Plasma Characteristics Of AC Gliding Arc In Vortex Flow And Its Application In Synthesis Of Mesoporous Nanocrystalline TiO₂

Gliding arc plasma has wide applications in gas conversion,pollution control and nanomaterial synthesis and so on.In this thesis,firstly,under the condition of constant arclength,the volt-ampere characteristics of alternation-current(AC)gliding arc plasma in vortex flow were studied by experiment and simulation.Then,in the case of the variable arc-length,the temporal and spatial evolution of the arc channel of AC gliding arc plasma in vortex flow was investigated Furthermore,the synthesis process of TiO₂ by AC gliding arc plasma in vortex flow was diagnosized.Finally,the one-step synthesis of mesoporous nanocrystalline TiO₂ by AC gliding arc plasma in vortex flow was studied.The main contents and results were summarized as follows:1.Volt-ampere characteristics of AC gliding arc plasma in vortex flow were investigated by experiment and simulation under the condition of constant arc-length.Three dimensionless factors,including extinction span factor,current lag factor and heating lag factor,were defined to characterize the discharge characteristics of AC gliding arc discharge in vortex flow.The effect of AC frequency on the dimensionless factors was also investigated.The extinction span factor decreases with the increase of ac frequency.At the lower AC frequency,the current lag factor and heating lag factor decreases and increases rapidly with an increase of AC frequency,respectively.When the AC frequency exceeds 40 kHz,they change very slowly.The dynamic volt-ampere characteristic curves simulated by arc channel energy equation are in a good agreement with the experimental results.The dynamic volt-ampere characteristic curve presents a double hysteretic loop,with positive and negative AC half-cycles in symmetry.In an AC halfcycle,the dynamic volt-ampere characteristic curve consists of five stages:extinguishing.reignition,volt-ampere rise,negative differential resistance characteristic and volt-ampere drop.With the increase of AC discharge frequency from 15 kHz to 91 kHz,the hysteresis loop of the dynamic volt-ampere characteristic curve gradually shrinks,and the rising and dropping stages of the volt-ampere characteristic curve gradually approaches,until they completely coincide into a straight line,and the negative differential resistance characteristic gradually disappears.2.The spatio-temporal evolution characteristics of arc channel of AC gliding arc plasma in vortex flow were investigated by means of flow field simulation,synchronous electrical measurement,ICCD image and high-speed imaging under the condition of variable arc length.The waveform of the peak value of AC voltage,Up,shows sawtooth-like with quasi-periodic change.With an increase of discharge gas flow rate,the gliding duration decreases gradually,and the minimum and maximum values of Up in the gliding duration increases.During the evolution of the arc channel,the jump phenomenon of the arc root of grounded electrode(GE)was observed.Through the combination of forward and backward jumps of the GE arc root,the forward and the net backward movements of arc channel are realized.The jump phenomenon of the GE arc root of can be ascribed to the caudal autotomy and regeneration of arc.When the arc is hard to be sustained,the arc auto-sheds the caudal part and regenerates by sprouting a new arc bud toward GE.This special behavior allows the main part of the arc to always remain to prevent the lose of the arc during arc evolution.The behavior of caudal autotomy and regeneration of arc.like a survival strategy of lizards,enables remaining the main part of the arc channel.This behavior is mainly caused by the phase difference between the instantaneous arc root of GE and PE.3.The diagnosis of synthesis process of TiO₂ by AC gliding arc plasma in vortex flow was carried out.Under the condition of air flow of 2 L/min,the arc channel glides with a constant arc-length.With the increase of air flow,the arc length during gliding process becomes variable,and the gliding duration decreases gradually.Four emission bands of TiO intermediate species in the visible region were observed during the synthesis process of TiO₂ by AC gliding arc plasma in vortex flow using the two precursors,titanium(IV)isopropoxide(TTIP)and Titanium(IV)bis(ammonium lactato)dihydroxide(TALDH):TiO ?(C3?—X3?).TiO?(cI?-a1?),TiO ?'(B3?-X3?)and TiO ?(A3?-X3?).Compared with OH(A2?+-X2?i)and N2(C3?u-B3?g),TiO?(A3?-X3?)is not affected by overlapping bands of other species and the rotational temperature can be obtained simply and reliably.The rotational temperature of TiO ?(A3?-X3?)was calculated as 3000 K,which was also used as gas temperature of arc channel,and the average electron energy of the AC gliding arc plasma in vortex flow is 1.1 eV.4.Mesoporous nanocrystalline TiO₂ was synthesized in one step by AC gliding arc plasma in vortex flow using TTIP as precursor.The disordered wormhole-like mesoporous structure of the synthesized TiO₂ sample with excellent thermal stability was confirmed by the H2 hysteresis loops of N2 adsorption desorption isotherms,as well as SEM,HRTEM and SAXS characterization.The synthesized mesoporous TiO₂ samples are highly crystalline and the mass ratio of anatase to rutile is about 75/25.When the discharge gas flow rate increases from 2 L/min to 5 L/min,the pore size of the synthesized TiO₂ sample decreases from 13.0 nm to 7.4 nm,the specific surface area increases from 30 m2/g to 98 m2/g,and the grain size of anatase decreases from 31.0 nm to 19.4 nm.The synthesized TiO₂ samples show high photocatalytic activity.In the photocatalytic oxidation of formaldehyde,the formaldehyde conversion of STTIP4 is as high as 82%;in the photocatalytic decomposition of water,hydrogen production rate of STTIP-4 samples is as high as 108 ?mol/h,which is higher than that of commercial P25 sample(89 ?mol/h).Using TALDH precursor added with ethanol,the mesoporous hollow microsphere TiO₂ was rapidly one-step synthesized by AC gliding arc plasma in vortex flow.The diameter of hollow microsphere TiO₂ is on the order of micron,the thickness of spherical shell is in the range of tens to more than one hundred nanometers,and the mesoporous size on the surface of spherical shell is in tens of nanometers.The addition of ethanol not only greatly promots the formation of mesopores on the spherical shell,but also significantly increases the crystallinity and transient photocurrent of the mesoporous hollow microsphere TiO₂ sample.