Discovered by Henry Cavendish in 1766 and then named by Antoine Lavoisier a few years later, hydrogen gas has been used by the human kind for many decades. Although artificially produced H2 was primarily used in aviation as the floatation gas for balloons and Zeppelins in the first one and half centuries after the discovery, nowadays gaseous hydrogen exploited by the society are mostly found in the fuel cells, which are emerging as a kind of promising clean energy source in the 21st century. Nowadays, industries, such as the coal fire power plants and oil refineries, use large amount of H2 at high temperatures while H2 is an explosive gas (e.g., back to the age of the Zeppelins, the
crash of the Hindenburg airship in 1937 in Manchester, New Jersey cost 36 lives), detecting hydrogen with trace concentrations at elevated temperatures are applied extensively in industries and society.
A hydrogen sensor is a gas detector that detects the presence of hydrogen. Such hydrogen gas sensors are used to locate leaks from any of those reservoirs. A modern hydrogen sensor should be low-cost, compact, durable, and easy to maintain as compared to conventional gas detecting instruments. Besides of portability, cheapness, sensitivity, and rapidity, durability at high-temperatures is an indispensable requirement for any materials/structures involved in the sensors demanded for H2 fuel cells.
Based on hydrogen sensors in commercial markets and scientific literatures, several detection methodologies and numerous sensing materials have been developed by difference researchers. Since the aim of the research is to develop on-chip sensors based on semiconductor technologies and micro-electro-mechanical systems (MEMS), only solid-state hydrogen sensors will be involved in the review.
What is hydrogen and why is it?
Hydrogen (H2) is the most abundant element in the universe and one of the most abundant on Earth. It is the ultimate fossil fuel candidate, with a high heat of combustion (142 kJ/g), low minimum ignition energy (0.017 mJ) and wide flammable range (4-75%), as well as high burning velocity. Due to the rapid consumption of fossil fuels, much attention has been paid towards H2 as one of the economical, non-conventional and best clean energy source/carrier for the many industrial applications. For example, the fuel cell technology uses gaseous H2 for the generation of power. Hydrogen power cars and buses are already in normal transit service in some of U.S. cities. Liquid hydrogen has been used for launching the space-shuttles. Hydrogen also finds applications in electronic, metallurgical, pharmaceutical, nuclear fuels, food and beverages, glass and ceramic industries and as well as the daily chemical industries, etc. Due to its strong reducing properties, every day millions of kilograms of hydrogen are used by hundreds of industries around the world. The combustion product of hydrogen is water, which is free from contamination and can be converted into hydrogen and oxygen again for periodic duties.
OPEN ACCESS
Due to the realization of the potential of hydrogen energy, further interest has grown into the production of large quantities of different forms of hydrogen, the enhancement of hydrogen storage capacity, and the development of safe transportation system for hydrogen.
Why we need a hydrogen sensor?
Hydrogen is a chemically reactive and highly explosive element which is tasteless, colorless and odorless that can't be detected by human beings. Due to its low ignition energy and wide flammable range, makes it as dangerous for transport, storage and uses as many other fuels. The safety and handling issues on hydrogen transportation and storage are the top priorities in the hydrogen industries. These concerns have been the prime motivation for people to develop a sensor or detector to indicate the appearance of hydrogen gas in a wide range of temperature and at difference concentrations. The sensor must have high sensitivity, selectivity, repeatability and rapid response times. For the development of efficient and reliable hydrogen sensors for process monitoring and leak detection applications, direct or in-situ monitoring of processes are used and require the sensors to be placed in environments where temperatures may exceed 500â-¦C. Hence materials and sensing structures capable of withstanding such conditions are being exhaustively investigated.
