Study On Preparation Techniques Of High-purity Spherical Silica Powder By Oxygen-Acetylene Flame Spheroidization Process

The paper mainly focused on studying the technique route in producing high-purityspherical silica powder using quartz and rice husk as raw materials. The spherical silica powderas a pivotal filler material has attracted more and more attention in Large Scale Integration (LSI)and ultra Large Scale Integration (VLSI) circuits packaging field with the amazing developmentof micro-electronic industries. In this study a new mechanical-chemical process was employed topurify for obtaining the ultra-fine, pre-shaped and high-purity quartz powder, when quartz rawmaterial was utilized. Otherwise, the rice husk was chosen to prepare low radioactivity sphericalsilica powder. Firstly, rice husk was pyrolyzed to ash that was used to prepare sodium silicate bya basic solution method. Then high-purity colloidal silica was prepared by a cationic-anion resinsalternative exchange process, and Na⁺,K⁺,Al³⁺,Fe³⁺,Cl^- were exchanged. Lastly, theas-prepared colloidal silica was granulated by the spray-drying granulation method, and thegranulation SiO₂ powder was calcined to improve its apparent density and fluidity. A noveloxygen-acetylene flame spheroidization route was employed to prepare spherical silica powders.In the developing technique special equipments were developed, for example, a boiling powdersfeeder, a spheroidization furnace, a flame burner, and so on. After the as-prepared high-purityquartz powder and calcined granulation SiO₂ powder were treated by oxygen-acetylene flame,the high-purity spherical quartz powder used in Large Scale Integration circuits packaging fieldand low radioactivity spherical silica powder used in ultra Large Scale Integration circuitspackaging field were obtained. The two type spherical silica powder and E-51 epoxy werecomposited respectively to prepare epoxy molding compound materials. The glass transitiontemperature, bending strength, linear thermal expansion coefficient of the composites wascharactered.The main achievements of the dissertation are as following: Firstly, structure, appearance, mineral compositions and mineral occurrence of the naturequartz raw material used in this work were studied in detail. Main and micro compositions of thequartz sand were determined quantitatively with chemical and instrumental methods. On the baseof traditional compound acids techniques, a milling and compound acids purifying process wasemployed to prepare high-purity quartz powders. Contrast of the two process, it need shortercompound acids leaching time and could get better purify effects in new mechano-chemistryprocess. At the optimal technique parameters, the content of SiO₂ was more than 99.98 %, thetrace amount of Al₂O₃ lower than 20×10^-6g/g, the total Fe₂O₃ lower than 5×10^-6g/g and theamount of other elements lower than 1×10^-6g/g in the as-purified quartz powders. At the sametime, the quartz raw material was ultra-fine grinding and the powders were pre-shaped by highenergy ball milling in the purify process. The as-shaped ultra-fine powder with good fluidity andpurity performances has met the oxygen-acetylene flame spheroidization process demand.Secondly, special oxygen-acetylene flame spheroidization equipments have been developedand new spheroidization route for quartz powder was founded successfully.The principle of powder transporting on traditional powder feeders was studied in detail andthe bottleneck problem for ultra-fine powder transporting was found. The powdersagglomerations and fluidness were two key reasons. On the basis of the principle of dynamicmechanics and gas dynamics, a boiling ultra-fine powders feeder was improved. The workingprinciple of the feeder accords as the shaking air, gravity of the powder and the pressuredifference between carrying gas and powder hopper. It was capable of feeding angle quartzpowder and granulated SiO₂ powder over a wide range of feed rates from 15g/min to 200g/min,over a wide range of powder size from 5μm to 50μm, regardless of morphology.Using siliconized silicon carbide (SISIC) with high strength, high wear resistance, hightemperature tolerance, high corrosion resistance, high anti-oxidization, high thermal shockresistance as raw material, a spheroidization furnace was designed by the solid grouting moldingand vacuum sintering technology. The reactive sintering silicon carbide is a high temperatureresistant and low linear thermal expansion material. The traditional water-cooling sandwich typespheroidization furnace could only maintain the furnace temperature lower than 1500K. Oncontrast, the new spheroidization furnace could maintain the furnace temperature about 2000K.The higher and stable temperature in the spheroidization furnace ensures a higher spheroidizationrate of the silica powder.Some improvements for the structure and nozzle size of the flame burner with an insidefeeding style had been done. So, the quartz powders could flight in the same direction tooxygen-acetylene flame and powder movement was axial. Because of the nozzle size increasing,the flame flow velocity decreased greatly. Contrast of the traditional vertical and outer feedingstyle or inclined and outer feeding style, one hand, this system could reduce the waste ofpowders, and on the other hand, the holding time of powder in flame burner and spheroidization was increased. Hence the spheroidization rate and the yield of spherical quartz powder wereincreased.The oxygen was chosen as the carrying gas to replace traditional N₂ or Ar during the powdersfeeding in oxygen-acetylene flame spheroidization process. An advantage is obvious in which thecarrying gas oxygen could be used as the assistant gas when acetylene burning. So, nounnecessary cooling gas went to the spheroidization furnace and the high-temperature of furnacecould be kept. It was a positive factor to increasing powder spheroidization rate.Thirdly, a mathematical model of the quartz powder spheroidization was established and themost efficient quartz powders spheroidization process parameters were determined.A heat energy transfer between