Tantalum powder

Electrolytic Capacitors. Tantalum, because of its high melting point of 2850°C, is produced as a metal powder. As such, it is molded, sintered, and worked to wire and fod, and used to budd certain types of tantalum capacitors (51). Other capacitors are made by compacting and sintering the tantalum powder. 

Fig. 3. Schematic of a reactor used to produce tantalum powder by the sodium reduction process.

The cooled reaction mass is extracted from the retort, cmshed and leached first with dilute mineral acid, and then with water to separate the tantalum powder from the salts. After drying and classification, the primary powder is ready for processing to sheet, rod, wire, or capacitor-grade powder. 

The dramatic improvements in the physical and chemical properties of tantalum powder produced by the sodium reduction process are evident in the lessening of chemical impurities (see Table 5). The much-improved chemistry reflects the many modifications to the process put in place after 1990. 

Table 5. Comparison of Impurity Levels in Tantalum Powders...

Post-Reduction Processing. The primary tantalum powder produced by the sodium reduction process is treated to convert the metal to a form suitable for use as capacitor-grade powder and feedstock for wire and sheet. 

Fig. 4. The moiphology of melted (a) and sodium-reduced (b) tantalum powders.

Wire. Tantalum wire is used primarily as the anode lead wire in soHd tantalum capacitors. Since the 1970s, the average weight of tantalum in a sohd tantalum capacitor has dropped from several hundred milligrams to less than 50 mg but the consumption of tantalum powder for capacitors has remained relatively constant because of the dramatic increase in the number of capacitors manufactured. The weight of wire per capacitor has remained relatively constant and thus wire consumption has increased steadily. 

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

Tantalum and tantalum alloys react with hydrogen, nitrogen and oxygen at temperatures above 300°C. Hydrogen is dissolved in the metallic matrix above 350°C and evolved at higher temperatures of about 800°C . The dissolved hydrogen embrittles the tantalum and its alloys. This effect can be used to prepare tantalum powder. 

Tantalum production has increased steadily and strongly since 1993. An optimistic forecast regarding the strongly increasing demand for tantalum capacitors caused excessive demand for tantalum powder in 2000, when the overproduction of capacitors led to a sharp shortage in tantalum powder. It is still difficult to predict when the electronics industry will return to balanced condition. 

Another application of tantalum strip solution is in the precipitation of potassium fluorotantalate, K2TaF7, which is used as a precursor in the production of tantalum powder by sodium reduction of melts. 

In the second part of the 20th century, the tantalum capacitor industry became a major consumer of tantalum powder. Electrochemically produced tantalum powder, which is characterized by an inconsistent dendrite structure, does not meet the requirements of the tantalum capacitor industry and thus has never been used for this purpose. This is the reason that current production of tantalum powder is performed by sodium reduction of potassium fluorotantalate from molten systems that also contain alkali metal halides. The development of electronics that require smaller sizes and higher capacitances drove the tantalum powder industry to the production of purer and finer powder providing a higher specific charge — CV per gram. This trend initiated the vigorous and rapid development of a sodium reduction process. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

Similar results were reported by Freidin et al. [568]. Moreover, a correlation was reported [360] between the particle size of tantalum powder obtained by electrolysis of fluoride - chloride melts and its electric conductivity. 

The main reason for the tantalum industry s drive toward the sodium reduction process is an increasing demand for tantalum powder by tantalum capacitor manufacturers. The modem tendency of the electronics industry to miniaturize their components calls for the improvement of tantalum powder... 

Despite the above disadvantages, some investigations show possible directions for further improvement and development of the process for the production of tantalum powder suitable for the manufacture of capacitors with no additional electron-beam melting and special crushing. 

Tantalum powder with a smaller particle size compared with that obtained by the regular process was obtained by thermal treatment at 700-950°C of a mixture containing KTaF6, NH4F and sodium [581]. Eighty three percent of the tantalum powder obtained appears to be smaller than 200 mesh. 

In order to produce coarse-grained tantalum or niobium powder it is recommended to perform the reduction of molten K2TaF7 or K2NbF7 with sodium or potassium in the presence of tantalum or niobium metal particles, which are added to aid nucleation [582]. It is reported that tantalum powder with an average particle size of 2.7-4.2 pm was obtained at a yield of 90.1-94%. 

The impact of the excess sodium on the properties of tantalum powder obtained by direct reduction of K2TaF7 with sodium in the stainless steel bomb reactor was investigated by Yoon et al. [583]. It was shown that the yield and amount of fines strongly depend on the amount of excess sodium present. With the increase in sodium excess, the proportion of the fine powder fraction (approximately 325 mesh) decreases appreciably and the yield of the process... 

