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Ultra-High-Purity Aluminum

Properties of high-purity aluminum are the low magnetic permeability, the absence of low-temperature brittleness, as well as the increased strength and plasticity at low temperatures. These explain the classical application of high- to ultra-high-purity aluminum for the stabilization of superconductors running at cryogenic temperatures as low as −269 °C.

• high-purity aluminum
• purification
• segregation

## 1. High- and Ultra-High-Purity Aluminum

The primary form of aluminum—from the Hall–Héroult process—has a purity range of 99.7 to 99.9%, with major impurities of iron (Fe), silicon (Si), zinc (Zn), and gallium (Ga). While this purity range is sufficient for most industrial applications and alloying, the use of aluminum in high-technology fields, such as semiconductor, electronics, superconducting, and so forth requires higher purity levels exceeding the ones obtained via classical aluminum production processes [1][2]. To reach such purity, several technologies have been developed. There is no officially standardized terminology for the various levels of purity in aluminum. As seen in Table 1 , different countries classify the aluminum following their own criteria [3][4].

Table 1. Classification of various aluminum purity terminologies in USA, China, Russia, and Japan, data from [2][3].
Country Designation/Category Purity (%)   Nines
USA Commercial purity 99.50 - 99.79   2N5 - 2N7
High purity 99.80 - 99.949   2N8 - 3N4
Super purity 99.950 - 99.9959   3N5 - 4N5
Extreme purity 99.996 - 99.999   4N6 - 5N
Ultra purity ≥ 99.999   5N+
China Primary Al 99.0 - 99.85   2N - 2N8
Refined Al 99.95 - 99.996   3N5 - 4N6
High purity Al ≥ 99.996   >4N6
Japan Second grade 99.950 - 99.990   3N5 - 4N
First grade 99.990 - 99.995   4N - 4N5
Russia Commercial purity 99.0 - 99.85   2N - 2N8
High purity 99.95 - 99.995   3N5 - 4N5
Ultra purity ≥ 99.999   5N+

Due to the extremely low amount of impurities in the base metal, its purity level is described as a function of “Nines”. For example, a base metal, in which the sum of the all targeted impurities is equal to 10 ppm, will have a purity of 99.999%, or 5N (“five Nines”). Moreover, a metal with 5 ppm of impurities would have 99.9995% purity and would therefore be described as 5N5.

## 2. High- and Ultra-High-Purity Aluminum

The primary form of aluminum—from the Hall–Héroult process—has a purity range of 99.7 to 99.9%, with major impurities of iron (Fe), silicon (Si), zinc (Zn), and gallium (Ga). While this purity range is sufficient for most industrial applications and alloying, the use of aluminum in high-technology fields, such as semiconductor, electronics, superconducting, and so forth requires higher purity levels exceeding the ones obtained via classical aluminum production processes [1][2]. To reach such purity, several technologies have been developed. There is no officially standardized terminology for the various levels of purity in aluminum. As seen in Table 1 , different countries classify the aluminum following their own criteria [3][4].

Due to the extremely low amount of impurities in the base metal, its purity level is described as a function of “Nines”. For example, a base metal, in which the sum of the all targeted impurities is equal to 10 ppm, will have a purity of 99.999%, or 5N (“five Nines”). Moreover, a metal with 5 ppm of impurities would have 99.9995% purity and would therefore be described as 5N5.

## 3. Main Applications from High-Up to Ultra-High-Purity Aluminum

The aluminum in its high purity has outstanding properties, such as high electric and thermal conductivity. Additionally, when an oxide layer is formed over its surface, high corrosion resistance and electrical insulation can be achieved. Such characteristics grant the usage of high-purity aluminum in integrated circuits, replacing Cu and Au as bonding wires for transistors, where thin films of high-purity aluminum are able to find applications as interconnecting lines in very large-scale integrated (VLSI) circuits [5][6][7].

Further properties of high-purity aluminum are the low magnetic permeability, the absence of low-temperature brittleness, as well as the increased strength and plasticity at low temperatures [8]. These explain the classical application of high- to ultra-high-purity aluminum for the stabilization of superconductors running at cryogenic temperatures as low as −269 °C [9].

Due to its beneficial performances, the application of this material in modern high-technology fields grows together with the advances of modern society. The eve-increasing power efficiency and zero-defect tolerances in the high-tech applications will most likely push the technical requirements and purity tolerances of high- and ultra-high-purity aluminum. The subsections below illustrate some classical utilizations of high-purity aluminum at specific purity ranges.

