# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 3 Heat Conduction Hydrogen Sensors
### Status of Research  
Thermally conductive hydrogen-sensitive materials include not only the precious metals Pt and Pd, which play a catalytic role, but also thermoelectric materials, which convert a temperature difference into electrical signals. [PERSON] et al. [39] showed that the catalytic activity of Pt film increases with the increase in film thickness, and when the film thickness exceeds 60 nm, the catalytic activity of Pt film reaches saturation and no longer increases with the increase in film thickness. In addition, the smaller the Pt particle size, the better the catalytic activity of Pt film, and when the film thickness exceeds 150 nm, the catalytic activity of Pt film is no longer affected by Pt particle size. Recently, [PERSON] et al. [40] showed that the catalytic activity of powdered catalysts is higher than that of thin film-type catalysts because the powdered material has a larger specific surface area and roughness, which is more tolerant to contact with hydrogen. They developed a micro-thermal heat conduction-type hydrogen sensor using a mixture of Pt and Al\({}_{2}\)O\({}_{3}\) powders as catalysts, which can detect hydrogen in the concentration range of 0.005-3% at room temperature, and found that the catalyst activity was maximum when the mass ratio of Pt and Al\({}_{2}\)O\({}_{3}\) was 2:3. Thermoelectric materials require a small heat capacity and fast heat transfer. [PERSON] et al. [41] developed a heat conduction-type hydrogen sensor by plating a Pt catalytic film on the 1/2 surface of NiO film, which can detect hydrogen in air at 0.025-10% concentration. Here, there is a good linear relationship between the detection signal and hydrogen concentration, and the films can quantitatively detect a higher concentration of hydrogen when nitrogen is used as the dilution gas. The conductivity of thermoelectric materials can be controlled via basic doping, such as Li, Na, Ni, etc. [PERSON] et al. [42] reported that a Ni film (30 nm) was sputtered on a SiGe film (3 \(\upmu\)m), and after annealing at 500-600 \({}^{\circ}\)C, the SiGe was transformed from an amorphous to a crystalline state. A Pt film (1 \(\upmu\)m) was then sputtered on the crystalline film to make a heat conduction-type hydrogen sensor. The research shows that the crystallinity of the thermoelectric material has a great influence on the hydrogen-sensitive performance of the element, and the greater the crystallinity, the better the hydrogen-sensitive performance.  
The design of the heat conduction-type hydrogen sensor is based on the theory that the thermal conductivity of hydrogen is greater. In a gas mixture including air, the thermal conductivity of oxygen is less than one-eighth that of hydrogen; thus, the thermal conductivity of the gas mixture is mainly determined based on the concentration of hydrogen. Heat conduction-type sensors are currently used in many applications due to their large measuring range, variety of gases measured, and high stability. At the same time, heat conduction-type gas sensors also have technical advantages, such as long life, good stability in a large detection range, and low cost. However, because the thermal conductivity sensor detects the concentration based on the temperature change in the thermal element, the measurement results will be affected to some extent by the selection of gas-sensitive materials and thermal conductivity, resulting in low accuracy, low sensitivity, large temperature drift, and other problems. To solve the problems of thermal conductivity gas sensors, their detection methods must be improved to overcome the defects of temperature drift [43; 44; 45; 46].