Introductory Chapter: Magnesium - A Perspective

1. Introduction

Magnesium (Mg) with atomic number 12 is situated in the II group and III period of the modern periodic table. Discovered in 1755 by Joseph Black, Mg derives its name from Magnesia, a district in Eastern Thessaly in Greece [1]. Introduced to a middle school student in the form of Mg wires burning with bright white flame (luminescent), Mg is a solid at RT (Figure 1), soft metal with a m.p. of 650°C and a b.p. of 1090°C and is the eighth-most abundant element in earth’s crust.

Figure 1.

(a) Magnesium powder (b) Burning magnesium wire.

Mg is well studied for its excellent properties in diverse fields as in land/air transportation, energy storage devices, catalysis, medical implants, food, nutrition, etc. (Figure 2).

Figure 2.

Schematic diagram showing varied applications of magnesium (Mg).

2. In transportation

One of the characteristic features of Mg, that makes it most explored in diverse fields, is its low density. With a density of 1.74 g/cc, it is lighter than aluminum, Al (2.7 g/cc). The low density of Mg and its alloys, rendered it to be studied extensively in the transportation industry, viz., automobiles and aerospace.

Traditionally, developments in magnesium alloys have been driven by industries like automobiles and aerospace requirements, for any material which is lightweight to operate with the huge demands of the industries (as in engines, gearbox, or casings of aircraft). The low density, only two-thirds that of aluminum, and dramatic improvements in anti-corrosion performance and mechanical properties in recent years have been attractive to designers to use Mg alloys in aerospace applications [2, 3]. Such alloys/components are now specified on projects like the McDonnell Douglas MD 500 helicopter/B52 Stratofortess (Figure 3). There are many applications of alloys of magnesium in aerospace industries [4, 5], gearbox parts in a helicopter (Westland Sea King) and wheels of aircraft, both in ZW3. Magnesium when it is forged, is also used in aircraft engine applications. In the future, magnesium forgings are most likely to be used in higher temperature applications.

Figure 3.

Examples of application for Mg alloys in (a) Boeing 747, wings, and seat components. (b) Aircraft door lock set made of magnesium alloy [7] (c) B-52s fighter-jet (wheel bearing) [6].

Besides the aerospace industry, another industry that has enjoyed the advantage of the low density of Mg is the automotive industry. In the 1920s, magnesium began to appear in the automotive industry. The lightweight metal began to be used in racing cars to add to their competitive edge in MRI153M alloy (Mg-Al-Ca-Sr based alloy). Years later, commercial vehicles such as Volkswagen Beetle started using magnesium-containing about 20kg (44.09 lbs) of total weight. The interest in magnesium use in automotive applications has increased over the past ten years due to the increasing environmental and legislative influences [7, 8]. Magnesium contributing to increased fuel efficiency, accelerated performance, and sustainability has led to their use in vehicles. Many automotive companies have found magnesium to be a suitable replacement for steel and aluminum, for their products (Figure 4). Audi, Daimler (Mercedes-Benz), Ford, Jaguar, Fiat, and Kia Motors Corporation are just a few. Magnesium is currently used in gearbox, front end and IP beams, steering column, driver’s airbag housings, steering wheels, seat frames, and fuel tank covers (AJ62A (98.8–91.8% Mg) and Magnox (Al 80))^.[6].

Figure 4.

Examples of applications for magnesium alloys in the automotive industry [8].

Using magnesium in automotive applications helps more than just weight savings. For a number of years, the desire to identify challenges, search for solutions, and explore new opportunities regarding the use of magnesium in vehicles has been growing. Magnesium usage on the front end provides a lower overall mass for the car. It allows for shifting the center of gravity towards the rear of the car, improving handling and turning capabilities. In addition, frequencies that reduce vibration and overall noise can be achieved by tuning magnesium-containing parts. A single cast piece of magnesium is preferably used to replace steel components in vehicles, thus providing material with additional strength and allowing housings to be cast into place. This castability involves the merits of low manufacturing cost and less tooling and gauges.

3. In medical industry

Not only lighter but Mg also form excellent alloys with Al and other metals due to its excellent mechanical, welding, and fabrication capabilities. Moreover, such alloys are widely studied for their improved mechanical strength, corrosion withstanding capabilities, etc., thus enabling them to be used as biocompatible alloys, energy storage devices, etc.

The Use of Mg-based products in the form of composites, and alloys in the medical industry is far widely studied. Ease of corrosion inhibition in the physiological conditions enabled exploration of Magnesium (Mg)-based biocomposites and alloys to be used in biomedical applications such as bone fixation, cardiovascular stents, hip joints, screws/pins, dental implants, etc. (Figure 5) In the Mg-based composites, the matrix materials are biomedical magnesium alloys based on Mg–Ca, Mg-Al, Mg–Zn, and Mg–REE alloy and the reinforcements are based on hydroxyapatite (HAP), calcium polyphosphate (CPP), and β-tricalcium phosphate (β-TCP) particles [9].

Figure 5.

Application of Mg-based products in Medical Implants.

4. In catalysis

Magnesium oxide is one of the most important metal oxides in catalysis. Though commonly used as support, its employment as a catalyst was also reported in viz., oxidative coupling of methane [10, 11], dehydrogenation–dehydration of alcohols [12], dehydrohalogenation of halogenated hydrocarbons [13], benzylation of aromatics [14], synthesis of pyranopyrazole derivatives [15], and Claisen–Schmidt condensation, etc. [16] Furthermore, the catalytic performance of MgO was improved by promotion by other metals [17] and addition of small amounts of iodine to the reactant mixture [14] for dehydrogenation of short-chain alkanes.

