Final answer:
APUs are composed of a small turbine engine and materials like silicon, common metals (copper, tin, gold), and uncommon elements (yttrium, gadolinium) to provide energy for various aircraft functions.
Step-by-step explanation:
The main components of a typical Auxiliary Power Unit (APU) include a small turbine engine designed to provide energy for functions other than propulsion when the main aircraft engines are not running. These components are essential for generating on-board electricity, providing hydraulic and pneumatic power, and sometimes even contributing to the starting of the main engines. Modern APUs utilize various materials in their construction, including silicon in electrical components, common metals such as copper for conductivity, tin in soldering, and gold for corrosion-resistant electrical connections. In addition, they include uncommon elements such as yttrium and gadolinium, which are often used in the manufacturing of high-temperature superalloys or precise electronic components.