An electrolytic capacitor is a capacitor where the dielectric layer is a metal oxide layer on the anode and the cathode is the electrolyte. The result is an extremely large capacity with a relatively high operating voltage, causing the popularity of such products.
History of the origin of electrolytic capacitors
The effect of electrochemical oxidation of a number of metals was discovered by the French scientist Eugène Adrien Ducretet in 1875 using the example of tantalum, niobium, zinc, manganese, titanium, cadmium, antimony, bismuth, aluminum and other materials. The essence of the discovery: when turned on as an anode( the positive pole of the power source), an oxide layer with valve properties grew on the surface. In fact, a similarity of the Schottky diode is formed, in selected works, n-type conductivity is attributed to aluminum oxide.
It turns out that the place of contact has rectifying properties. Now it is easy to assume further, if we recall the qualities of the Schottky barrier. This is a low voltage drop when turned on in the forward direction. For capacitors, low means impressive value. As for the reverse inclusion of electrolytic capacitors, people have heard about the dangers of such experiments. The Schottky barrier develops increased leakage currents, due to which the oxide layer begins to degrade immediately. A significant role is assigned to the tunnel breakdown. The flowing chemical reaction is accompanied by the release of gases, providing a negative effect. Theorists say that this phenomenon leads to heat.
The name of the invention of the electrolytic capacitor is 1896, when on January 14th Karol Pollak filed an application with the Frankfurt patent office. So, on the anode of the electrolytic capacitor, the oxide layer is built up under the action of a positive potential. The process is called molding, in the conditions of modern technology development lasts for hours and days. For this reason, the growth or degradation of the oxide layer is not noticeable during operation. Electrolytic capacitors are used in electrical circuits with a frequency of up to 30 kHz, which means the time of changing the direction of the current in tens of microseconds. During this period, nothing will happen to the oxide film.
At first, in Russian practice, industrial production of electrolytic capacitors was not considered economically viable. Scientific journals even considered how to set up production. Such notes include an article by Mitkevich( Journal of the Russian Physico-Chemical Society, Physics No. 34 for 1902).The electrolytic capacitor in question consisted of a flat aluminum anode and two iron cathodes located on the sides. The design was placed in a 6-8% solution of baking soda. Forming was carried out with a constant voltage( see below) 100 V to a residual current of 100 mA.
The first serious developments in the domestic ownership of capacitors with liquid electrolyte relate to 1931 and were created by the laboratory of P. A. Ostroumov.
The ability of valve metals with an oxide film to straighten the current varies. The tantalum quality is most pronounced. Perhaps due to tantalum pentoxide, characterized by p-type conductivity. As a result, a change in polarity leads to the formation of a Schottky diode connected in the forward direction. Due to the specific electrolyte selection, the degrading working layer of the dielectric can be restored right in the process. On this historical excursion is completed.
Production of electrolytic capacitors
Metals, oxides of which are characterized by rectifying properties, called valve by analogy with semiconductor diodes. It is easy to guess that oxidation leads to the formation of a material with n-type conductivity. This is considered the main condition for the existence of a valve metal. Of the above, only two have clearly pronounced positive properties:
The first is used much more often due to the relative cheapness and prevalence in the Earth's crust. Tantalum is used in extreme cases. The build-up of the oxide film occurs in two ways:
- The first method is to maintain a constant current. In the process of increasing the thickness of the oxide resistance increases. Consequently, a rheostat is included in the circuit in series with the capacitor during molding. The process is controlled by the voltage drop at the Schottky junction, if necessary, the shunt is adjusted so that the parameters remain constant. At the initial stage, the forming speed is constant, then an inflection point occurs with a decrease in the parameter; after a certain interval, the further growth of the oxide film proceeds so slowly that the technological cycle is considered completed. At the first bend, the anode often starts to spark. Accordingly, the voltage present is called analogously. At the second point, the sparking sharply increases, the further forming process is inexpedient. And the second bend is called the maximum voltage.
- The second method of forming the oxide layer is reduced to maintaining a constant voltage at the anode. In this case, the current decreases exponentially. Voltage is chosen below the spark voltage. The process goes to a residual forward current, below which the level no longer falls. Then the molding ends.
The correct electrolyte selection plays a large role in the molding process. In industry, this boils down to the study of the interaction of corrosive media with aluminum:
- Representatives of the first group of electrolytes, this includes boric, citric acid and borax, almost do not dissolve aluminum and oxide. Massively used in the manufacture of electrolytic capacitors. Long molding leads to a voltage drop of up to 1500 V, which determines the thickness of the dielectric layer.
