H₂sensors made of Pd films on micro-structured templates and supersonic cluster beam deposited oxide films

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H₂sensors made of Pd films on micro-structured templates and supersonic cluster beam deposited oxide films


Author: Zhao, Meng
Title: H₂sensors made of Pd films on micro-structured templates and supersonic cluster beam deposited oxide films
Degree: Ph.D.
Year: 2013
Subject: Hydrogen -- Industrial applications.
Detectors -- Materials.
Gas detectors -- Design and construction.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xxi, 182 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2616062
URI: http://theses.lib.polyu.edu.hk/handle/200/7033
Abstract: Hydrogen (H₂) is widely used in industrial applications and renewable energy research. H₂ is a highly explosive gas, such that H₂sensing technology is particularly envisaged for monitoring leakage and preventing explosion. From a thorough literature review, it was found that the performance of most commercialized H₂ sensors cannot reach the projected requirements for future applications. New sensor materials are needed to be developed to enhance the sensor response; shorten the response time; and improve selectivity for the detection of a target gas. They are also required to be miniaturizable. In recent years, two types of H₂ sensors attract great attention. They are: (i) Palladium- (Pd-) based sensors utilizing the H₂ induced lattice expansion (HILE) effect to close the structural discontinuity in the sensor to produce percolation-like resistance drop; and (ii) Nano metal oxide (MOx) sensors utilizing the large sensor area and porosity to achieve high sensor response and fast response rate. However, difficulties were encountered in the development of both sensor types, which greatly hinder their practical applications. This project consists of the following three major parts with respective considerations to alleviate the problems. The first part of the study is to fabricate and test a Pd-based HILE-type H₂ sensors designed to have the following features. (i) They have structural discontinuity in the form of isolation gaps with precisely controllable dimension, such that the sensors' performance is also controllable as a consequence. (ii) The Pd-based sensor elements were made on a compliant substrate, so that when the sensor element underwent a HILE process, the substrate can be deformed more readily to accelerate the sensor response. The compliance of the substrate also helps to release part of the stresses induced when reacting with H₂, such that the sensor's performance is more stable. (iii) The fabrication method employed was compatible to silicon-based micromachining technique, so that the sensor was miniaturizable and mass-producible. In practice, these ideas were realized by depositing a Pd film onto a single silicon (Si) pillar or a 2-dimensional Si pillar array, with the pillar(s) to have large height-to-thickness ratio up to 30. It was verified experimentally that the sensor response was controlled by tuning the dimension of the spacing around the pillars and the thickness of the Pd layer. The response time can be shorter than 1 s. The sensor output remains stable over quite many numbers of switching cycles. The sensors were produced with Si micromachining techniques and were demonstrated to be miniaturizable. Their sensor performance was compared with other existing HILE-type H₂ sensors. The effects of adding of a gold/chromium (Au/Cr) buffer layer for stabilizing the sensor structure, operation temperature and ambient gas on the H₂ sensing performance were also investigated.
The second part of the study was initiated by a finding from literatures. Many MOx-based H₂ sensors were claimed to have nanostructures in terms of either having nano-sized gas sensing species or a highly porous structure. These two conditions are expected to facilitate achievements of stronger sensor response and faster response rate respectively, but no one single "nano" sensor type reported to date can satisfy all of them concomitantly. In our work, we solved this difficulty by using supersonic cluster beam deposition (SCBD) to produce Pd-coated nanocluster-assembled highly porous tungsten oxide films (Pd/SCBD WO₃). Structural analysis shows that an SCBD WO₃ film is constructed of loosely connected genuine WO₃ nanoclusters of 35 nm in diameter, and the film structure has a porosity as high as 66%. Therefore, the Pd/SCBD WO3 film has a larger sensor response and shorter response time than those of most reported nano MOx-based sensors. The sensor signal is stable over at least 2400 switching circles. Other performances, such as selectivity, relative humidity dependence and ambient pressure dependence were also studied. Possible physical mechanisms involved in the detection processes are discussed. In the third part of the study, we prepared Pd coated sputtered WO₃ (Pd/sputtered WO₃) films to examine whether the oxide films produced by sputtering technique can be made to have satisfactory H₂ sensing performance comparable to that of other nano-structured highly porous H₂ sensor materials. The substrate temperature and sputtering pressure were varied when preparing the oxide films. The structure and H₂ sensing performances of the Pd/sputtered WO₃ film samples were investigated as functions of these two parameters. Efforts are made on correlating the film structure and the H₂ sensing properties. Results were compared with those of Pd/SCBD WO₃ films to examine the feasibility of making high-quality H₂ sensors with sputtering technique.

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