Hydrothermal synthesis is a method used to create substances using hot water solutions under high pressure. This process is also called the "hydrothermal method." The word "hydrothermal" comes from geology, the study of Earth's materials. Scientists who study rocks and minerals have been researching how substances form under high heat and pressure since the early 1900s. George W. Morey at the Carnegie Institution and later Percy W. Bridgman at Harvard University helped develop tools and techniques to safely handle materials under these extreme conditions. In general, a process is considered hydrothermal if it uses water heated above 100 °C (212 °F) and pressures higher than 1 atmosphere.
In material science, hydrothermal synthesis is used to grow single crystals. At very high temperatures (over 300 °C) and pressures (over 100 atmospheres), minerals that usually do not dissolve in water can become soluble. This process happens inside a special steel container called an autoclave. The container holds the starting material, called a "nutrient," along with water. A temperature difference is created between the ends of the container. At the hotter end, the nutrient dissolves into the water, and at the cooler end, it forms a layer on a seed crystal, gradually growing the desired crystal.
The hydrothermal method has advantages over other crystal-growing techniques. It can produce crystals that would not form when melted. It also works well for materials that release a lot of vapor when heated. This method is especially useful for creating large, high-quality crystals while keeping control over their composition. However, the method has disadvantages. It requires expensive equipment like autoclaves, and it is difficult to see the crystal growing if a steel container is used. Some autoclaves made of thick glass can be used up to 300 °C and 10 bar.
History
The first recorded use of hydrothermal methods to grow crystals was by German geologist Karl Emil von Schafhäutl in 1845. He created tiny quartz crystals using a pressure cooker. In 1848, Robert Bunsen grew crystals of barium and strontium carbonate at 200 °C and 15 atmospheres of pressure. He used sealed glass tubes and a solution of ammonium chloride ("Salmiak") as a solvent. In 1849 and 1851, French crystallographer Henri Hureau de Sénarmont grew crystals of various minerals through hydrothermal synthesis. In 1905, Giorgio Spezia reported growing larger crystals. He used sodium silicate solutions, natural crystals as seeds, and a vessel lined with silver. By heating one end of the vessel to 320–350 °C and the other end to 165–180 °C, he produced about 15 mm of new crystal growth over 200 days. Unlike modern methods, the hotter part of the vessel was at the top. During World War II, a shortage of natural quartz crystals from Brazil led to the development of a commercial hydrothermal process for growing quartz crystals in 1950 by A. C. Walker and Ernie Buehler at Bell Laboratories. Other important contributions were made by Nacken (1946), Hale (1948), Brown (1951), and Kohman (1955).
Uses
Many different types of compounds have been made using hydrothermal conditions, which involve high temperatures and pressures. These compounds include elements, simple and complex oxides, tungstates, molybdates, carbonates, silicates, germanates, and others. Hydrothermal synthesis is often used to create synthetic quartz, gems, and other single crystals that have commercial value. Crystals that have been successfully grown include emeralds, rubies, quartz, alexandrite, and others. This method has proven to be very effective in discovering new compounds with special physical properties and in carefully studying the physical and chemical behavior of complex mixtures under high temperatures and pressures.
Equipment for hydrothermal crystal growth
The crystallization vessels used are called autoclaves. These are typically thick-walled steel cylinders with an airtight seal that must remain strong under high temperatures and pressures for long periods. The material of the autoclave must not react with the solvent used. The closure is the most important part of the autoclave. Many types of seals have been created, with the Bridgman seal being the most well-known. In most cases, solutions that cause steel to corrode are used in hydrothermal experiments. To protect the inside of the autoclave from corrosion, protective inserts are often placed inside. These inserts can match the shape of the autoclave and fit directly inside (called contact-type inserts), or they can be "floating" inserts that only cover part of the interior. Inserts are made from materials such as carbon-free iron, copper, silver, gold, platinum, titanium, glass (or quartz), or Teflon, depending on the temperature and type of solution being used.
Methods
This is the most commonly used method in hydrothermal synthesis and crystal growing. Supersaturation is created by lowering the temperature in the crystal growth area. The nutrient is placed in the lower, hotter section of the autoclave, which is filled with a specific amount of solvent. The autoclave is heated to form a temperature difference between sections. The nutrient dissolves in the hotter area, and the hot, saturated solution moves upward through the autoclave due to the movement of the liquid. The cooler, heavier solution in the upper part moves downward, while the more concentrated solution rises. The solution becomes supersaturated in the upper area because of the lower temperature, and crystals begin to form.
In this method, crystallization happens without a temperature difference between the growth and dissolution areas. Supersaturation is created by slowly lowering the solution’s temperature in the autoclave. A drawback of this method is the challenge of controlling the growth process and adding seed crystals. Because of these issues, this method is rarely used.
This method relies on the difference in solubility between the material being grown and the starting material. The nutrient is made of compounds that are not stable under the growth conditions. The solubility of the unstable phase is higher than that of the stable phase, so the stable phase forms as the unstable phase dissolves. This method is often used together with one of the other methods described earlier.
Compared to traditional methods, this newer method is expected to produce materials with similar structure and biological properties at room temperature, but in less time.
Use beyond material science
Hydrothermal synthesis is useful for green chemistry because it uses water instead of chemical solvents made from fossil fuels. This process is between supercritical water (very hot and pressurized) and regular room-temperature water. Under hydrothermal conditions, water’s ability to mix with nonpolar substances increases, making it easier for these substances to dissolve. Also, water splits into more hydrogen and hydroxide ions, which helps speed up chemical reactions.
In industry, hydrothermal methods are often used to process plant and animal materials. For example, they break down keratin in animal feathers into smaller, easier-to-digest parts. However, this process can also damage some amino acids, which are important building blocks in proteins. Hydrothermal conditions can also turn waste materials like sewage sludge and straw into useful products.