Alternative Materials of Tantalum in the Fabrication of Electrolytic Capacitors

Abstract

Tantalum is one of the most important mineral used to manufacture of capacitors. Tantalum powder is used to manufacture capacitors having high voltage and capacitance. Tantalum capacitors are of high quality and don’t suffer from degradation due to aging. The global availability of tantalum has reduced overtime resulting to an increase in price. Tantalum costs are expected to increase by 200% every year. This necessitates the use of alternative materials to manufacture these capacitors.

This study evaluated the possible materials that can replace tantalum. Materials that can be used to replace tantalum include: niobium, ceramics, Multilayer Ceramics (MLC) and aluminum polymer. To select the best replacement material, each of these materials was evaluated based on vital capacitor properties. From the analysis, aluminum polymer was selected as the best replacement material.

Introduction

Tantalum metal was discovered in 1802 and its use quickly rose during the 19^th century. The use of tantalum for industrial applications is based on its good mechanical properties and corrosion resistance. Tantalum is used to fabricate numerous industrial and chemical equipments. The material is also widely used to fabricate heat exchangers, pipes, chemical equipments and other mechanical tools. In the 1950’s, most of the tantalum was used to fabricate industrial materials and equipments (Andersson, Reichert & Wolf, 2000). However, it was discovered that tantalum could be used to manufacture capacitors and other electronic components.

Capacitors manufactured from this metal have excellent properties and don’t suffer from degradation. Currently, 60% of tantalum metal is used to manufacture capacitors. Tantalum has high strength, good thermodynamic stability, is insensitive to high temperatures, has high volumetric capacitance and is resistant to corrosion. These properties make tantalum the best materials to manufacture capacitors.

Though tantalum has excellent properties, its use to manufacture capacitors is rapidly declining due to its unavailability and costs limitations. Tantalum is one of the rare minerals found on earth and is limited in supply. Its supply declined in 2008, 2000 and 1997. It is expected that the supply of tantalum will continue to decline and this will increase its price in the long run. In addition to its short supply, the American government also passed a bill in 2010 that discourages industries from purchasing minerals which come from conflict and war zones. Most of the tantalum is mined in the Democratic Republic of Congo and is controlled by militant groups. The ban on such minerals is expected to further decline the supply of tantalum.

In the paper, an evaluation of materials that can replace tantalum in capacitor manufacture was carried out. These substitute materials were evaluated in terms of their mechanical, electrical and chemical suitability and their ability to outperform tantalum in capacitor manufacture. The substitute materials costs were also compared to tantalum.

The electrolytic capacitor performance and the use of tantalum

Capacitors have several unique properties that make them suitable for charge storage and other electrical applications. These properties dictate the type of materials and technology to be used in the manufacturing processes. Some of the fundamental properties include:

High volumetric efficiency

The material used to manufacture capacitor should have high volumetric efficiency. The volumetric capacitance is defined by the capacitance and voltage (CV) value of the material. The volumetric efficiency on the other hand is given by (CV/g) where g is the weight in grams (Deshpande, 2013). The electric capacitance is based on

Where ε0 the permittivity of material in vacuum, εr is the dielectric material permittivity, S is the surface area of the anode material and d is the dielectric thickness. Tantalum powder used to make capacitors has a CV/g value of 50000; this means that a capacitor made from this process should ideally have 50000µF. However, due to the dielectric materials, which exhibit a much lower CV/g, these capacitors have a CV/g value that ranges between 200-1000 CV/g.

Dielectric properties

Materials used to manufacture capacitors have matching dielectric materials. When considering the capacitor materials, the properties of the dielectric should also be considered. Tantalum capacitors use their own oxide, that is, tantalum pent-oxide (Ta₂O₅) as a dielectric material. The thickness of the dielectric is paramount during materials selection as capacitance is inversely proportional to the dielectric thickness. Tantalum capacitors have high dielectric constant and this enables the device to be small. Other tantalum capacitors use magnesium oxide as the dielectric material. The use of this oxide is however being replaced by other dielectrics with superior properties (Deshpande, 2013).