As present, there are many commercially available hydrogen sensors, including electrochemical, semiconductor, metallic, thermoelectric, optical, acoustic and etc. Among them, so far the semiconductor type sensors can fulfill our requirements of sensor characteristics that stated above. Nevertheless, this type of sensors, especially the Si based substrate manufacturing processes, still suffers however from high operation temperature, which results in high power consumption and potential safety hazards. Moreover, the cross selectivity to other combustible or reducing gases is another critical issue, which should be restricted to enhance the sensing accuracy.
Figure 1 shows the growing number about relevant publications about semiconductor hydrogen sensors.
Fig. 1: Publications about semiconductor hydrogen sensors since 1996 according to an enquiry in Thomson Reuters ISI Web of Knowledge.
Background of Study
Nowadays, with the increasingly serious environmental pollution and raised of the awareness in environmental conservation in many countries, have push many scientists and engineers making efforts in producing green products, which are environmentally friendly. Looking a clean, renewable energy source becomes an urgent need to overcome the conundrum. The shortage and polluted impacts of fossil fuels has reinvigorated interest in the advancement of its alternatives.
Hydrogen, the most abundant gases element in natural, will be the most promising candidate. Today, hydrogen has many important applications such as it's used in the processes of many industries that include chemical, petroleum, food and semiconductor. Furthermore, the negative environmental impacts of burning fossil fuels, coupled with rising oil prices, have lead to renewed interest in these clean energy technologies, especially those involving hydrogen. Molecular hydrogen can become efficiency energy carriers for use in fuel cells and combustion engines.
It's being much doubts about hydrogen's safety have prevented it from fulfilling its potential as a fuel source. According to A. Trinchi et al., Its low mass and high diffusivity makes it difficult to store. Hydrogen is a highly active and flammable chemical element in concentrations ranging from 4% to 90% by volume, with its lowest explosion limit being 4.1%, making the need for placing hydrogen sensors near high concentration storage facilities essential. It is also a major cause of corrosions whereby it degrades the mechanical properties (strength and durability) of metals. This is especially significant at elevated temperatures, where it is termed as high temperature hydrogen attack (HTHA) and results in embrittlement. For example, the tensile strength of steel alloy 4140 can be reduced by as much as 67% when exposed to hydrogen at a pressure of 41MPa and a temperature of 300K. For optimal operations and safety issues, the use of hydrogen in advanced power production will required a real-time monitoring of the hydrogen concentrations via an online gas sensing system that is reliable and can survive the harsh and chemically corrosive operating conditions within the combustion engines. Hydrogen gas sensors would form an integral part of such systems incorporating hydrogen as a fuel. These electronic devices are important in various applications for safety reasons. It is of great attention mainly for detection of hydrogen leakages. The desired gas sensors must function at temperatures up to 800 â-¦C and at least 4000h of operating lifetime in the presence of gases containing significant amounts of hydrogen, and maybe other types of hydrocarbon gaseous. Hence, materials and sensing structures capable of withstanding such conditions are being exhaustively investigated. High sensitivity, selectivity, reliability, fast response time for real-time monitoring, long-term stability, low hydrogen concentrations workable, and cost effectiveness are additional essential factors for the sensing system.
Harsh environment, in general, is termed as extreme environment, hostile environment, rough environment or "unfriendly" environment. These environments are always referred to the situation involving high temperature, high frequency, high power, high electromagnetic interference (EMI), intense vibrations, erosive flows, high radiations and high aggressive media exposure. In addition, harsh environment is related to the condition which is likely to cause significant corrosion-related degradation. Typically, harsh environment applications are related to automotive (main aim), aerospace, avionics, micro-propulsion, turbo-machinery, industrial process control, nuclear power, communication and well-logging industries. In these industries, not only the operational temperatures are high but also the inability to provide cooling system. Thus, this would cause conventional pure Si-based hydrogen sensing electronic systems to fail.
