not completely fixed values, and the most appropriate energizing voltage should be selected according to the actual situation.

1.5 The relationship between the structure of the energizing oxide film and the energizing solution

🥦Alumina often exists in three crystal forms. After high temperature treatment, alumina is a-type, which belongs to orthorhombic system, and the crystal structure is the most compact; at lower temperature, Y-type can be obtained, which is a cubic crystal system with spinel structure. , the density is slightly lower than that of type a, and it can also be converted to type a at high temperature: there is also β type, which belongs to the hexagonal crystal system, with the lowest density and the worst hardness, in which a small amount of other alkaline oxides (such as ,0 and O). In addition, according to the X-ray diffraction refinement analysis, it is found that there are ɧ- (equiaxed crystal system), p- (crystal system uncertain) X- (hexagonal crystal system), δ- (tetragonal crystal system) θ- (monoclinic crystal system) ) are equivalent.

🥬The aluminum anodic oxide film is energized in different types of electrolytes, and the oxide film layers with different structures will be obtained. According to the electrolysis results of aluminum in it, the electrolyte can be divided into three categories: the first category is the electrolyte that neither dissolves aluminum nor dissolves the oxide film, generally it is an oxyanion weak acid or weak acid ammonium salt, such as boric acid (ammonium), Adipic acid (ammonium), citric acid (ammonium); the second type is the electrolyte that can both dissolve the oxide film and generate the oxide film, generally an oxyanion strong acid or medium strong acid, such as sulfuric acid, phosphoric acid, oxalic acid; the third The type is an electrolyte that does not grow an oxide film and dissolves aluminum, generally an anionic acid or salt that does not contain oxygen, such as hydrochloric acid, halide. Because energization requires the growth of an oxide layer, the energization process does not use a third type of electrolyte. The first type of electrolyte can energize the oxide film with a dense structure, and the second type of electrolyte can obtain an oxide film with a porous structure.

1.5.1 The first type of electrolyte that dissolves neither aluminum nor oxide film

🥒The first type of electrolyte that does not dissolve aluminum and aluminum oxide film, the most common are boric acid and borate aqueous solution, as well as adipic acid and adipic acid salt aqueous solution, azelaic acid and azelaic acid salt aqueous solution used in recent years, in addition It is an organic acid with a hydroxycarboxylic acid HO-0 COOH group and its salt aqueous solution, such as citric acid, tartaric acid, and salicylic acid.

🥕If energized at room temperature or above, an amorphous film will be obtained, whose structure is mainly amorphous and contains very small amounts of aluminum oxide, crystallites and aluminum hydroxide, which is different from natural The structure of the oxide film is similar. If it continues to be heated to about 100 °C, the energized amorphous film will be transformed into a structure type dominated by aluminum oxide crystalline phase.

🧅If it is energized at a temperature of about 95 C, it can directly generate a dense oxide film structure mainly composed of Al2O3 crystal, which is suitable for the working medium of electrolytic capacitors. In the manufacturing process, sometimes after energization and drying of moisture, high temperature heat treatment at about 400~500 ℃ is carried out to completely convert the film structure into a crystalline type to meet the needs of high working voltage capacitors. For low-voltage capacitors, considering that the oxide film may be thickened due to heat treatment, the capacitance may decrease, and some do not undergo high-temperature baking, but the capacity stability will be improved after such high-temperature treatment.

🥔Sometimes the energized aluminum oxide film is not sufficiently resistant to hydration, and the aluminum oxide may dissolve and form an aluminum hydroxide layer. It has the properties of a porous film, which can make the insulating properties of the oxide film lose. Correspondingly, the capacitor will gradually lose its capacitance and the leakage current will also increase sharply. The general solution is to perform high temperature heat treatment after the membrane is energized to convert the abdominal layer into an aluminum oxide membrane. In addition, a small amount of phosphates can also be added to the energizing solution or immersed in a phosphate solution after energization. Since aluminum ions in the oxide film react with phosphate radicals to form aluminum phosphate, oxygen-containing substances (such as water) can be hindered. The migration of the oxide film inside, so that the porous aluminum hydroxide cannot be generated, significantly improves the hydration resistance and moisture resistance of the oxide film. In addition to phosphates, commonly used anti-hydration inhibitors include silicates, arsenates, periodates and tungstates, etc., but generally lower-cost phosphates are used in the production of aluminum electrolytic capacitors.

