Salt & water is the “solution”

Oxygen is the basis of life on earth but sometimes it is unwanted for materials processed at high temperatures. Thanks to the age-old technique of food preservation by salt, oxidation of materials can also be avoided.
Salt & water is the “solution”

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What is the world really made up of? : Metals, ceramics and polymers are the basis of everything around us. The evolution of human civilization is closely related to the development of materials. Even the historical eras: Stone age, Bronze age, Iron age are named after the “materials” that dominated the times. Every major discovery in this world is driven by the development of materials.

Every material has its unique properties. Metals are ductile and tough whereas ceramics are hard and brittle. If ceramics were not brittle then we would not have the space shuttle Columbia disaster (18 years ago). A piece of debris broke off the external tank and hit the edge of the left-wing during lift-off, causing a hole of 40 cm on the heat shield. During the re-entry of the spaceship into the earth's atmosphere, hot gases damaged the internal structure of the wing leading to the death of seven astronauts and the failure of the space mission. The heat shield was made up of carbon fiber reinforced carbon which is a ceramic and brittle in nature. There is a need for the development of a material that is ceramic and not brittle. This would allow the material to be used at high temperatures like ceramics and have mechanical stability like metals.

In recent years, a new class of material has been discovered - MAX phases. This material has properties corresponding to both metals and ceramics. Titanium silicocarbide (Ti3SiC2), as it is referred to, is the topic of my research. The elements are prone to oxidation and the reaction leading to the formation of Ti3SiC2 takes place at high temperatures. Carrying out such reactions in air oxidizes the material resulting in the degradation of the properties. A very well-known example of room temperature oxidation is the formation of rust on steel.

Like Ti3SiC2, many metals and ceramics are prone to oxidation at their processing temperatures which is above 1000°C. To prevent the spontaneous oxidation of such materials, an inert atmosphere is normally used. A furnace operating at a high temperature with a controlled atmosphere like nitrogen or vacuum costs resources and carbon emissions. I was looking for a “solution” to process all the oxidation-prone materials in air at a relative lower temperature. Very often we think in a complicated way to find a solution to a difficult problem. Lost in the thoughts of trying to find a “solution”, we overlook the answers which are lying around us. I found my answers in the kitchen:

From my childhood, I am fond of cooking and accompany my mother in the kitchen. Once I saw her making sauerkraut with shredded white cabbage, saltwater, and an airtight container. As inquisitive as I was, I started asking about the fermentation process, why-what-how-when, all sorts of questions. I couldn’t wait to see the end result. After around twenty days, I ran to the basement of my house to check the jars. They all looked very good except one. The fermented sauerkraut had mold growing inside the jar. I also saw her putting the chopped potatoes in water to prevent browning which is caused due to the oxidation of starch. It is the same with With various experiences, I understood that oxidation-prone materials can be placed under a non-reactive liquid to avoid contact with oxygen.

I wanted to find a way to prevent the oxidation of materials at a very high temperature (>1000°C) with a liquid. Water converts to steam above 100°C and hence was not an option for my experiment. I soon realized that the melting point of the kitchen salt (Sodium Chloride) is 801°C and I decided to use only salt and no water. Theoretically, the molten salt can protect the material above 801°C up until the boiling point which is 1465°C. The 801-1465°C temperature window is quite wide and most of the materials can be processed in this temperature range. When heated above 801°C, the salt forms a melt pool and the material is submerged under the liquid salt, just like the sauerkraut in saltwater. There was only mild oxidation observed which was related to the oxidation before the salt melted (<801°C). The problem was partially solved and the real challenge was to prevent oxidation below the melting point of the salt.

Schematic of Molten Salt Shielded Synthesis (MS3)

A different choice of salt was necessary to tackle the oxidation from room temperature to the melting point of the salt. Potassium bromide was the magic salt that made it possible to avoid oxidation even below its own melting point. This is because it has the unique property to be pressed into dense form at room temperature. Using this property, a gas-tight cladding was formed around the material which blocks the contact with oxygen at temperatures below the melting point of the salt. It was finally possible to process oxidation prone materials in the air without the need of a protective atmosphere and expensive machines. Various other materials were processed with the same salt as a proof of concept.

As the basis of cooking is chemistry, very often scientists inspire themselves with simple kitchen practices to answer bigger questions. Taking an example of baking a cake, we need wheat flour which is a powder. We cannot bake with whole wheat. Similarly, we also need ceramic powders for the fabrication of complex shapes. Within my example of Ti3SiC2, the reaction of Titanium, Silicon, and Carbon leads to a solid block of material that needs to be pulverized to obtain powders. The grinding process consumes energy and the use of salt avoids the grinding step all together. After reaction at high temperature, the salt grains sit in between titanium silicocarbide grains. The salt is simply washed with water to obtain free-flowing powder without any grinding step.

 With the increasing demand for hi-tech materials, there is also a need to address global issues like the climate crisis and clean energy. Unfortunately, the processing of materials at high temperature is energy-intensive and not environment friendly. The temperature of the reaction is reduced as the ingredients can swim across each other in the molten salt. The salt doesn’t take part in the reaction but simply assists in the reaction. It is the same as mixing water with sugar and corn syrup to make candies, the ingredients can mix in a more homogeneous way in a liquid medium. The reduction in the reaction temperature results in energy savings.

This process can prove to be revolutionary for industries involved in the manufacturing of such oxidation-prone materials and can open doors for future materials research which will make the ductility of ceramics possible. This will also create new possibilities for space exploration as materials are the key to sustenance of life outside earth.

 Thanks to my mother as an endless source of inspiration, oxidation is now a thing of the past.

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