Study On Properties Of Functional Materials Prepared By High Pressure DBD Plasmas

The dielectric barrier discharge (DBD) can produce non-equilibrium plasmas in awide range of working pressure from several hundreds Pa to atmospheric pressure. Todeposit functional materials assisted by DBD plasmas is an advanced method whichhas a brilliant future because of its various advantages involving the simplicity of theexperimental setup, the significantly lower consumption of the feed gas and theelectric energy, environment friendly, quick deposition rate, as well as the large-areadeposition on various substrates at room temperature.In this work, the plasma setup has a novel designed reactor which the films weredeposited in the downstream zone by DBD plasmas. It can avoid the direct strikingcaused by DBD filament and streamer discharge;meanwhile films were formedquickly by taking the advantage of plenty of radicals generated by DBD discharge.Also, the working pressure used in this work is between 200Pa～50kPa, three kindsof functional materials including diamond-like carbon films(DLC), carboxyl(-COOH)functional films and carbon nanotubes (CNTs) has been synthesized successfullywhich proved the practicality of the apparatus. Through synthesize and measurementof the films properties, the mechanisms of the polymerization was discussedpreliminarily. The main content of this work are as follows:I. Deposition of DLC films(1)Using CH4,Ar and H2 mixtures as feeding gas, DLC films were polymerizedin DBD plasmas in the downstream zone under the pressure between 200～500Pawhich was reported the first time and apparent DLC feature were observed by FTIRand Raman test. The appropriate process to deposit DLC film is 200℃ astemperature,H2 to CH4 ratio between 1:1～1:2, 500Pa as working pressure.(2)The substrate temperature has less effect whereas CH4 concentration has astrong influence on the film properties. The water contact angle (WCA) value and thecontent of diamond phase of the DLC film increases with the increasing of CH4concentration, but the roughness increases as well.(3)DLC film are favorably formed in CH4 rich feeding gas in the DBD plasmadownstream zone which showed a quick deposition rate of the high pressure DBD,butmore flaws appeared in the film and reduced the compactness and uniformity of theDLC film.II.Polymerization of carboxyl functional films(1)Polymerized acrylic acid (PAA) films were deposited in DBD plasmas indownstream zone using acrylic acid as monomer;Water contact angle test showedthat the PAA film has a minimum degree of 3o and really has super hydrophilicsurface ,and numerous polar groups including hydroxyl, carboxyl and carbonyl haspolymerized on the PAA films which proved by FTIR results.(2)The PAA film properties polymerized in various working pressures (300～2200Pa) and discharge voltages(3.6～4.4kV)were investigated. The FTIR and XPSresults showed that high COOH retention (Ca. 33.5%) of PAA film can be depositedin high pressure (2200Pa) and medium discharge voltage (3.6kV) condition, whichcan achieved the same result produced by prolonged pulsed plasma.(3)High density –COOH contained PAA film can be deposited in the DBDplasma downstream zone successfully which showed a unique feature of such plasmasetup, but the stability of the PAA film immersing in water should be improved future.III.Synthesis of carbon nanotubes(1)Using CH4 as carbon source, H2, ammonia water, NH3 and water mixtures asetching and dilution gas, carbon nanotubes (CNTs) were synthesized on 13nm Nicatalyst coated silicon substrate under the temperature of 630～750℃. Temperatureplays an important role on the growth of CNTs but varied with gas mixtures. Theappropriate temperature to form CNTs was 750℃ for CH4/H2/NH3 mixtures, but630℃ for CH4/H2/ammonia water or CH4/H2/NH3/H2O,respectively.(2)In 15% CH4 gas mixtures, CNTs can grown fast when adding a small additionof ammonia water to CH4/H2 mixtures approximately at 3‰;but adding moreammonia water, CNTs can not formed even no any carbon product.(3) In 15% CH4 gas mixtures, CNTs can grown well within an addition of 3% NH3,but no any product found when adding the NH3 content over 8% ,the reason maybe asthat the etching rate of N contained gas discharge is far higher than the speed ofcarbon formation.(4)In 15% CH4 and 8% NH3 gas mixtures, it is found that a small addition of wateraffects the formation of CNTs significantly. The addition of water between a contentof 2‰ and 6‰ can enhanced a fast growth of CNTs, but no any products can begrown when there is no water added or the water content over 17‰.The mechanismof water assisted growth of CNTs should be investigated further.