Material of the month: Titanium
Titanium is a transition metal named after the Titans – powerful deities in Greek mythology. It has the highest specific strength of any metal and, in its unalloyed condition, is as strong as some steels yet 45% lighter. Titanium is used in a range of applications, from aeroplanes to fireworks to replacement hip joints.
Titanium is the ninth most abundant element in the Earth’s crust and the seventh most abundant metal overall. However, in nature it is always bonded to other elements. It is present in many minerals but is most often extracted from rutile and ilmenite. In 2011, a total of 6.7Mt of these two minerals were mined: 19% of this total in Australia, 17% in South Africa and 10% in Canada. In the same year, around 190,000t of titanium metal was produced, mostly in China, Japan and Russia.
Titanium reacts readily with carbon to form a carbide, so it cannot be extracted by the common method of reducing its oxides with coke. In this context, reduction means the gain of electrons or removal of oxygen. Incidentally, titanium reacts with oxygen in air to form an oxide coating that protects the bulk metal from further oxidation. This layer is thin when it first forms (1–2nm) but continues to slowly grow to reach a few millimetres after several years.
Clergyman and amateur geologist William Gregor first discovered titanium in 1791 in Cornwall after coming across a metallic oxide that he could not identify. In 1795, a Prussian chemist by the name of Martin Heinrich Klaproth independently discovered the same oxide and named the metal titanium.
But pure titanium was not prepared until more than a century later when, in 1910, Matthew Hunter produced it using a process now referred to as the Hunter process. The method involves mixing rutile, which is a mineral consisting of titanium dioxide, with chlorine and coke. When extreme heat is applied, the carbon in the coke reacts with the oxygen in the oxide, and titanium tetrachloride and carbon dioxide are formed. The tetrachloride is then reduced with sodium to form sodium chloride and high quality titanium.
In the 1930s, William Justin Kroll produced metallic titanium using the Kroll process. This is more economical than the Hunter process and has, for the most part, replaced it. In this process, the titanium tetrachloride is obtained and then fractionally distilled (molten components separated by their boiling points) to remove impurities. It is then reduced with magnesium to obtain pure titanium in a spongy mass form that can then be cast into ingots.
Another process for creating titanium, known as FFC Cambridge, was later developed by Tom Farthing, Derek Fray and George Chen in Cambridge in the 1990s. This method has been patented and commercialised by Sheffield-based company Metalysis. It is an electrochemical process, transforming metallic oxides into metallic powders by electrolysis. Currently, titanium is expensive because of the costly multi-stage Kroll process. FFC Cambridge is a more efficient process and therefore an attractive alternative. In addition, different types of powder can be produced and used in a variety of advanced powder metallurgy manufacturing methods, which reduce material waste compared to conventional metal forming techniques. Metalysis is currently working with commercial partners to test the suitability of its powders for a variety of applications.
Most of the titanium that we use is actually not in its metallic form but in the compound titanium dioxide. This is widely used as a white pigment because of its high brightness and extremely high refractive index. It is also used in sunscreen because it blocks UV rays. The most popular use for metallic titanium – accounting for two thirds of its use – is in the aerospace industry, where it was first used during the Cold War. Other uses include consumer products such as spectacle frames, jewellery, body piercings and architecture, such as the Guggenheim Museum in Spain.
There are many benefits of using titanium in the medical world and the most common medical-grade titanium alloy consists of 6% aluminium and 4% vanadium. Bodily fluids do not corrode titanium, it is non-toxic and it is biocompatible. It can exist in harmony inside the body, not only by exhibiting corrosion-resistance but also without causing any unwanted reactions from human cells. Interestingly, the lower Young’s Modulus of titanium compared to steel is suitable for orthopaedic applications, because it is more closely matched to that of bone, meaning loads are more easily shared. Among other advantages, this reduces the risk of fractures at implant boundaries.
Titanium has an innate ability to join with human bone, an unusual property referred to as osseointegration. The human body’s natural bone and tissue bond to the titanium implant so it ends up firmly anchored. This phenomenon was discovered by accident in the 1950s by Per-Ingvar Branemark. The Swedish surgeon and research professor had implanted a titanium monitoring device into a rabbit’s bone and, after the experiment, found that he could not remove the device because it had integrated with the bone.
Titanium has become a fundamental material for orthopaedic rods, pins and plates, for both routine surgery (including hip replacements) and emergency surgery (such as skull reconstruction following trauma). Titanium screws are used in dental implants and these act like the root of the tooth, with osseointegration playing an essential role. Titanium is also used in replacement heart valves and prosthetic limbs. It is used extensively in surgical instruments because of its hardness, bacterial resistance and low density compared to stainless steel, reducing surgeon fatigue.
A highly desirable requirement of medical implants is that they are of the fit-and-forget kind and, once fitted, never need to be replaced or maintained. Titanium is lightweight yet durable, with implants lasting upwards of 20 years. This is important as people are living longer than ever before, and joint replacement surgery and dental implants are becoming more commonplace. An added bonus of titanium is that it is non-ferromagnetic, so patients with titanium implants can be safely examined with magnetic resonance imaging (MRI) if they require this procedure in the future.
The use of titanium in medical applications is an excellent example of how materials science can drastically improve quality of life for millions of people. It also demonstrates how the modern medical and engineering worlds are very much intertwined, offering a lot of scope for interdisciplinary work that pushes the boundaries of both industries.