About quartz glass, material properties

The production method of fused quartz glass can be:

electrically fused, flame fused, plasma fused and electric-arc fused. The raw material can be natural quartz sand SiO2 and synthetic chemical-silicon tetrachloride(SiCl4). The combination of production methods and raw material composes the main 6 types of our quartz glass: EN (electrically fused natural quartz) series; FN (flame fused natural quartz) series; PN (plasma fused natural quartz) series; FS (flame fused synthetic quartz) series; PS (plasma fused synthetic quartz) series; AN (electric-arc fused natural quartz) series.

Electrically Fused Quartz Glass

Electric fusion is the most commonly used melting process for manufacturing quartz glass. This process uses resistance Heaters to melt natural quartz sand into quartz glass tubes, rods, blocks etc.. There are two methods of electric fusion, continuous fusion and batch fusion. In the continuous process, quartz sand is continuously poured into the upper part of a tungsten crucible surrounded by electric heating elements. The inside crucible is maintained in a neutral or slightly reduced atmosphere that keeps silica from reacting with the refractory metal while the silica is melted inside. The melted quartz exits at the forming part at the bottom of crucible which is shaped into tube, rod and so on. The OH can be reduced by annealing. In the batch fusion process, several tons of raw material are placed inside a refractory vacuum chamber containing graphite heating elements to produce tubes, rods and so on. The OH is usually below 2ppm. The low OH makes electrically fused quartz glass has a high infrared transmission. The electrically fused quartz glass usually has certain bubbles and drawing lines on the glass surface.

Flame Fused Quartz Glass

Flame fusion is a 2-step process. The first process is the natural quartz sand or synthetic SiC14 is melted in hydrogen/oxygen (H2/O2) flame into a solid round ingot. The second process is to further shape the ingot into various shape and dimension. Flame fused quartz glass contains a abundant amount of hydroxyl (OH) as a result of the direct contact between the H2/O2 and the silica raw material. This OH content can not be reduced by annealing and its presence lowers the viscosity and infrared transmission. The flame fused quartz has no drawing lines and very low bubble content.

Plasma Fused Quartz Glass

Plasma fusion is a 2-step process. The first process is to melt the natural quartz sand or synthetic SiCl4 in a plasma flame into a solid round ingot. The second process is to further shape the ingot into various shapes and dimensions. Plasma fused quartz has low OH usually below 5ppm, so it has good infrared transmission. The plasma fused quartz has very low bubble content and has no drawing line.

Electric-arc Fused Opaque Crucibles

Electric-arc fusion is the commonly used process for manufacturing crucibles which are used to grow monocrystalline silicon ingots.

Thermal properties

One of the most attractive features of quartz glass is its very low thermal coefficient of expansion (CTE). The average CTE value for quartz glass at about 5.0 × 10-7/ °C is many times lower than that of other common materials. To put this in perspective, imagine if 1 3 blocks of stainless steel, borosilicate glass and quartz ware were placed in a furnace and heated by 500 °C. The volume of the stainless steel block would increase by more than 28 liters and that of the borosilicate block by 5 liters. The quartz block would expand by less than one liter. Such low expansion makes it possible for the material to withstand very severe thermal shock.

It is possible to rapidly quench thin particles of quartz glass from over 1000 °C by plunging them into cold water without breakage. However, it is important to realize that the thermal shock resistance depends on factors other than CTE such as surface condition (which defines strength) and geometry. The various types of fused silica and fused quartz have nearly identical CTE’s and thus can be joined together with no added risk of thermally induced breakage.