the flame and powders, a temperature variation rule of thepowder, a speed variation law of the powder and the minimum energy of quartz spheroidizationwere studied in detail. And the mathematical model of quartz powder spheroidization wasestablished as:Where P is the particle density, t is the time required for spheroidization, dp is the powder size,H_m is the enthalpy of silica,λ_p is the thermal conductivity of gas, T_g is the gas temperature, T_p isthe powder temperature, Nu is the Nusselt number,εis the emissivity of silica powder andσisthe Stefan Boltzmann constant.During the preparation process of spherical quartz powder, the influencing factors tospheroidization rate, for example, the feeding rate, the flow rate of burning gas and assistant gas,the pressure of burning gas and assistant gas, the frequency of shaking gas, the size and sizedistribution of quartz powder, the morphology of the powder were studied. At optimumspheroidization technological parameters, the flow rate of burning gas 10L/min, the flow rate ofassistant gas 20L/min, the flow rate of carrying gas 5L/min, the feeding rate 60g/min, thehigh-purity quartz powders with the average size 10μm were treated by oxygen-acetylene flame.And, the spheroidization rate of the as-prepared spherical powder sample was 95%, the purity ofthe sample more than 99.9%SiO₂, the apparent density 0.87g/cm³ and the fluidity from 67/50g to70s/50g. After spheroidization, the average size of sample was bigger, the size distributionnarrower, the powder denser, the surface of the powder smoother and the fluidity better.Fourthly, a low radioactivity spherical silica powder using the rice husk as the raw materialswas obtained by a chemical-physical method and the basic process parameters were optimized.The rice husk was pyrolyzed fully at 600℃and the ash was used to prepare sodium silicate bya basic dissolving method. The most efficient decomposition can be obtained when the ratio of ash to the sodium hydroxide solution (20% in W/V) weight is 1:3, the temperature wascontrolled at 140℃, and the heating time was lasted for 4 hours. At these conditions theconversion rate of the SiO₂ in rice husk exceeded 95%. During the preparation procedures ofhigh-purity colloidal silica, the method and technique of exchanging Na⁺, K⁺, Al³⁺, Fe³⁺ and Cl^-from the sodium silicate were investigated in details.The cationic-anion resin alternative exchange was an efficient type, the particle size ofcolloidal silica is from 20 to 30nm and the amount of Na⁺ is lower than 5×10^-6g/g. Then, thecolloidal silica with the solid content of 30% was granulated by the spray-drying granulationmethod with the feeding rate 10mL/min. Because of the hollow-core structure of granulationSiO₂ powders, the apparent density was lower and the fluidity was insufficient. To improve theseproperties, the granulation SiO₂ powders was calcined at 1000℃for 1 hour. At last, the calcinedSiO₂ powders were treated by the oxygen-acetylene flame, and the spheroidization rate of thelow radioactivity spherical silica powder sample was more than 95%, the purity more than99.99%, the size distributed from 1～4μm and the content of U 0.05×10^-9g/g.Fifthly, the Epoxy Molding Compounds (EMC) was prepared separately by both themechanical disperse and the ultrasonic disperse using the spherical high-purity quartz powder,the low radioactivity spherical silica powder and the common quartz powder as the fillers. Inwhich, filling amount of spherical high-purity quartz powder was 80%, more than two otherpowders, and Epoxy Molding Compounds' properties were the best. But, for the higher purityand lower radioactivity of low radioactivity spherical silica powder, it had obvious advantage asthe filling materials in the ultra Large Scale Integration circuits packaging field. When the fillingamount of spherical high-purity quartz powder was 80%, the Epoxy Molding Compounds'bending strength was 146 MPa, the linear thermal expansion coefficient 8.5×10^-6K^-1 and theGlass transition temperature increasing 34℃. The Differential Scanning Calorimetric (DSC),Kissinger equation and Ozawa equation were employed to detect the kinetics of co-curing system.The apparent reactive activation energy (â–³E) of spherical quartzpowder/E-51/MeTHPA/DMP-30 was 78.52kJ/mol, and the order of reaction n=0.917. Using thespherical quartz powder and low radioactivity spherical silica powder as the fillers, the linearthermal expansion coefficient was greatly reduced and the thermal stability was improved, whilethe mechanical properties were maintained.On the whole, a new oxygen-acetylene flame spheroidization quartz powder and calcinedgranulation SiO₂ powder was developed and a series of special equipments were developed. Inwhich, the rice husk was used as the raw material to prepare the low radioactivity silica powderin the first time. The experimental conditions and technique parameters were optimized with thedetection of results and processes by analytical instruments. Recent researches and developmentsof actuality in the China and international spherical silica powder of actuality produced werebrief reviewed. Several processes and their features for spherical quartz powder, for example, the high temperature fused quartz jet process, the high frequency plasma spheroidization process, thedirect current arc plasma spheroidization process and the carbon electrode high-temperaturespheroidization process were analyzed and compared. The oxygen-acetylene flamespheroidization process is a simple-equipment, easy-control and lower energy cost process.Contrast of the traditional ethyl orthosilicate or silicon tetrachloride hydrolysis methods for lowradioactivity spherical silica, the chemical-physical process taking the rice husk as raw materialwas a higher yields, a lower raw materials cost, a nontoxic and easy industrial production method.Therefore, the oxygen-acetylene flame spheroidization process has advantages over some othermethods and it should be a potential cost-effective process for mass production of spherical silicapowder. Besides, this method can be expanded to preparation of many other spherical powders.