Introduction

Hafnium dioxide is white or gray powder. It is insoluble in water, hydrochloric acid, nitric acid and other common inorganic acids, and slowly dissolves in hydrofluoric acid to form fluorohafnate. It reacts with hot concentrated sulfuric acid or bisulfate to produce hafnium sulfate. Mix and heat with carbon in the presence of chlorine to obtain hafnium tetrachloride. It reacts with potassium fluorosilicate to generate potassium fluorohafnate.

Physical properties

Hafnium oxide (HfO2) is a white crystalline powder. Pure hafnium oxide exists in three forms, one in the amorphous state and the other two in the crystalline state. When unstable compounds such as hafnium hydroxide and hafnium oxychloride are calcined at <400°C, amorphous hafnium oxide can be obtained. Continue to heat the hafnium oxide to 450-480°C, and start to transform into monoclinic crystals, and continue to heat to 1000-1650°C to gradually increase the lattice constant, and transform into a monomer of 4 hafnium oxide molecules. At 1700~1865°C, it begins to transform into a tetragonal crystal system. Adding a small amount of hafnium oxide such as magnesium oxide, calcium oxide, and manganese oxide to hafnium oxide can form a solid solution of face-centered cubic lattice at temperatures above 1500°C. If 8%~20% calcium oxide is added to hafnium oxide, the lattice constant α increases from 0.5082nm to 0.5098nm accordingly. If the amount added reaches the formation of CaHfO3, the crystal structure is transformed into a rhombohedral system. Monoclinic hafnium oxide has a density of 9.68g/cm3, a melting point of 3031K, and a boiling point of 5673K.

Chemical properties

The chemical properties of hafnium oxide are similar to those of zirconium oxide, and its activity is related to the calcination temperature. The higher the calcination temperature, the lower the chemical activity. Amorphous hafnium oxide is easily dissolved in acid, but crystalline hafnium oxide does not react even in hot hydrochloric acid or nitric acid, but only dissolves in hot concentrated hydrofluoric acid and sulfuric acid. After crystalline hafnium oxide is dissolved in alkali and hydrochloric acid, it is easy to dissolve in dilute acid. At 1100°C, hafnium oxide and lithium hafnate. At higher than 1500°C, hafnium oxide interacts with alkaline earth metal hafnium oxide and silicon dioxide to form hafnate and hafnium silicate. It forms a series of solid solutions with silicon oxide above 1800 °C. The hafnium salt can be hydrolyzed to obtain amphoteric hafnium hydroxide. Hafnium hydroxide can be dried at 100°C to reach HfO(OH)2, and then it can be converted into hafnium oxide when the temperature is raised. Hf2O3 and HfO can be formed during the carbonization process, but there are few studies on this.