Consequently, there is a need for semiconductors with good thermal stability and wide bandgap for stable electronic properties at elevated temperatures. M.T. Soo et al. investigated that Silicon Carbide (SiC) is able to deal with this harsh environment application when we used it as a substrate to develop hydrogen sensors. These wide bandgap semiconductors offer great potential to fabricate active high-temperature electronics and micro-systems for applications in very-high-temperature regimes (more than 300â-¦C). Furthermore, wide bandgap semiconductors may offer additional advantages in terms of high power and high frequency applications. However, the most mature, in terms of film growth and process technology, among all of the wide bandgap semiconductors stuff is SiC. Therefore, development of SiC-based hydrogen sensor is of strong concern.
SiC has additional attractive features compared with other wide bandgap semiconductors. SiC substrates are commercially available, it has known device processing techniques and it has an excellent ability to grow a good quality of thermal oxides. As a result, SiC is now in the forefront of high voltage and power semiconductor research. There are almost 250 poly-types of SiC that have been discovered. For different poly-types, the bandgap ranges from 2.2 eV for the cubic configuration to 3.3 eV for the hexagonal configuration. This range of wide bandgap allows high temperature operation up to 1000â-¦C. This property of SiC allows hydrogen sensors based on this material to be integrated with high-temperature electronic devices on the same chip. Moreover, it has excellent thermal conductivity (3-4.9W/cmK), chemical inertness and radiation hardness.
Most of the SiC-based hydrogen sensors are grouped into field effect devices, which properties are determined largely by the effect of an electric field on a region within the devices. The unique working principle of SiC field effect devices makes it a gas sensor with high sensitivity and good selectivity towards a variety range of gases, such as hydrogen or hydrocarbons. Furthermore, these SiC-based devices able to perform as rapid sensors over a broad range of temperature, where the response is in the order of milliseconds (ms). Gas sensors of this structure have a great stability and reliability for harsh environment applications. Among these field effect devices, metal-oxide-semiconductor (MOS) SiC-based hydrogen sensor is preferred. MOS capacitor hydrogen sensors are very simple to fabricate. They consist of junctions of metal, semiconducting and insulating materials. Direct measurements of current, capacitance and conductance are made as a function of bias voltage, and from them numerous parameters (including barrier height, interface state density, etc.) could be determined. Thus, it is preferred in our research works.
Field effect hydrogen gas sensors based on Si substrates have operating temperatures limited to below 200 â-¦C. High temperature operation, particularly for long periods, places great strain and demands on the sensors. Consequently, research efforts have intensified in the last decade into materials that not only allow for increased operating temperatures, but which are also compatible with Si fabrication technologies. SiC has emerged as the leading candidate for field effect based sensors. In addition, to its compatibility with Si, its wide band gap, chemical inertness and stability make it ideal for high temperature operation in harsh/extreme environments.
Fig. 2: Hierarchical organization chart of solid-state hydrogen gas sensors with respect to the compatibility to operating temperatures. The maximum tolerable temperature ranges for the devices are marked in different colors.
Statement of the Problem
We are already get the prototype of the sensors (die) which provide by W. M. Keck Micro-fabrication Facility at Michigan State University of United State. Now, we will conduct both the lab and real-environment test on those samples. Those samples with varieties doping diameter (or concentrations), oxide thickness and metal gate material will be tested under difference concentrations of hydrogen gas. But now we will focused on by set the concentration of hydrogen in 0.2% and 2% respectively. By using the workstation that provide by Keithley (software include), we are able to study the characteristics of those samples by analyzed the C-V curve that generates from the Keithley semiconductor characterization system.
After that, those sample prototypes must be sealed in a ceramic or polymer which normally a final stage in integrated circuits fabrication. Then, in addition with an external signal (capacitive) detected and amplifier circuits, we are able to test the ready-to-use sensor system in real-environment, such as in hybrid or hydrogen fuel-cells car engine. There are four key issues to be met by any hydrogen detector, if it is to gain wide acceptance for use within the hydrogen infrastructure (production, storage, transportation, and utilization).