1.5.2 The second type of electrolyte that can both dissolve and grow oxide films

🍠In some electrolytes such as oxalic acid, sulfuric acid, and phosphoric acid, two opposite reactions of dissolving the oxide film and energizing the oxide film occur simultaneously, that is,

(1) Anodizing process of aluminum:

(2) The process of aluminum oxide being dissolved:

🥐At the beginning, a dense aluminum oxide film will be formed on the surface of aluminum, but due to the dissolution of the electrolyte, many parts of the surface of this dense film begin to corrode, and the current is concentrated here to try to repair these local damages, which makes the electrolyte nearby. The temperature is higher than the general part, but it makes the dissolution

Acceleration, the result is many small holes. As shown in Figure 4-9.

🥯The electrolyte can continue to contact the aluminum through the holes and continue to energize the oxide film. In this way, the oxide film grows on one side and is etched on the other side, and the final thickness of the film is determined by the balance relationship between these two processes. The energized oxide film through this process is full of honeycomb-like pores. Its structure is roughly like many hexagonal prisms (single cells), each single cell consists of a central hole and pore walls and dense The bottom of the hole is formed, as shown in Figure 4-10.

🍞The size of a single cell is related to the type of electrolyte, energizing voltage and temperature, but has little to do with the length of energizing time. The size of the single cell has a linear relationship with the energizing voltage. When the voltage increases, the size of the single cell increases, and the number of holes decreases accordingly. The volume occupied by the holes and its energizing voltage have the relationship in Figure 4-11. Raising significantly reduces the pore volume. In addition, the pore volume also increases with the increase of temperature, decreases with the increase of current density, and is related to the type of electrolyte and its concentration. When energized in oxalic acid or chromic acid and phosphoric acid, the number of pores is much less than that in sulfuric acid, but the diameter of the pores is larger. The diameter of the hole is mainly determined by the type and temperature of the electrolyte, and has nothing to do with the magnitude of the energizing voltage. For example, it is about 6~12 nm in sulfuric acid, but it can reach about 30 nm in other acids.

🥖Since a lot of heat is released during the energizing process of the porous film, the pore wall tends to locally form a hydrated oxide film. Partially hydrated porous membranes with a “crystal green” structure, the thickness of which can even reach the micron level, but still retains a very thin dense membrane under the porous membrane. In the manufacture of electrolytic capacitors, this step is called “pre-energization”. Electrolytic capacitor anodic oxide film growth technology.On this basis, the next step is continued to generate a dense oxide film corresponding to the energizing voltage value. This process is commonly known as “enabling”.

🌭The pre-energized porous film can protect the dense film generated by the subsequent energization process from mechanical damage, and isolate the working medium from direct contact with external operating objects; it can absorb the working electrolyte and help to repair the dielectric film; It can reduce the dissolution of the dense film in the working electrolyte

🍔Therefore, the service life of the capacitor is extended and the growth rate of the leakage current is reduced; it can also shorten the energization time and reduce the power consumption, and shorten the aging time to a certain extent. Then adding this process also increases the production cost. On the other hand, if the thickness of the porous film layer is too large, the anode surface area and the corrosion coefficient value will be greatly reduced, so that the capacitance will drop significantly. Therefore, low-voltage capacitors do not use a pre-energizing process, and general high-voltage capacitors are rarely used. An exception is high-voltage bulk electrolytic capacitors for energy storage. The country guarantees reliability because it needs to withstand the electric shock of hundreds of thousands of repeated charging and discharging.

🥪It is worth mentioning that the porous film aluminum foil obtained by pre-energizing in oxalic acid or sulfuric acid, if treated with boiling water, the porous film will be transformed into a larger diaspore film: it will make countless small particles on the film. The holes are filled and become a sealing oxide film.

🍭The study found that if the hydrated sealing film continues to be energized in the first type of electrolyte, the obtained composite film has better performance than the pure oxide film, and can operate in a wide temperature range (-100~+150℃) It maintains quite good capacitance address stability, and the insulation quality is better than that of paper capacitors. In addition, compared with the dense film, the oxide layer of the gift film can grow thicker, so this station capacitor can work at a higher voltage (hundreds of volts), and it also has a self-healing effect. However, the purity requirements of raw materials and water in this process are very strict, so the cost issue needs to be weighed.

1.6 Corrosion of hydrated oxide film on the surface of aluminum foil

🍿A hydrated oxide film can be formed by the action of metal aluminum and moisture. After the uncorroded aluminum foil is annealed, the surface will accelerate the formation of this oxide, and the corroded aluminum foil will also form this hydrated oxide film in deionized water.

1.6.1 The structure of the hydrated oxide film

🍩The structure of the hydrated oxide is closely related to the water temperature: if the water temperature is lower than 75 °C, a film mainly composed of (water alum) or amorphous aluminum hydroxide will be formed, and its insulation performance is not good; if the water temperature is higher than 75 °C Above ℃, a thin layer mainly composed of (water and aluminum in or in the form of AI100H) and other reaction products will be formed. With the increase of the water temperature and the prolongation of the reaction time, the thickness of the hydrated oxide film and the percentage of the diaspore content will increase. between 300mm. The higher the temperature, the more diaspore content and the less content. If the film prepared in 155 ℃ water vapor, its diaspore content is the most.

