Development of novel low-temperature selective hydrogen gas sensors made of palladium/oxide or nitride capped Mg-transition metal hydride films

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Development of novel low-temperature selective hydrogen gas sensors made of palladium/oxide or nitride capped Mg-transition metal hydride films

 

Author: Tang, Yu Ming
Title: Development of novel low-temperature selective hydrogen gas sensors made of palladium/oxide or nitride capped Mg-transition metal hydride films
Degree: Ph.D.
Year: 2011
Subject: Gas detectors.
Thin films.
Palladium.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: 169 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2522648
URI: http://theses.lib.polyu.edu.hk/handle/200/6566
Abstract: Palladium capped Mg-based transition metal alloy film (Pd/Mg-TM) is a potentially useful hydrogen gas (H₂) sensing material, which can operate at low temperature for detection of H₂ leakage in an environment to ensure safe use and storage of the gas. The Pd layer catalytically dissociates hydrogen molecules, and the hydrogen atoms produced can enter (hydridation) or be detached (dehydridation) from the alloy layer. These processes are reversible, such that the film is switchable between a metal state and a hydride state, giving rise to substantial changes in its optical transmittance/reflectance and electrical resistivity. Unlike a conventional metal-oxide (MOx) H₂ sensor, hydridation of an Mg-TM film is associated with relatively low enthalpy, and hence can perform at temperature much lower than the operation temperature of an MOx sensor (typically around 500°C or above). As such, an Mg-TM based sensor does not experience undesired annealing effect during operation, and hence is much more stable and durable. Furthermore, the detection selectivity of a Pd/Mg-TM film versus other reducing gases is superior to most conventional MOx-type hydrogen sensors. In this project, we systematically investigated the H₂ sensing properties of Pd/Mg-TM films. The emphases of our work and the major results obtained are briefly delineated in the following. 1. The dependences of H₂ sensing properties of Pd/Mg-nickel (Ni) films on hydrogen partial pressure (PH₂) and temperature were investigated with a specifically designed measurement system. At 60°C, the changes of the resistance (R) and optical transmittance (T) with He-Ne laser as incident light of a Pd/Mg-Ni film were observed with PH₂ increased with a very slow rate (quasi-static mode). During measurements, the film in the measurement chamber was made to get contact and react with a H₂-argon (Ar) mixture, where the gas was admitted into the chamber very slowly. The relationship obtained is referred to as the resistance-pressure isotherm (RPI) and optical transmittance-pressure isotherm (TPI) respectively. Each plot shows three different stages. In Region I of low PH2, both R and T increase linearly with increasing PH2, with relatively fast response and recovery times. In Region II of moderate PH₂, both R and T increase more rapidly and non-linearly with PH₂. In Region III of even higher PH₂, the increasing rates of the responses with increasing PH₂ become slower. The durability of the gas sensing response were investigated by exposing the film to air and H₂-Ar (or H₂-air) mixture with a fixed H₂ content alternatively. This testing method is referred to as the cyclic mode of measurement. Results show that the H₂ sensing response is most stable in Region I, because the hydrogen content in the film varies in a low range such that hydrogen atoms are only incorporated interstitially without affecting the network. As such, only a solution of hydrogen is formed, which is denoted as the α-phase of Mg-Ni alloy. In Region II, more hydrogen atoms are incorporated into the film, such that transitions between the α-phase and β-phase occur. The gas sensing responses are stronger, but H-induced volume expansion and contraction of the film material occur, which initiate cracking of the film surface and subsequent invasion of oxygen from the surrounding. The oxygen atoms tend to react with Mg atoms and a surface oxide layer is formed to degrade the reproducibility of the gas sensing response of the film. In Region III, the film is mainly in the β-phase. The difference between the specific volumes of the alloy phase and β-phase are great (over 30%), such that cracking of the Pd cover layer and the alloy layer is more severely, and the gas sensing properties deteriorate very fast with increasing number of switching cycles. Data obtained at various measurement temperatures reflect that degradation of the gas sensing response of a Pd/Mg-Ni film is associated with a decrease in the enthalpy and entropy, and an increase in the activation energy of the hydridation processes. Combining with compositional and structural data, one expects to gain insights on the mechanisms governing the stability of switching responses of R and T.
2. Referring to the results on the gas sensing responses detected, we recognized that the durability of the gas sensing responses of Pd/Mg-Ni films was still a problem requiring further improvement. Three remedial schemes were implemented and the results are briefly described in the following. Increase the thickness of the Pd cover layer (tPd). In particular, the H2 gas responses of two Pd/Mg-Ni (35 nm) films with 5 and 10 nm Pd cover layers respectively obtained in cyclic tests were compared. The test was performed by exposing a film to a 4% H₂-argon (or 4% H₂-air) mixture. The gas responses the one with a smaller tPd started to degrade substantially after around 400 cycles, but that of the one with a larger tPd can last to over 2000 cycles without significant deterioration. This suggests that a thicker Pd cover layer can obstruct the invasion of oxygen into the alloy layer more effectively, and thus decelerates the formation of surface oxide. It also shortened the response time substantially, but the drawback was to cause a reduction in gas detection sensitivity. The next one was to increase the Ni content near the film surface before the addition of the Pd cover layer. This approach is based on the intention of obstructing the diffusion of the Pd atoms in the cover layer into the Mg-Ni layer and to alloy with Mg atoms. Results show that both the sensitivity and stability are prominently improved. The drift in enthalpy and entropy associated with the hydridation process are greatly suppressed. In addition, data of X-ray photoelectron spectroscopy (XPS) analyses showed that diffusion of Pd into the Mg-Ni layer was substantially reduced, explaining the observed improvement of the stability of the gas responses of the film. The third approach was to dope iron (Fe) into the Mg-Ni layer by means of co-sputtering when depositing the alloy film. Results show that stability of the gas sensing responses of the Fe-doped Pd/Mg-Ni films was also improved. The reason leading to the improvement was found to be similar to that of the surface Ni-enriched Mg-Ni film, namely by suppression of alloying between Pd and Mg atoms, but the switching rate of a Fe-doped one was found to even faster. 3. To further examine the possible mechanisms responsible for the hydridation of a Pd/Mg-Ni film, we tried to construct a simple four-step kinetic model consisting of physisorption, chemisorptions, surface penetration and hydride formation to describe the process. A set of differential equations was established, which was expected to embrace the mechanisms controlling the temperature and PH₂ dependences of the absorption and desorption kinetics. The model was justified to be effective in describing the hydridation of a Pd/Mg-Ni film by reproducing the trends of the temperature and PH₂ dependence of the gas sensing responses observed experimentally. The model is not only useful in helping one to gain physical insight of the switching process of an Mg-Ni film, but may be also valuable in giving guidelines for optimizing the operation condition of the film for Hc detection. 4. The response of the film to the variation in the relative humidity in the detected environment was observed. The presence of moisture was found to be influential to the degradation of the gas sensing properties of an Mg-Ni film, because water molecules can react with Mg atoms to form certain compounds, which eventually weaken the H-induced switching properties of the substance. This assertion is proposed according to the results of atomic force microscopy (AFM) analyses, which indicated that the surface morphology of the film sample after experiencing a certain number of switching cycles in a humid environment became very rough. We further found that the influence of moisture can be alleviated to a certain extent by adding a SiO₂ or SiNx layer on a Pd/Mg-Ni film. The durability of a film with such a water-resistant layer was found to be prominently improved.

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