Performance - sensors must respond to the presence of hydrogen well before the explosive limit (4% H2 in air) is reached. This requirement dictates that a premium is placed on detecting small quantities of hydrogen in the ambient atmosphere (prefer detection limit of 0.5% or better). The sensor must also respond quickly (prefer response time of 1 second or less), so that corrective action or evacuation can occur before the explosive limit is reached. Fast response times are also desired for diagnostic study of hydrogen transportation systems (vehicles, electrolyzers, storage containers, etc.).
Lifetime - sensors must have a usable lifetime consistent with the application for which it is intended. For transportation applications that must be at least the time between scheduled maintenance (minimum 6 months, prefer 1 year or more). In this respect the sensor must be operational with no active effort for a minimum of that period, while exposed to ambient conditions.
Reliability - sensors must indicate the presence of hydrogen reliably. That is, they must perform to some specification, each and every time they are exposed to hydrogen over the lifetime of the sensor. Response must not drift outside acceptable limits over that lifetime. Functionality of the sensors should be easily verifiable, but there will be a low tolerance for false alarms. Sensors should be able to survive multiple excursions to hydrogen concentrations above the explosive limit without damage.
Cost - sensors and their controllers must be reasonably priced, so that their inclusion within the hydrogen infrastructure can be ubiquitous. A worthy goal is $5 per sensor and $30 per controller. As long as performance, lifetime and reliability are not compromised, less expensive is better.
Also, the demands on hydrogen sensors can be summarized as follows:
Indication of hydrogen in concentration range, 0.01-10% (safety) or 1-100% (fuel cells)
Safe performance, i.e. explosion proof sensor design and protective housing
Reliable results of sufficient accuracy and sensitivity (uncertainty < 5-10% of signal)
Stable signal with low noise
Robustness including low sensitivity to environmental parameters such as:
temperature (-30-80â-¦C (safety), 70-150â-¦C (fuel cells))
pressure (80-110 kPa)
relative humidity (10-98%)
gas flow rate
Fast response and recovery time (<1s)
Low cross sensitivity (e.g. hydrocarbons, CO, H2S)
Long life time (>5 years)
Low power consumption (<100 mW)
Low cost (<100 per system)
Small size
Simple operation and maintenance with long service interval
Simple system integration and interface
Summary
In recent years, many investigations are carrying out to obtain gas sensors which are characterizing with small size, high response and recovery time and working near high temperature and harsh environments. Large amounts of publications related to hydrogen gas sensors because hydrogen is one of the most dangerous and combustible gases. Besides, hydrogen is one of the best candidates of energy carriers, and because of limited resources of coal and natural gas, hydrogen will become one of the main energy carriers in the near future.
The reputation of hydrogen as the next generation energy delivery agent, supplementing electricity in use today, has been firmly established. Hydrogen-based economy has come around the corner while fuel-cell cars often feature prominently in news media as evidence.
It is generally recognized that building the infrastructure - not the vehicles - is the task to bring about the realization of hydrogen economy. For example, safe storage and delivery of hydrogen is a necessity that stands in line before public acceptance of using hydrogen, much like electricity today, as an energy medium. Electricity is a form of energy, and hydrogen is a matter. This fundamental difference calls for implementation of an entirely different infrastructure that is non-existent today. In fact, many of the technologies to build such infrastructures are under development themselves, of which hydrogen sensor technology is one area of interest pertaining to our research title.
The ability to detect and quantify the amount of gaseous hydrogen present is fundamental to all aspects of hydrogen processes. Unlike electricity, which is essentially confined to its carrier, hydrogen is a gaseous matter at atmospheric conditions and can escape from its containment to cause an explosion hazard. Hydrogen sensing is also required as a means of monitoring and controlling hydrogen-based processes used in, for example, fuel cells. Because the point-of-use nature of hydrogen-based energy economy, the demand for hydrogen sensors is very great in terms of quantity and variety because each applications may have specific requirements for sensor characteristics. The quantity demand also translates into a requirement for low production cost so that needs, rather than costs, are the determining factor for sensor use.