🍪Because diaspore is a relatively insoluble oxide film, it can provide corrosion protection to aluminum foil before and after energization, and can shorten the energization time and improve the performance of capacitors.

1.6.2 Influence of impurities

🌰A small amount of impurities in deionized water can have a great impact on the performance of the hydrated oxide film. The general impurities are mainly chloride ions and silicon dioxide, containing more than 2 ppm (2X, 2 parts per million), the film energized in the water, the performance of the capacitor will be unstable, and the gas ions will affect the performance of the hydrated oxide film. impact is more pronounced. Therefore, in order to ensure that the pH value of deionized water is between 5 and 6, it is generally adjusted by adding airless phosphates to make the hydration film grow well.

1.6.3 Effects on medium and high voltage foils

🥜For high-voltage and medium-voltage (above 160 V) capacitors, if a thick hydrated film has been formed on the etched aluminum foil before energization, the dielectric film will grow under the hydrated film during energization, and part of it will grow. The hydrated membrane is converted into a dense membrane, which not only reduces the current dissipation during energization energy consumption, improve the energization efficiency, and the energized composite film is higher than the pure dense film in terms of electrical strength. In order to achieve this effect, the formation of the hydration membrane should be as close as possible to the energizing process, otherwise the hydration membrane will gradually “age” after a long time, and it is difficult to completely transform into a dense film in which some water molecules or hydroxides remain. They absorb energy under an AC electric field (DC superimposed AC), increasing the capacitor losses. With the hydrated membrane, the energization of the high-voltage anode foil does not need to be pre-energized, and there is no effect on the corrosion coefficient.

1.6.4 Effect on low voltage foil

🥎The presence of a hydrated membrane is disadvantageous for low-voltage anode foils. Since it is naturally generated in the process and in a humid environment, even after etching and cleaning, the thickness is generally greater than 10 nm, and this thickness can reduce the voltage by about 2V (1~1.4 nm/V). This makes it very difficult to fabricate large-capacity anode foils with extremely low operating voltages (below 3V for enabling voltages).

1.6.5 Influence on the negative electrode foil

🎾When the negative electrode foil is stored in humid air, a hydrated film of about 10-30nm will be formed, and this part of the hydrated film layer will lead to a significant decrease in the negative electrode capacitance.Electrolytic capacitor anodic oxide film growth technology. In capacitors, the effective capacitance of low-voltage capacitors is greatly reduced due to the series relationship between the negative electrode capacitance and the anodic oxide film capacitance. Overdose must be prevented.

1.7 Electrolytic capacitor anodic oxide film growth technology：Rectification effect of anodized film

🌋The anodic oxide film energized on the valve metal has unidirectional conductivity. When the anode metal is connected to the “ten” of the power supply voltage (the negative electrode of the capacitor is connected to the “one” pole of the power supply), the current value is very small, correspondingly, the insulation resistance of the oxide film can be considered to be large; but when the one “pole of the power supply voltage is connected to the “one” pole of the power supply After the anode metal, the current is very large, the capacitor continues to heat up, and eventually fails due to damage. At this time, it can be considered that the insulation resistance of the oxide film is very small. This is the conductance asymmetry of the oxide film, also known as “unidirectional conductivity” “”rectification effect” “has polarity and so on.

There are two main theoretical explanations for the rectification effect.

1. The theory of p-n junction of anodic oxide film

💰The p-n junction concept of semiconductors can be applied to the anodic oxide film.Electrolytic capacitor anodic oxide film growth technology. Both P-n exist at the interface between the oxide film and the electrolyte. The entire anodic oxide film is an n-type semiconductor, and excess metal atoms are distributed asymmetrically in the film. Near the outer surface of the film, the composition of the film conforms to the stoichiometric ratio of the oxide film.

⏳When the base metal is connected to the positive electrode of the power supply, the electrolyte provides a very thin oxygen ion layer (ie, an electric double layer composed of anions) at the oxide film/electrolyte interface, which can be considered as a P-type semiconductor with hole conductivity. A P-n junction is formed. The electrons in the n-type region and the holes in the p-type region will diffuse to each other, so that the thin layer lacking electrons in the n-type region is positively charged, and the thin layer lacking holes in the p-type region is negatively charged, and a thin layer is generated between the two thin layers. The electric field forms a blocking layer to prevent the continued diffusion of electrons and holes, as shown in Figure 4-12.