Characteristics Of Large Area Discharge Plasma And Mechanism Study Of AlN Powders Prepared By It Assisting High Energy Ball Milling

Atmospheric pressure (AP) large area dielectric barrier discharge (DBD) diffuse plasmas are showing an important application prospect in nano powder synthesis, surface modification, biological medicine, environmental engineering and other fields, because it does not need to run under vacuum, and can realize continuous work online. The focus on research work of paper is carried out by two ways:One is that a bipolar nanosecond high voltage pulse is employed to generate stable and large area air DBD diffuse plasmas using array needle-plate electrode configuration at AP. The other is that a large area surface discharge plasma along PTFE rod surface in ball mill vial excited by a sine AC power is obtained at AP, and using it assisting high-energy ball mill has successfully prepared AlN nano powder as well as to also deeply study its synthesis mechanism. The main researches contents are given as follow:1. In single needle-plate electrode configuration, air diffuse DBD plasma excited by bipolar nanosecond pulse voltage has been successfully obtained at AP, by increasing the number of needle electrode, and successfully obtain a large area nanosecond pulse air diffuse discharge plasma. The optical spectra of plasma and the waveforms of pulse voltage and current are recorded. The rotational and vibrational temperatures are determined by comparing the experimental and best fitted spectra of N2+(B2∑u+→X2∑g+,0-0) and N2 (C3Πu→B3Πg,0-2), and the effects of pulse peak voltage, pulse repetition rate, discharge gas gap, dielectric thickness and needle electrode number on discharge uniformity, emission intensity of N2(C3Πu→B3Πg,0-0), discharge power, power density, plasma temperatures and discharge plasma area are also studied. The results show that there are respectively only an effective discharge breakdown in positive and negative pulse discharge of bipolar nanosecond pulse, and the single discharge duration is about 70 ns. The emission intensity of N2(C3Πu →B3Πg,0-0) increases with the rising of the pulse peak voltage and pule repetition rate, but decreases with increase of discharge gas gap, dielectric thickness and needle electrode number. In addition, the average power and power density with the increase of number of nendle electrode gradually becomes large and small, respectively, however, both average power and power density increase with the rising of pulse peak voltage. Plasma gas temperature gradually decreases with with the rising of dielectric thickness and needle electrode number. Meanwhile, the discharge plasma area presents obvious shrinkage with increase of dielectric thickness and needle electrode number, but presents the obvious lateral extension with the rising of needle electrode number. At the same time, the discharge plasma still keeps good uniform characteristics. When increasing the number of needle electrode, discharge breakdown voltage is decreased and the discharge peak current is increased obviously. When the pulse peak voltage is kept at 34 kV, discharge plasma area has reached approximately 70 ×50 mm2.2. In N2at AP, the pure Al2O3 powder as precursors, a large area surface discharge plasma excited by sine AC power is obtained, and using it assisting high-energy ball mill Al2O3 powder has successfully prepared AlN powder. Respectively using discharge plasma assisting high-energy ball mill (p-milling) and common ball mill (c-milling) to activate Al2O3 powder, and then compare the micro-morphology, particle size, crystallite size and lattice distortion of milled Al2O3 powder as well as the morphology, particle size and conversion rate of synthesis of AlN powder. And the synergy mechanism of discharge plasma in the ball mill process of is discussed in detail. The results show that Al2O3 powder under p-milling can get smaller grain size, particle size and larger lattice distortion and the specific surface area compared to Al2O3 powder under c-milling. The activation energy of synthesis of AlN by p-milling Al2O3 powder calculated by Kissinger equation is about 371.5 kJ/mol, which is far lower than the value of c-milling AI2O3 (457 kJ/mol) and the value of raw Al2O3 powder (529 kJ/mol). In addition, the AlN conversion rate under p-milling Al2O3 powder is significant higher than that under c-milling Al2O3 powder at the same annealing temperature. Meanwhile, the synthesis of AlN by p-milling Al2O3 has better crystallinity, crystal morphology and uniform distribution. This suggests that the p-milling precursors can obtain larger reactivity, and effectively reduce the annealing temperature and improve the conversion rate of AlN in the nitriding process, the main reason is because heating effect and high energy particles bombarding effect of discharge plasma in milling process.3. The Al2O3/C mixture system as precursors, respectively using p-milling and c-milling to activate Al2O3/C mixture, the phase transformation and microstructure of milled Al2O3/C mixture as well as conversion efficiency and particle size of sysnthesis of AlN are analyzed deeply. By compared with the results of milled Al2O3 powder, it is found that the refined Al2O3 particles and C particles in mixture parcel mutually to form cladding and composite mixture. The refined rate of p-milling Al2O3/C mixture is reduced, the corresponding crystal internal defects and distortion are also decreased. However, at the same annealing conditions, conversion efficiency of sysnthesis of AlN under Al2O3/C mixture by p-milling and c-milling is significant higher than that under p-milling Al2O3 powder, what is more, compared with p-milling Al2O3 powder, p-milling Al2O3/C mixture shorten milling time of 10 h to make the AlN conversion rate reach 100%. In addition, the synthesis of AlN by p-milling Al2O3/C mixture has more uniform particles distribution and smaller particle size. These differences are mainly because C particles play a significant role of lubrication in ball mill process, which can effectively reduce collision impact between powder and mill ball, and decrease the input of energy from ball mill machinery on mixture, hence, the refined rate is reduced, however, the cladding and composite small particle structure formed by p-milling Al2O3/C mixture greatly increase the effective contact area and shorten the average diffusion path, meanwhile, the smaller precursor particles are highly active state, thus accelerate the nitriding rate and conversion efficiency.4. The Al/DN mixture by p-milling with annealing has successfully synthesized AlN nano powder. Researches are mainly focused on the phase structure transformation process, microstructure, and particle size of p-milling products as well as the chemical reaction mechanism between Al and DN. The results show that when the milling time is less than 8 h, the main phases in p-milling products are Al and DN. Continue to increase the milling time, the diffraction peaks of DN gradually fade away and AIN phase diffraction peaks gradually appear, when milling time reaches 14 h, conversion efficiency of synthesis of AIN has reached 90%, and the specific surface area is about 52 m2/g. The main reaction mechanism is that the solid organic nitrogen source DN contains plenty if nitrogen groups, during the ball mill process, the -NH2 functional group in DN molecular fractures first, which leads to the DN molecular become unstable and further discompose and lose nitrile groups, meanwhile, the refined and highly active Al atoms react with free nitrogen groups and finally form Al≡N functional groups.