High temperature stability

Capacitors should ideally have high temperature stability. This makes the capacitor useful in high temperature environments such as engines, aviation, industries and in military systems. The melting point of Tantalum is 3000⁰C and therefore, capacitors made from it have high thermal stability.

High reliability

Before the introduction of tantalum capacitor, most capacitors were developed using wet foil technology and the dielectric was in liquid form. This liquid electrolyte could leak from the capacitor and this affected its reliability and functionality. The development of tantalum capacitor enabled the use of solid electrolyte and this improved the capacitor stability as the electrolyte could not escape, evaporate, solidify or dissociate with time. For capacitors to be reliable, the dielectric and anode should be stable and not susceptible to dissolution and corrosion. Tantalum does not chemically react with copper and this makes the interface between capacitors, computer chips and other devices with copper wires more reliable (Deshpande, 2013).

Capacitor size

The size of the capacitor is a major consideration. Current digital systems require small surface mount capacitors that have high voltage and high capacitance. This is not achievable with most liquid electrolytic capacitors. Tantalum capacitors are made from pressed powder making them small and they can be used in digital surface mount technologies.

Electrical performance

This is also one of the most fundamental aspects of capacitors. Capacitors should have low resistance so that they can be applied in low voltage applications. Capacitors should have low Electric Series Resistance (ESR), should be able to operate at high frequencies and have good dissipation factor. Tantalum for example has very low ESR as compared to wet electrolytic capacitors and can be used for low voltage applications.

Replacement materials

As aforementioned, there is need to indentify new materials that can replace tantalum in the manufacture of electrolytic capacitors. Though tantalum has unique properties as compared to other materials, its supply is limited and the costs are also high. Some of the materials that can be used to replace tantalum include (Nishino, 1996).

1. Ceramic materials: these include ceramic materials such as ceramic capacitors and Multilayer Ceramic (MLC) (Qiquan, Caspar, & Edwards, 2005).
2. Aluminum polymers: they include solid polymer electrolytic capacitors and surface mount aluminum capacitors.
3. Niobium powder.

Evaluation of the possible replacement materials

The ceramic, aluminum capacitors were compared to tantalum so as to evaluate the differences and similarities and also identify a suitable replacement of tantalum materials. Niobium capacitors were not considered as the material is in short supply is expensive and has very few suppliers. The evaluation of tantalum, aluminum polymer and ceramic capacitors is outlined below.

• Size: both tantalum and ceramic capacitors are nearly the same size. This makes them easily applicable in digital and surface mount technology. Aluminum capacitors are slightly larger than tantalum capacitors.
• Temperature coefficient: the temperature coefficient of the materials is nearly the same. Both aluminum polymer and tantalum capacitors can operate at high temperatures. Ceramic capacitors are mostly used for low temperature applications.
• Short circuit: all capacitors are susceptible to short circuits. Such failure increases the ESR as the current damages the internal structure of the anode and electrolyte. Tantalums are particularly affected by increase in current.
• Equivalent Series Resistance (ESR): capacitors made from tantalum and MLC have low ESR and can operate at low voltages. Aluminum polymer exhibit a higher ESR making them unsuitable for low voltage applications (Ming, Yan & Zhang, 2007). MLC capacitors can however achieve lower ESR due to the ability to change length and width during fabrication.
• Power de-rating: though tantalum capacitors have higher power ratings, they suffer from damage due to high current. Aluminum and MLC capacitors have improved the power rating and are not affected by large currents.
• Volumetric efficiency: before the development in MLC capacitors, the ratio of volumetric efficiency of tantalum to ceramic was 100:1. With the development of MLC, this ratio has changed to 2:1. Currently ceramic capacitors use thin dielectric layers and this increases the capacitance per unit volume (Qiquan, Caspar, & Edwards, 2005). Aluminum polymer capacitors also exhibit high CV/g values (Ming, Yan & Zhang, 2007).
• Polarity: even though tantalum and ceramic caps use electrostatic charging to build up voltage, ceramic capacitors are non polar while tantalum caps are electrolytic. Thus, the voltage applied to tantalum cap must create a positive charge at anode and negative charge at cathode. Aluminum capacitors are polarized and similar to tantalum.
• Ignition failures: tantalum capacitors have high ignition failure. High currents can cause the capacitor to burn due to oxygen formation at the electrodes. This does not occur for aluminum and ceramic capacitors.

