Effects Of Particle Size Distribution Of Raw Material On The Reliability Of Electrical Porcelain

The particle size and distribution of electrical porcelain materials，especially thesize and number of coarse particle，affects the quality of the electrical porcelain blankand follow-up process directly，thus affecting the performance and reliability ofelectrical porcelain products. Using the raw material of fine grinding，less coarseparticles and narrower particle size distribution helps to reduce the content of coarsegrains and pores in the ceramic structure and improve the uniformity of the structure，which has important significance to improve the performances and operationreliability of electrical porcelain products.At present， the majority of electrical porcelain manufacturers still use thetraditional ball milling to grind raw material，which has the advantages of simpleoperation and stable running. However，the lean materials of raw materials(bauxiteand feldspar，etc.) which are high hardness are difficult to grind，result in that it willneed a very long time to achieve the required range of particle size and distribution ifsimply use ball milling in the normal production. In addition，ball milling has lowgrinding efficiency，high energy cost，a fairly large number of coarse particles andwild particle size distribution in its slurry. It not only constrains the efficiency ofelectrical porcelain manufacturers， but also affects the stability of quality andoperational reliability for electrical porcelain products. The working principle ofstirred bead milling is that the motor drives the mixer shaft to rotate at high speed，thus drive the Al2O3porcelain ball which has a diameter of2~3mm to strike rawmaterials at high speed in order to achieve the purpose of refining particle anduniform dispersion，which is based on the principle of stirred milling and sand milling.It can make raw materials continually grind，which is helpful to improve grindingefficiency.Based on the raw materials and formulation of an electrical porcelain enterprisein Ping xiang，This paper used single ball milling and ball milling combined withstirred bead milling to prepare a series of different particle size distribution of slurry.The grinding effect of different grinding methods was analyzed by laser particle sizeanalyzer. The experiment found that the grinding method of ball milling for8hcombined with stirred bead milling for1min reduced the particle size and number ofcoarse particles in the slurry and narrowed the particle size distribution range effectively when its average particle size close to the slurry of normal production. Itimproved the grinding efficiency and saved the energy consumption significantly.According to the normal production process，the slurry of different particle sizedistribution was made of strips of φ22mm×140mm. A part of the strips was sintered at1270℃in the factory’s kiln which was reducing atmosphere. Another part of thestrips was sintered at1260℃、1280℃、1300℃and1320℃in the laboratory’s siliconcarbide resistance furnace which was oxidizing atmosphere respectively. The flexuralstrength，electric breakdown strength，volume density and open porosity of differenttemperatures and atmosphere were tested. XRD and SEM were respectively used todetect the phase composition and to observe the microstructure. The effect of particlesize distribution of raw material on the sintering properties for electrical porcelainblank was analyzed. The results showed that the sintering temperature of blankbecame lower with particle size and distribution range decreasing. The bendingstrength of porcelain sintered in oxidizing atmosphere was increased by5%to10%than that of sintered in reducing atmosphere. Compared to the samples of single ballmilled for14h and ball milled for8h combined with stirred bead milled for1minsintered in reducing atmosphere，the content of corundum and mullite of the samplessintered in oxidizing atmosphere was higher. The ceramic microstructure was moreuniform and the grain size is smaller. The growth of secondary mullite was morecomplete.The paper also studied the influences of particle size distribution on theperformances and reliability of electrical porcelain. With the decrease of particle sizedistribution range，both the bending strength and the electrical strength of the samplessintered in oxidizing atmosphere increased accordingly. The dispersion of bendingstrength was gradually decreased.The grain size and pores in the microstructure were smaller. And the shape ofpores tended to regular spherical. Their structures became more densification. Theflexural strength and the electrical strength of the sample which was ball milled for8hcombined with stirred bead milled for1min were increased by10%and8.5%respectively after sintering than those of normal production. The deviation value ofbending strength is reduced by35.7%. The volume density was enhanced by0.8%andthe apparent porosity was reduced by16.7%. And the content of corundum andmullite was higher and the secondary mullite was more complete.In addition，the processing properties of blank ball milled for14h and ball milledfor8h combined with stirred bead milled for1min were compared. The dry body strength of the latter increased by14%. Both the blanks were made of the samenumber of XWP3-70products according to the normal production process in aelectrical porcelain manufacturer of Ping xiang. The time of mud squeezing andstaleness was slightly longer for the latter. They had no big difference in the formingand drying process. The measured average tensile strength was close to each other，but the deviation of the latter reduced by50%. The percents of pass for products were85%and87.3%respectively.