- Chromic, sulfuric, succinic and oxalic acids dissolve alumina well, but do not affect the metal. A distinctive feature of the molding is a relatively thick dielectric layer. Moreover, with further expansion does not occur a significant decrease in current or voltage increase. Such a process is used to form electrical capacitors with relatively low performance( up to 60 V).Hydrates and salts of the acid used are mixed with aluminum oxide in porous structures. These processes can be used for protective purposes. Then the molding goes according to the previous scheme( the first group), and is completed as described. A protective layer of hydroxides protects the oxide from destruction during operation.
- The third group of electrolytes consists mainly of hydrochloric acid. These substances are not used in the molding process, they dissolve aluminum and its salts well. But willingly used for cleaning surfaces.
For tantalum and niobium, all electrolytes fall under the classification of the first group. The capacity of the capacitor is determined mainly by the voltage at which the molding is completed. Polyhydric alcohols, glycerin and ethylene glycol salts are used in a similar way. Not all processes follow the scheme described above. For example, when aluminum is molded in a solution of sulfuric acid using the direct current method, the following sections of the graph are distinguished:
- A rapid increase in voltage is observed for several seconds.
- Then, at the same rate, a decline to the level of about 70% of the peak was observed.
- A thick, porous oxide layer builds up during the third stage, and the stress grows extremely slowly.
- In the fourth section, the voltage increases sharply before the occurrence of a spark breakdown. Molding ends.
A lot depends on technology. The thickness of the layer, and therefore, the operating voltage and the durability of the capacitor, is influenced by the electrolyte concentration, temperature, and other parameters.
capacitor. Electrolytic capacitor design.
. The plates are usually not flat. For electrolytic capacitors, they are often coiled into a tube, coiled. On the cut, it resembles a Tesla coil with the consequences. This means that a capacitor has a significant inductive resistance, which in this context is considered parasitic. Electrolyte-impregnated paper or fabric is placed between the plates. The body is made of aluminum - the metal is easily covered with a protective layer, is not affected by the electrolyte and removes heat well( remember about the active component of the resistance of the anode).
These are dry electrolyte capacitors. Their key advantage in decent use of volume. There is no excess electrolyte, which reduces the weight and size at the same electrical capacity. Despite the characteristic name of the electrolyte is not dry, rather, viscous. They are impregnated with gaskets of fabric or paper, located between the plates. By virtue of the electrolyte viscosity, the body is allowed to be plastic or paper; a resin seal is used for sealing. As a result, the technological cycle of manufacturing products is simplified. Historically, dry electrolyte species appeared later. In domestic practice, the first mentions occur in 1934.
At the end of foreign electrolytic capacitors there are cross-cut notches through which the internal volume is squeezed out. This is in case of an accident. Such a damaged capacitor can be easily noticed with the naked eye and replaced in time, which speeds up the repair. Crash marking helps to avoid accidents and incorrect polarity. At the cathode on imported, a white stripe is drawn along the entire height, with minuses spaced apart, and for domestic ones, crosses( pluses) are on the opposite side.
To increase emissivity, body color is dark. Exceptions to the rule are rare. Such a measure increases the heat transfer to the environment. When the voltage on the worker( molding) is exceeded, there is a sharp increase in current due to ionization, a strong sparking on the anode develops, a dielectric layer partially penetrates. The consequences of such phenomena are easily eliminated in the design and with the housing used as a cathode: capacitors with liquid electrolyte occupy relatively much space, but they remove heat well. But perfectly manifested when working at low frequencies. What causes the specific use as a filter power supply( 50 Hz).
These cylindrical electrolytic capacitors are not arranged as shown above, without paper tabs. In some models, the case plays the role of a cathode, the anode is located inside, it can be of arbitrary shape so that the maximum nominal capacity is ensured. Due to mechanical processing and chemical etching, designed to increase the surface area of the electrode, the parameters can be raised by an order of magnitude. The design is typical for models with liquid electrolyte. The capacity of the structure under consideration varies when the industry releases from 5 to 20 µF at an operating voltage of 200 - 550 V. Due to the increase in the resistance of the electrolyte with decreasing temperature, capacitors with liquid electrolyte and casing are used as cathodes mainly in a warm microclimate.