Table 1 below shows a summary of the comparison between ceramic, aluminum and tantalum capacitors.

Table 1: comparison between different kinds of capacitors.

Alternative materials selection

From the analysis, it can be seen that the main materials that can replace tantalum capacitors are aluminum polymer, multilayer ceramic and niobium. Though niobium powder has very high volumetric efficiency ranging from 70000CV/g and is nearly comparable to tantalum. Its use is limited due to availability issues and lack of suppliers. As a result, niobium was not analyzed. From the analysis both ceramics and aluminum polymer can replace tantalum though they are not as effective as tantalum capacitors. From the analysis, aluminum was selected to replace tantalum for capacitor manufacture. The main properties of aluminum polymer capacitors include:

• Low ESR which are comparable to tantalum
• High volumetric efficiency comparable to tantalum
• Relatively small size
• High operating temperature range
• Low costs as compared to tantalum
• Readily available, aluminum is the third most abundant mineral in the earth’s core
• aluminum polymer capacitors have high capacitance and can operate at high voltages

Evaluation of the environmental costs of the selected material

The main impacts of aluminum polymer capacitors lies in the materials used to manufacture them. These are aluminum and plastic polymer. Aluminum has several negative impacts on plant, animals and sea life. Aluminum is harmful to the aquatic animals causing death to fish. Al⁺³ ions are poisonous to plant life, fungi and other organisms that live in the soil. The environmental costs of aluminum are associated with it production and disposal. Most aluminum products are recycled rather than disposed. The main environmental costs associated with using aluminum include

Cost analysis of the new material

The costs of an aluminum polymer capacitor is about 0.05$ per capacitor. These capacitors are usually supplied in batches of 1000. The relative cost of an equivalent tantalum capacitors is about 0.1$ -1$. Thus, the cost of tantalum capacitor is approximately twice the price of an aluminum polymer capacitor. The main processes associated with the production of aluminum capacitor are listed below

1. Purchasing of the ore;
2. Metallization;
3. Film slitting;
4. Winding the foils;
5. Impregnation;
6. Healing;
7. Adding the contact layer;
8. Flattening;
9. Welding the terminals;
10. Coating and lacquering;
11. Testing the finished capacitors;
12. Packaging and transportation.

Conclusion

This study evaluated the potential materials that can replace tantalum in the manufacture of capacitors. Though tantalum produces high quality capacitors, its use is associated with high costs due to unavailability of the material. Niobium powder, aluminum polymer and ceramics can replace tantalum in capacitor manufacture. After evaluating these materials, aluminum powder was selected as the best replacement to tantalum. This is because aluminum capacitors are polarized can operate at high voltages and have high volumetric efficiency. It should also be noted that multilayer ceramic capacitors can also replace tantalum in manufacture of capacitors with low capacitance.

References

Andersson, K., Reichert, K. & Wolf, R. (2000). Tantalum and tantalum compounds. New York: Wiley publishers.

Deshpande, R. (2013). Capacitors: Technology and Trends. New York: McGraw-Hill.

Ming, H., Yan, S. & Zhang, Z. (2007). Study on Solid Electrolytic Capacitors of Conducting Polymer & Aluminum. Electronic components and materials, 12(1),14-2.

Nishino, A. (1996). Capacitors: operating principles, Current Market and Technical Trends. Journal of Power Sources, 60(2),137–147.

Qiquan, F., Caspar, J. & Edwards, D. (2005). Dielectric Properties and Microstructures of Ba(Ti,Zr)O₃ Multilayer Ceramic Capacitors with Ni Electrodes. Journal of the American Ceramic Society, 88(6), 1455–1460.