Last Updated on April 24, 2023 by You Ling
Over the past few decades, piezoelectric materials have played a crucial role in the field of aerospace engineering. One of the primary factors in reducing aircraft failures is their health monitoring. For this purpose, various types of sensors are specifically designed for aircraft. Operating all these sensors requires a significant amount of energy, and hence, harvesting techniques have been adopted. This article discusses the applications of piezoelectric materials in health monitoring, aircraft sensors, and energy harvesters. It elaborates on how piezoelectric materials are used at various stages in the aerospace industry.
Main Applications of Piezoelectric Ceramics in Aerospace
1、Structural Health Monitoring (SHM)
Piezoelectricity is used for Structural Health Monitoring (SHM), which checks the integrity of mechanical structures during their use. This is especially important where safety is a major concern, such as transport structures, infrastructure, and building structures. Continuous monitoring has apparent economic advantages compared to conventional inspections, such as those requiring scheduled interruptions of service or even dismantling selected components of aircraft.
How it works:
Sensors used in SHM are fixed or integrated into the structures, where they continuously monitor them. SHM systems can be either “active” or “passive.” “Passive” systems only “listen” to the eventual noise generated by crack propagation (especially in the case of composite structures) or changes in structural frequency response. “Active” systems generate acoustic waves within the material. When the high-frequency acoustic pulse emitted by the transducer encounters materials with different acoustic impedances (density and speed of sound), they are partially absorbed or reflected. “Active” SHM employs different methods, with the most common ones being the analysis of transmission between two transducers (“pitch-catch”) or listening to the reflected sound (“pulse-echo”). If a defect is present in the structure, the reflected signal will increase, while the emitted signal will decrease. Detection, localization, characterization, and assessment of the defect require complex signal processing and analysis.
Piezoelectric actuators, in particular, have provided significant improvements for many aerospace applications, ranging from satellite control to jet propulsion.
Piezoelectric ceramics are used in micro-thrusters for satellites, where the micro-thrusters are used for positioning and stabilizing the satellite. Naturally, there is a high focus on the reliability and functionality of the products used in space.
How it works:
Micro-thrusters can use various technologies, but the one of interest here is the so-called “cold gas micro-thrusters.” In this method, the thrusters generate very small and controlled forces (<500µN) by ejecting gas stored in a high-pressure tank (usually nitrogen). This requires both precise control of the propellant pressure in the circuit and rapid, accurate “dosing.” The piezoelectric actuator integrated into the valve ensures both these functionalities. For redundancy, the micro-thrusters are equipped with multiple piezoelectric actuators. The motion of the piezoelectric actuators ensures fast, precise control of the flow rate during operation.
As the exploration of space intensifies, the need to understand this new environment becomes increasingly important. For years, NASA has been testing the feasibility of using piezoelectric sensors to detect the impacts of space particles and debris. Given the capabilities of piezoelectric components, NASA designed an impact piezoelectric ceramic sensor to detect micrometeoroids and sub-millimeter orbital debris that are usually difficult to track. The impact piezoelectric sensors work by utilizing thin piezoelectric strips or plates. When space debris impacts the piezoelectric sensor, the particle collision generates vibrational waves on the piezoelectric strip or plate. The vibrational motion from the debris impact creates an electrical signal proportional to the applied strain, allowing the piezoelectric sensor to capture the location of the impact and use this information to identify velocity and direction data. The proportional charge generated by the impact can also be used to determine the particle material and density. Overall, the impact detection capabilities provided by piezoelectric ceramic sensors open the door to a comprehensive understanding of the space environment.
All engines are based on the Carnot cycle principle, which means the higher the gas temperature, the higher the efficiency. This requires increasing the thrust-to-weight ratio of aviation engines and reducing fuel consumption, and raising the turbine inlet temperature is the key. As a result, research on high-temperature structural ceramics and ceramic matrix composites has become one of the critical technologies for high thrust-to-weight ratio aviation engines.
Ceramic bearings are widely used in the aerospace industry, with properties such as high-temperature resistance, cold resistance, wear resistance, corrosion resistance, magnetic insulation, and high-speed rotation. They are developed for the harsh conditions of the aerospace industry, such as adjustment, heavy load, low temperature, and unlubricated conditions, and represent a perfect combination of new materials, processes, and structures.
