Tantalum powder is powdered metal tantalum, dark gray, or silver-gray, which is an important raw material for the preparation of capacitors and tantalum materials. In this article, let’s take a deeper look at the tantalum powder preparation methods. Before going any further, let’s figure out the types of tantalum powder.
Capacitor tantalum powder is generally divided into three series: low-voltage series (working voltage is no more than 25V), medium-voltage series (working voltage is 32-40V), and high-voltage series (working voltage is no less than 50V).
Capacitor-grade tantalum powder is used to manufacture tantalum capacitor cores. Its quality is mainly measured by its physical properties, chemical composition, and electrical properties.
The physical properties of capacitor-grade tantalum powder mainly include average particle size, grain shape, fluidity, and forming density. Low-voltage series products should have a small average particle size, complex particle shape, and low forming density, while high-voltage products should have a larger average particle size, simple particle shape, and higher forming density. The fluidity determines whether the tantalum powder can be formed by an automatic forming machine. Therefore, with the rapid increase in the production of small chip tantalum capacitors, people have paid much attention to the fluidity of tantalum powder.
The chemical composition has a direct effect on the electrical properties of tantalum powder. The high impurity content, especially the high content of phosphorus, boron, oxygen, carbon, potassium, sodium, and iron, will increase the leakage current of tantalum anodic oxide film and decrease the breakdown voltage, thus making the electrical properties of tantalum powder worse.
However, the content of certain elements in tantalum powder is not as low as possible. It was found that the incorporation of certain selected elements can improve certain electrical properties of tantalum powder. It has been proved in practice that adding trace phosphates to tantalum powder can inhibit the shrinkage of tantalum block during sintering, thus increasing the specific weight capacitance of tantalum powder; adding trace aluminides with high dielectric strength can improve the breakdown voltage of tantalum powder.
Electrical properties refer to the DC leakage current, breakdown voltage, and capacitance of the tantalum anode block after the tantalum powder is weighed, compacted, vacuum sintered, and anodized. All these properties are not only determined by the intrinsic properties of the amorphous tantalum pentoxide anodic oxide film but also closely related to the physical properties and chemical composition of the tantalum powder.
Metallurgical-grade tantalum powder is used to prepare various tantalum materials or tantalum powder as alloy additives, so its quality is mainly measured by physical properties and chemical composition. As important physical performance indexes, the particle size should pass through a 150-mesh standard sieve, and the loose density should be 3～6g/cm3. In order to avoid the loss of chemical composition during vacuum sintering and melting, the oxygen content should be controlled at about 0.2%. Since tungsten, molybdenum, niobium, and other high melting point metals cannot be removed in the vacuum melting process, their content should be strictly controlled.
The methods for preparing tantalum powder include the metal thermal reduction method of tantalum pentoxide, carbon reduction method, molten salt electrolysis method, hydrogen reduction method of tantalum halide, metal thermal reduction method, and metal thermal reduction method of tantalum fluorine-containing complex, etc.
At present, the sodium reduction method of potassium fluorotantalate is mainly used in the industry to prepare metal tantalum powder, and the reaction formula is: K2TaF7+5Na=Ta+2KF+5NaF
The specific method is to mix K2TaF7 with NaCl or KCl in a certain proportion and put it into the reduction tank, under the protection of argon, inject sodium at 300-800°C and stir to reduce. After the reduction product comes out of the furnace, it is washed with water to remove fluoride and excess sodium, washed with hydrochloric acid to remove iron and other metal impurities, and washed with hydrofluoric acid to remove oxygen and silicon. Then, it is washed with water until it is neutral and dried to obtain the raw powder of sodium-reduced tantalum powder. After the raw powder is sieved, blended, and vacuum condensed, capacitor-grade or metallurgical-grade sodium-reduced tantalum powder is prepared.
The capacitor-grade tantalum powder prepared in this way is mainly used for low and medium-voltage series. This kind of sodium-reduced tantalum powder is pressed, vacuum sintered, and sintered into a sintered tantalum strip, and then hydrogenated and ground into a sintered tantalum powder. This capacitor-grade tantalum powder is mainly used for medium and high-voltage series. If the tantalum strip is first smelted into a tantalum ingot with a vacuum electron beam, then hydrogenated and ground, the electron-bombarded tantalum powder with the simplest grain shape is produced. This capacitor-grade tantalum powder is mainly used for high-voltage series.
Thank you for reading our article and we hope it can help you have a better understanding of the types and preparation methods of tantalum powder. If you want to know more about tantalum powder, we would like to advise you to visit Advanced Refractory Metals (ARM) for more information.
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