Ultra-fine particle sensors will improve air quality
Plasmas, which consist of charged particles and are one of the four states of matter, are used in high-tech industrial systems such as in machines lithographic to make microchips or in ultrafine particle sensors (UFP) to measure the concentration of tiny particles (less than 0.1 micrometers) that could be harmful to human health.
"Due to their small size , UFPs can lodge deep in the lungs and then enter the bloodstream, causing irreversible tissue damage and disease, ”says Tim Staps, Ph.D. researcher in the Complex Ionized Media group at the Department of Applied Physics.
While industrial UFP sensors have been around for some time, there are several problems that need to be overcome before they are common in society. Their timely availability would also be a great advantage as the Rijksinstituut voor Volksgezondheid en Milieu (RIVM) estimates that the number of deaths in the Netherlands due to the inhalation of particulate matter (including ultrafine particles) emitted by cars and other processes is between 7,000 and 12,000 per year.
The first problem is the cost as laboratory-scale devices are priced at 10,000 euros, too expensive for cars, while the large-scale use of ultrafine particle technology is limited by the lack of legislation. In addition, many technical problems are also hindering their large-scale use.
“In addition to making compact UFP sensors, they shouldn't require ongoing maintenance either. For example, industrial sensors have to be checked every 100 hours, which is not feasible for cars, ”says Staps. “Sensors also need to be sensitive as UFP concentrations in the air can be low and difficult to measure.”
“Accurate UFP sensors could protect people's well-being by monitoring indoor air quality in buildings and in workplaces where UFPs are on the agenda. To combat the high concentrations of UFP, heating, ventilation and air conditioning could therefore be introduced. The bottom line, however, is to accurately measure UFPs and then act on their detection. ”
For his doctoral research, Staps and his colleagues have developed ways to accurately measure the surface charge of particles. “First, we charge the particles using a plasma by guiding the electrons and ions of the plasma to the particles, and then we measured the amount of charge carried by the particles. It is the total charge of all particles that gives a measure of their size and concentration "explained Staps.
To measure the charge of particles, Staps turned to the resonance spectroscopy of the microwave cavity (MCRS), a technique used since the 1950s to measure free electrons in a gas under vacuum conditions. Staps and his colleagues adapted the technique for use under normal conditions of air pressure and density.
“In a vacuum, electrons can travel many meters before colliding with a gas or dust particle . Under normal conditions this distance decreases dramatically and the signals produced when an electron hits a gas or dust particle are much smaller than in a vacuum. Thus, we designed a new configuration that minimizes the effects of external vibrations and other sources of signal noise. "
In both vacuum and atmospheric conditions, Staps and the researchers found that collisions between particles and free electrons in the plasma determined whether the particles were charged or not. “Such observations are important for understanding the physical process behind the charging and discharging of particles. But the same data can also be used to develop new theories to describe charging processes under conditions of varying pressure. "
Detecting collisions between electrons and particles is one thing, but Staps and the researchers needed to measure the electrons bound to the particles and the presence of negative ions (which are the final dust particles) in the plasma. But to measure the charges, the electrons must be freed from the particles, and to do this, the researchers used lasers in combination with MCRS.
"The laser approach, known as photo-signaling, involves firing a large number of photons against particles. It is important to emphasize that the energy of the photon exceeds the binding energy that confines the electron to the surface of the particles. This is a very unique way to detect the charge on particles in a vacuum and the presence of negative ions at atmospheric pressure. ”
Staps is extremely optimistic that his research could provide an excellent starting point for the development of future particle sensors. "To make accurate ultrafine particle sensors, we need to understand how small particles are charged and then use this data to formulate new theories about plasma-based charging of nanoparticles," said Staps.
"These insights can accelerate the advancement of sensor technologies and the production of these sensors on an industrial scale is likely to happen very soon. But engineering challenges remain, one of which is that plasmas interacting with air become contaminated fairly quickly. ”
In addition, the development of accurate sensors can help industries minimize particle production ultrafine from processes, thereby improving air quality and reducing the health risks of those who work and live near systems that produce UFP.