This class of material is required by the manufacturing of optoelectronic storage media, such as CDs, DVDs, and so forth. Furthermore, the aluminum can be processed into the electrical conductor of the computer storage hard disk via the cathode-sputtering process [10].

## 4. The Production of High- and Ultra-High-Purity Aluminum

The high-volume and industrial production of high-purity aluminum follow two main routes: three-layer electrolysis and fractional crystallization. While both processes can individually reach 4N8 purity, higher purity levels can be obtained when employing them in series. Alternative routes, such as vacuum distillation and organic electrolysis, are reported in the literature and can be used for low volume production and/or highly specialized applications.

The availability of public information regarding price, production volume, and main consumers are very scarce and quite often regarded as a trade secret among the industry. The small amount of public information available was compiled in the Table 2 [10].

Table 2. The production capacity of high- and ultra-high-purity aluminum in the world in 2003.
Country Three-Layer Electrolysis (kt/a) Segregation (kt/a) World Total (kt/a)
Japan 5.2 34.3 39.5
Norway 8.0 8.0
Russia 15.0 15.0
China 28.0 5.0 33.0
United States 20.0 20.0
Germany 4.5 7.0 11.5
France 2.0 2.0
World total 62.7 66.3 129.0

The price of high-purity aluminum increases exponentially according to its purity degree (see Table 3 ). According to the London Metal Exchange (LME), the price of 2N7 Al was 2.45 $/kg in June 2021 [11]. Despite the variance between suppliers and the format (ingot, foil, etc.), a price range for several aluminum purities was obtained from the company Laurand Associates Inc, where, for the same period, the price per kilogram of pellets ranges from 275 US$ for 4N up to 900 US$for 6N purity [12]. Table 3. The price per kilogram of aluminum in the western market in 2021, data from [11][12]. Purity Price (Western Spot Market,$/kg)
2N7 2.45
4N 275
4N6 300
5N 600
6N 900

This entry is adapted from 10.3390/met11091407

### References

1. Leroy, M. Alpha Rays Emitting Impurities in Ultra Pure Aluminum Evolution Through the Successive Refining Steps. J. Phys. IV 1995, 5, C7-99–C7-110.
2. Zhao, H.; Lu, H. The Development of 85kA Three-Layer Electrolysis Cell for Refining of Aluminum. In Proceedings of the TMS Aluminum Committee at the TMS 2008 Annual Meeting & Exhibition, New Orleans, LA, USA, 9–13 March 2008; pp. 533–540.
3. Lindsay, S.J. Very high-purity aluminum: An historical perspective. JOM 2014, 66, 217–222.
4. Wang, Z. The properties of high pure aluminum (Part A). Light Met. 2004, 8, 3–6. (In Chinese)
5. Head Electronic Co Limited. Guideline for Wire Bonding. Available online: http://www.headpcb.com/html/2018/news&blog_0612/157.html (accessed on 28 June 2021).
6. Sarkar, J.; Saimoto, S.; Mathew, B.; Gilman, P.S. Microstructure, texture and tensile properties of aluminum-2 at.% neodymium alloy as used in flat panel displays. J. Alloy. Compd. 2009, 479, 719–725.
7. Onishi, T.; Iwamura, E.; Takagi, K.; Yoshikawa, K. Influence of adding transition metal elements to an aluminum target on electrical resistivity and hillock resistance in sputter-deposited aluminum alloy thin films. J. Vac. Sci. Technol. A 1996, 14, 2728–2735.
8. Wang, Z. Talk about the purity aluminum (II). Met. World 2004, 4, 36–37. (In Chinese)
9. Zhang, J.; Sun, B.; He, B.; Mao, H.; Chen, G.; Ge, A. Principle and control of new-style purification equipment of 5N high purity aluminum. Chin. J. Mech. Eng. 2006, 42, 64–68. (in Chinese).
10. Wang, Z. Production, market and application of high-purity aluminum in the world. Nonferr. Met. Process. 2004, 33, 1–6.
11. The London Metal Exchange. London Metal Exchange Daily Price for Primary Aluminum. Available online: https://www.lme.com/en-GB/Metals/Non-ferrous/Aluminium#tabIndex=0 (accessed on 28 June 2021).
12. Laurand Associates Inc. Price of High- and Ultrahigh-Purity Aluminum. Available online: https://highpurityaluminum.com/products/pellets-slugs-shot?variant=19365073256559 (accessed on 28 June 2021).
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