6、Infrared Camouflage Stealth Materials
Ceramic infrared camouflage stealth technology is achieved by using infrared functional ceramics to reduce or change the target’s infrared radiation characteristics, thereby achieving low detectability. This material can alter infrared radiation properties, has low infrared emissivity within atmospheric window bands, and blends the target’s infrared characteristics with the surrounding environmental background in an infrared field, maximizing the reduction of the target’s infrared signature.
Moreover, the use of ceramic components in engines can reduce heat generation and suppress infrared radiation emission, providing infrared stealth functionality.
7、Antenna Radome Transmissive Materials
Aerospace transmissive materials (antenna radomes) are multi-functional materials designed to ensure that communication, telemetry, ignition, guidance, and other related systems can operate normally under harsh environmental and climatic conditions. Porous silicon nitride ceramics have a low dielectric constant and dielectric loss, low density, good thermal insulation, suitable strength, and long service life. Their low radar wave absorption rate per unit thickness compared to other ceramics makes them an excellent choice for aerospace transmissive materials.
To maximize the service life of satellite batteries, ceramic separator materials must be used. Ceramic separators are made by vacuum-finely mixing rare earth and other composite materials, then sintering at high temperatures. They are resistant to strong acids and bases and do not dissolve in chromic acid solutions.
9、Aircraft Brake Discs and Rocket Engine Anti-Oxidation Ceramic Coatings
Carbon/carbon composites, due to their unique properties, have been widely used in the aerospace field, such as rocket engine nozzles, throat linings, and aircraft brake discs. However, they have a fatal flaw: they oxidize in oxygen environments above 400℃, leading to a rapid decline in material performance. Therefore, anti-oxidation protection measures are necessary for carbon/carbon composites in high-temperature oxygen environments. Anti-oxidation ceramic coatings, with their excellent physical and chemical stability, provide a solution.
When rockets break through the atmosphere, they experience significant external friction, affecting internal sensors such as temperature and pressure sensors. In addition to significant forces, a large amount of heat is generated externally. Pressure sensor accuracy is critical; if the internal circuit board is damaged due to external forces, the pressure sensor is rendered useless. Zirconia ceramic substrates, with their high wear resistance and compressive strength, prevent damage.
11、Aerospace Vehicle Shell Ceramic Coatings
HfB2, ZrB2, and ZrC are used for ultra-high-temperature ceramic coatings. As hypersonic vehicles advance, the need for surface anti-ablation and anti-atmospheric erosion increases. Ultra-high-temperature ceramics, such as HfB2, ZrB2, and ZrC, provide indispensable contributions to enhancing vehicle surface resistance to ablation and erosion.
12、Infrared System Windows
These ceramics, composed of yttrium oxide and magnesium oxide, were jointly invented by young scholars from Far Eastern Federal University, experts from the Far Eastern Branch of the Russian Academy of Sciences Institute of Chemistry, the Ukrainian National Academy of Sciences Institute of Single Crystals, and the Chinese Academy of Sciences Shanghai Institute of Ceramics. It is reported that this material can be used to make infrared system windows for aerospace equipment, allowing more than 70% of infrared light below 6000 nm to pass through.
Considering the emphasis on battlefield survivability in the design of armed helicopters, ceramic composite lightweight armor materials are used in seats and critical helicopter parts. The special ceramics involved are mainly alumina ceramics and boron carbide ceramics.
The former Soviet Union’s “Mi-28” boldly used special ceramics as protective materials for the fuselage and cockpit, equipped with two layers of armor plates. A certain thickness of titanium alloy plate was located between the two layers of armor, while a large amount of special ceramics was used on the outside.
In summary, ceramics play a crucial role in various aerospace applications due to their unique properties, such as high-temperature resistance, wear resistance, and high strength. Key applications include aviation engines, ceramic bearings, infrared camouflage stealth materials, antenna radomes, satellite batteries, aircraft brake discs, rocket engine anti-oxidation coatings, ceramic substrates, aerospace vehicle shell ceramic coatings, infrared system windows, and helicopter armor. These advanced ceramic materials contribute significantly to enhancing the performance, safety, and reliability of aerospace equipment, enabling them to operate in extreme environments and withstand challenging conditions. The development and utilization of these materials will continue to drive progress in the aerospace industry, paving the way for future innovations and breakthroughs.