Using alkoxy magnesium Mg(OR)₂ and dialkyl magnesium Mg(alkyl)₂ species as Grignard reagents in organic synthesis is a fundamental textbook example of organic synthesis [12, 18]. However, the utilization of magnesium as a catalyst in asymmetric synthesis is dramatically undeveloped compared to that of almost all other transitional metals, despite its relatively higher natural abundance than all these other metals combined. Magnesium catalysts are widely used in chemical transformations, and their catalytic use in asymmetric synthesis is highly advisable. For enantioselective reactions, magnesium salts such as Mg(OTf)₂, Mg(NTf₂)₂, and Mg(ClO₄)₂ are used as Lewis acid catalysts. Different magnesium reagents can give rise to chiral magnesium catalysts. One method involves coordinating complex formation using chiral ligands and a strong Lewis acid magnesium salt, described as “the magnesium catalytic strategy with a fixed magnesium salt” [19].

5. In energy storage

Magnesium is under research in the energy sector as a possible replacement or improvement on lithium-ion batteries in certain applications. In the early 1990s, the potential use of magnesium batteries was recognized based on V₂O₅, TiS₂, or Ti₂S₄ cathode materials and magnesium metal anodes [20]. Magnesium has a (theoretical) energy density per unit mass under half that of lithium (18.8 MJ/kg vs 42.3 MJ/kg) in comparison to that of lithium as an anode material. Still, a volumetric energy density is around 50% higher (32.731 GJ/m³ vs 22.569 GJ/m³), making its use advantageous [21]. Magnesium anodes do not exhibit dendrite formation compared to metallic lithium, except in certain nonaqueous solutions and at current densities below ca. 1 mA/cm². Such dendrite-free Mg deposition allows magnesium metal to be used without an intercalation compound at the anode, thus raising the theoretical maximum relative volumetric energy density to around five times that of a lithium graphite electrode [22]. Additionally, modeling and cell analysis have indicated that magnesium-based batteries may have a cost advantage over lithium due to the abundant magnesium on earth and the scarcity of lithium deposits [20, 23].

Another viable emerging energy storage technology under research is the Magnesium-Sulfur rechargeable battery (Figure 6) that uses magnesium ion as its charge carrier, magnesium metal as the anode, and sulfur paste as a cathode. To increase the electronic conductivity of the cathode, sulfur is usually mixed with carbon to form a cathode composite. Currently, efforts on rechargeable magnesium battery research are underway at Apple, Toyota, and Pellion Technologies [6] and in several universities.

Figure 6.

Schematic diagram of Mg-S batteries showing the working principle and formation of magnesium-polysulfides (Mg-PS), which passivate the anode surface [20].

Successful research outcome would be of great interest since, in theory, the Mg/S chemistry can provide 1722 Wh/kg energy density with a voltage of ~1.7 V, addressing most future energy challenges.

6. In thermoluminescence

Use of magnesium silicates and borates (for example, MgO, Cr³⁺/ Be-doped MgO, Mg(SiO₃)_n, MgO(B₂O₃)₃, Li₂O-MgO-B₂O₃, Sr₃MgSi₂O_8, etc.) as thermoluminescent materials are reported [24]. In this phenomenon exhibited by certain crystalline materials, previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon heating the material [25]. TL glow curves for 2 mg Sr₃MgSi₂O₈ substance exposed to radiographic radiation by 254 nm UV source show a resolute single peak around 123°C [26]. A glow curve deconvolution technique was used for analyzing acquired glow curves. Moreover, magnesium orthosilicate intensity of TL glows curves shows a dose-response linear relationship extending to 20 Gy. To a certain degree, TL features of this phosphor are conditional beside Mg₂SiO₄ is suggested to consider a suitable substance for dosimetry [27].

7. In food and nutrition

Magnesium is the fourth most abundant metal ion, showing important physiological functions in the human body. Magnesium balance is maintained by renal regulation of magnesium reabsorption. According to the United States Food and Nutrition Board recommended daily allowance for magnesium is 420 mg for adult males and 320 mg for adult females, respectively. Green vegetables, nuts, seeds, and unprocessed cereals are rich sources of magnesium; besides, some magnesium is made available in fruits, fish, meat, and milk products. However, consumption of processed foods, demineralized water, and agricultural practices using soil deficient in magnesium for growing food, lowers the consumption levels than the recommended amounts, resulting in magnesium deficiency leading to hypomagnesemia, cardiac or neurological disorders [28].

8. Conclusions

Magnesium displays its utility in diverse fields ranging from materials to medical fields, as discussed in the preceding sections. Improvements in processing technologies have wide scope for increased use of materials based on magnesium, which may revolutionize the transportation sector in terms of energy and cost savings. Corrosion withstanding capabilities in biological media, improved mechanical strength, and low density make Mg and its alloys to be used as potential candidates in medical implants. Not only in medical and material fields, but the contribution of Mg in the energy sector is also no less, as Mg batteries are being explored as an alternative to Li ion batteries. Similarly, improved catalytic efficiencies based on Mg catalysts are advantageous for chemical industries too. In the coming years, sustainable life is not far away with the involvement of magnesium and its products in almost all fields of daily life.

Acknowledgments

AV like to thank CSIR SRF (09/013(0825)/2018-EMR-I) for financial support. SSS gratefully acknowledges IoE incentive for Sr. Faculty (Dev. Scheme No. 6031), Banaras Hindu University, Varanasi, India.