Materials of construction for thermistors: identify what thermistors are typically made of, given their large negative temperature coefficient and semiconductor behavior.

Introduction / Context:
Thermistors are temperature-sensitive resistors widely used for precise temperature measurement, compensation, and control. Their hallmark is a strong, often negative, temperature coefficient of resistance (NTC), enabling high sensitivity over limited ranges compared with metal RTDs.

Given Data / Assumptions:

• Focus on common NTC thermistors used in instrumentation.
• Behavior: semiconductor with exponential R–T characteristic.
• Construction involves sintered materials and passivation.

Concept / Approach:
Typical thermistors are formed from sintered mixtures of transition-metal oxides such as manganese, nickel, cobalt, copper, or iron oxides. These ceramic semiconductors exhibit strong temperature-dependent conductivity due to carrier concentration changes. In contrast, ultra-pure metals (e.g., platinum) are used in RTDs, not thermistors. Iron–copper or nickel–chromium are alloy systems for specific resistors or thermocouples (not thermistors).

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer datasheets list compositions and sintering processes for NTC thermistors; their Beta constants characterize the R–T curve.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing thermistors with metal RTDs or with thermocouples; each uses different materials and principles.

Final Answer:
Metal oxides