The reduction of K2TaF7 can also be performed using sodium vapors [584]. This process is conducted at an Na pressure as low as 0.1 torr, which enables the removal of interferring gases such as N, O and H20. The interaction begins at 350°C. The temperature further increases up to 800°C to prevent the condensation of sodium and the formation of colloidal tantalum powder. The product of the interaction is removed from the reactor after cooling and treated with boiled HC1 and HF solutions. The method enables the production of coarse grain tantalum powder with 99.5% purity. 

The leaching process aims to remove the salts from the metal - salt mixture but also enables to achieve additional purification of the tantalum powder. Keller and Martin [586] found that the application of a leaching solution containing 0.1-10% HF and 0.5-10% H202 leads to a decrease in the oxygen content of the final tantalum powder obtained from the reduction of K2TaF7 with sodium. 

Significant improvement of tantalum powder properties was achieved by the application of molten alkali halides as solvents for potassium heptafluorotantalate, K2TaF7. Variation of the initial concentration of K2TaF7 in the melt, stirring and rate of sodium loading enable a well-controllable production of tantalum powder with a wide variety of specific charges. Heller and Martin [590] proposed the use of a reactor equipped with a stirrer in 1960. Fig. 142 shows a typical scheme of the reactor [24, 576]. All metal parts of the reactor are made of nickel or nickel alloy. 

Final product - capacitor grade tantalum powder... 

The oxygen level in primary tantalum powder can be also increased by adjustment of the reduction process parameters [594] or by controlled stepwise additions of sodium to the reactor [595]. 

The concentration of K2TaF7 in the initial melt is the main parameter controlling the particle size and surface area of the reduced primary powder [598]. Typically, the increased concentration of K2TaF7 leads to the formation of coarse tantalum powder. According to Yoon et al. [599], the diluent prevents a strong increase in the temperature of the melt that is caused due to the exothermic effect of the reduction process. Based on the investigation of the reduction process in a K2TaF7 - KC1 - KF system, it was shown that increased amounts of diluent lead to a decrease in particle size of the obtained tantalum powder. 

Yoon et al. [600] investigated the influence of the process temperature on the yield of tantalum powder and amount of fine fraction obtained. The reduction was performed using K2TaF7 dissolved in a KC1 - KF melt. The melt temperature varied in the range of 800-980°C. The analysis of the obtained results shows that the yield of the process increases at higher temperatures. In addition, higher temperatures lead to a decrease in the amount of fine fraction of tantalum powder produced. Nevertheless, it was noted that solely changing the process temperature does not ensure an improved yield. 

Kim et al. [601] investigated the influence of both temperature and the excess amount of sodium compared with the stoichiometry of the interaction. The molten system K2TaF7 - KC1 - KF was used for the experiments and the temperature varied in the range of 800-980°C. The excessive amount of sodium ranged from -10% to +10%. It was found that increasing either the temperature or the excess amount of added sodium led to an enhanced yield and increased the particle size of the tantalum powder. Optimal conditions were found to be 920°C and 5% excess reductant. 

Based on available results, it can be summarized that the particle size of tantalum powder increases (specific charge decreases) with the increase in temperature, K2TaF7 concentration and excess sodium. In addition, an increase in the specific surface area of the melt and Na/K ratio also leads to the formation of coarser tantalum powder. The most important conclusion is that for the production of finer tantalum powders with higher specific charges, the concentration of K2TaF7 in the melt must be relatively low. This effect is the opposite of that observed in the electrochemical reduction of melts. 

Higher temperatures, increased K2TaF7 concentrations and other factors mentioned above shift the equilibrium in Equation (174) to the left and lead to the formation of coarser tantalum particles. Form this point of view, it can be concluded that smaller hexacoordinated complexes, TaF6 lead to the formation of coarser tantalum powder, whereas the predominant presence of larger heptacoordinated complexes ions initiates the formation of finer particles. 

The uniformity of tantalum powder is also a veiy important parameter of capacitor-grade tantalum powder. The loss of powder uniformity can initiate during the regular reduction process due to varying conditions at the beginning and end of the reduction process. At the end of the process, the concentration of tantalum in the melt is very low, while the sodium content increases. Based on the complex structure model of melts, it should be noted that the desired particle size of the powder is formed at the veiy beginning of the process, while the very fine fraction forms at the end of the process, independent of the initial content of the melt. The use of special equipment enables to perform a continuous reduction process with simultaneous loading of K TaFy and sodium, which can influence the improved uniformity of the primary powder [592,603,604],... 

---