Researches

• 1. Introduction
  
• 2. Energy technologies using plasma
  
  • 2.1 Plasma processing of dye-sensitized solar cells
    
  • 2.2 Spark ignition of hydrogen
    
• 3. Enviromental technologies using plasma
  
  • 3.1 Removal of gaseous environmental pollutants
    
  • 3.2 Water treatment
    
• 4. Plasma medicine
  
• 5. Fundamental study of plasma reactions
  
  • 5.1 Laser diagnostics of plasma
    
  • 5.2 Development of plasma simulation
    

We use chemical reactivity of atmospheric-pressure discharge plasma (streamer discharge, dielectric barrier discharge, plasma jet, atmospherci-pressure glow discharge, spark discharge) for energy technologies, environmental pollution controls, and medicine. In addition, we study the reaction mechanism of the plasma using laser spectroscopy and simulation.

Plasma is generated using a high-voltage (1-100 kV) electrical discharge in a gas (e.g. air, argon). In the plasma, electron-impact reactions with gaseous atoms/molecules produce a number of chemically active species such as ions (N2⁺, O2^-), excited species (N2(v), N2(A), O2(a), O(D)), and radicals (O, N, OH). As a result, the plasma has very high chemical reactivity with low heat load, where the gas temperature is about 300 K.

We improve the conversion efficiency of dye-sensitized solar cell (DSSC) using plasma treatment on TiO2 nanoporous electrode. The amount of dye molecules on the TiO2 film is increased by the plasma treatment due to the surface chemical modification on the TiO2 film by plasma reaction. Necking of TiO2 nanoparticles is also improved by the plasma treatment. DSSC is one of next-generation solar cells. In Japan's PV roadmap towards 2030 (PV2030+) by New Energy and Industrial Technology Development Organization (NEDO), the target of module and cell conversion efficiencies of DSSC by 2025 are 15% and 18%, respectively.

       

(Left and center) dye-sensitized solar cell. (Right) plasma treatment.

Combustion efficiency and stability of automobile and aircraft engines can be improved using plasma. Plasma dissociates hydrocarbon fuel molecules and oxygen molecules to produce radicals. It results in acceleration of combustion. Plasma also has the effect of ignition like spark-ignition in automobile engine. Related with these technologies, we investigate plasma-combustion reaction of spark plasma in flammable gas (hydrogen). This research is also related to safety of electrostatic hazard of hydrogen used for fuel cells.

   

(Left) Experiment on spark ignition of hydrogen.
(Right) Measurement of OH radicals in spark-ignited hydrogen-air mixture using laser-induced fluorescence (LIF).

We decompose gaseous environmental pollutants such as NOx, SOx, and volatile organic compounds (e.g. benzene, trichloroethylene, and toluene) using dielectric barrier discharge (DBD) plasma. In the plasma, the pollutants are decomposed as the following processes:

1. Electrons of 1-10 eV collide with molecules in the flue gas, leading to dissociation, excition, and ionizion of molecules.
2. Resulting active species (radicals (O, N, and OH), ions, and excited particles) react with pollutants to remove them.

We use plasma for water treatment. Plasma in water produces a large amount of OH radicals from H2O. OH radical is a very strong agent to decompose chemical organic products and bacteria in water. Conventional water treatment using OH radical without discharge is called advanced oxidation process (AOP). In the conventional AOP, OH radical is produced by UV photodissociation of dissolved H2O2 or UV photodissociation of O3 through singlet O(¹D). The discharge in water, which produces a large amount of OH radicals, is a new excellent AOP.

We study the treatment mechanism of plasma medicine. Recently, it has been proved that plasma is efficacious for cancer treatment, blood coagulation, wound healing, tissue regeneration, skin regeneration, dental clinic, etc. It is believed that active species (OH, O, NO, O2*) are effective for the medical treatment. However, the treatment mechanism is not known. We do not know whether the plasma medicine can be actually used for human care. So we have just started to study the treatment mechamism of plasma medicine, particularly focusing on the study of the effect of active species on medical treatment.

       

(Left) plasma for medicine. (Center) Experiment using cells (joint research).
(Right) Measurement of OH radicals using two-dimensional laser-induced fluorescence.

We have measured various active species in the atmospheric-pressure plasma. This fundamental study is essential for developing the plasma application technologies, where active species produced in the plasma play important roles. For example, in the NOx removal process, NOx is removed by chemical reactions with active species of O, OH, and N. The active species are produced by pulsed discharge of about 0.1 us, reacting with NOx with time constant of 1 us to 1 ms in the postdischarge time as
        N + NO → N2 + O,
in a reducing atmosphere. In an oxidizing atmosphere, NOx is removed by reaction with O and OH. The list of our measurements is shown below. We have measured spatio-temporal behavior of these active species in the atmospheric-pressure plasma.

       

(Left) Laser equipments. (Right) OH density measurement in the postdischarge period of pulsed corona discharge using laser-induced fluorescence.

       

(Left) List of active species using laser spectroscopy. (Right) Measurement of rapid decomposition of environmental pollutant NO molecules using a pulsed plasma using laser-induced fluorescence.

We have developed a plasma simulation that can simulate discharge generation and active species productions and reactions. This simulation is a strong tool to develop plasma application technologies.

       

(Left) simulated electric field strength and electron density in streamer discharge.
(Right) comparison of measured (upper) and simulated (lower) streak photographs of streamer discharge.