In this blog post, we’ll explore the principles and characteristics of solid-solid reactions—where solids react with one another—and how they’re applied in everyday life.
Let’s think back to our science lab classes from elementary school through high school. If you paid close attention in class, you’ll likely remember various experiments related to the solid, liquid, and gas states. Through these experiments, we learned about reactions between liquids, as well as between solids and liquids, liquids and gases, and gases and gases.
But have you ever conducted an experiment involving a reaction between two solids? Most of us probably haven’t. Reactions that occur between solids are called solid-solid reactions, and we’ll explore them now.
First, let’s take a brief look at solids. Among the three states of matter—solid, liquid, and gas—a solid is a state in which particles maintain relatively fixed positions unless an external force is applied, and it retains a definite shape and volume. So how can two solids that don’t move relative to each other react?
Let’s imagine a couple who are friends. For the two of them to meet in person and go on a date, they must have feelings for each other. When two solids react, temperature plays exactly that role.
As the temperature rises, the energy possessed by the reactants also increases. For a chemical reaction to occur, the reactants must possess sufficient energy. The minimum energy required to initiate a reaction is called activation energy, and the reaction proceeds only when particles with energy greater than the activation energy collide.
Furthermore, as the temperature rises, the rate at which atoms or ions diffuse within the solid increases, thereby accelerating the reaction rate. The reaction rate must be sufficiently fast to minimize the phenomenon where the growth of reactant and product particles hinders the reaction. Here, particle growth refers to the phenomenon in which small particles gradually grow into larger ones by combining with each other or diffusing. The method of producing desired products by utilizing solid-state reactions at high temperatures is called the solid-state reaction method.
The solid-state reaction method is widely used, primarily in the manufacture of magnetic materials. Magnetism refers to the property of a material to possess a magnetic field or to respond to a magnetic field. A representative example is ferrite. Manganese-zinc ferrite, produced by mixing oxides of manganese, zinc, and iron in specific ratios, exhibits magnetic properties that vary depending on the composition ratio of manganese and zinc. Generally, zinc oxide (ZnO), manganese oxide (MnO), and iron oxide (Fe₂O₃) are mixed in appropriate proportions and then sintered at temperatures of approximately 1,000°C or higher. Sintering is a process in which a powdered material is heated to a temperature below its melting point, causing the particles to bond together and form a dense solid. Manganese-zinc ferrite manufactured in this way is used in various electronic components, such as transformers, inductors, power supplies, and telecommunications equipment.
This may raise a question: Wouldn’t it be easier to raise the temperature further to melt the solid into a liquid state and then carry out the reaction? So why do we insist on carrying out the reaction in the solid state? There are two main reasons.
First, the process is relatively simple, allowing for the efficient production of the desired product. Consequently, it is less costly and more economical than other manufacturing processes. Second, there is no need to use a separate solvent, and the drying process can be omitted, further reducing additional process costs.
As such, magnetic materials manufactured using the solid-state reaction method are utilized in a wide range of fields. Although we may not usually realize it, they can be easily found in our daily lives. Typical examples include various electronic components used in automobiles and magnets attached to refrigerators.
At first glance, the principles behind materials produced by the solid-state reaction method may seem somewhat difficult and distant. However, in reality, they are widely used in various electronic devices and household items around us, and this is one of the key material manufacturing technologies underpinning modern industry.
