Quartz Material

What Quartz Material Is

Quartz material is amorphous silicon dioxide, chemically the same SiO2 as the mineral quartz but with no long-range crystal order. The silicon and oxygen atoms form a continuous random network of corner-sharing tetrahedra, which is why the material is glassy and optically isotropic, showing no birefringence. This distinguishes it cleanly from crystalline quartz, the trigonal mineral used in oscillators, wave plates, and piezoelectric elements. When a buyer says "quartz tube" or "quartz crucible," the part is almost always glassy fused quartz, not a crystal.

The generic term covering all glassy SiO2 is vitreous silica. Industry then splits it by raw material. Fused quartz is made by melting naturally occurring crystalline silica, either selected quartz rock or high-purity sand, in an oxyhydrogen flame or an electric furnace. Synthetic fused silica is made by oxidizing or hydrolyzing a silicon-bearing gas, most commonly silicon tetrachloride (SiCl4), so the glass is built up atom by atom from vapor. Historically the words also tracked appearance, with "fused quartz" meaning transparent ware and "fused silica" meaning opaque ware, which is why the labels still cause confusion. The reliable practice is to read the specific grade designation rather than the generic noun.

What makes quartz irreplaceable is a rare combination of four properties in one material: a very low coefficient of thermal expansion of about 0.5 to 0.55 x 10^-6 per Kelvin, a high softening point near 1660 to 1710 degrees Celsius, transparency from the deep ultraviolet (around 180 nanometres) through the visible and into the near infrared (around 3 micrometres), and near-total chemical inertness. No competing transparent material combines all four. Borosilicate glass expands ten times more and softens hundreds of degrees lower; sapphire transmits well but is far costlier and crystalline; most ceramics are opaque. Quartz sits in a unique niche.

The industrial history is long. Vitreous silica was first produced in the laboratory in the 1830s and commercialized for tubing and labware in the early 1900s once oxyhydrogen torches could reach the fusion temperature. Synthetic fused silica by flame hydrolysis emerged in the mid-twentieth century and became strategically important with deep-UV photolithography, where lens and photomask substrates must transmit 248 nanometre (KrF) and 193 nanometre (ArF) excimer laser light with extreme uniformity. Today the semiconductor supply chain consumes large volumes of both high-purity synthetic silica for optics and natural fused quartz for diffusion tubes, wafer carriers, and etch chamber parts.

The scale of use spans laboratory cuvettes weighing a few grams to optical lithography boules and furnace liners weighing hundreds of kilograms. Form factors include tubing and rod, plates and windows, crucibles and boats, polished optical blanks, and fiber preforms (the starting glass for optical fiber is essentially ultra-pure synthetic silica). Because the same chemistry serves such different duties, selection is never about the material alone; it is about matching the right grade and form to the temperature, wavelength, purity, and chemistry of the application.

Grade Families and Classification

The single most useful classification is by manufacturing route, because the route fixes the purity and the hydroxyl content, and those two numbers determine almost everything else. There are three main families: electrically fused natural quartz, flame fused natural quartz, and synthetic fused silica. The table below summarizes how they differ on the parameters that matter for selection.

Electrically fused quartz is melted from natural crystalline silica in an electric resistance or arc furnace under a dry, often partial-vacuum, atmosphere. Because no hydrogen flame touches the melt, the hydroxyl content stays very low, typically under 30 ppm and often below a few ppm. Low hydroxyl raises viscosity, so these grades sag less and resist devitrification better at high temperature, making them the benchmark for the hottest furnace and diffusion-tube duties and for infrared optics. Heraeus markets electrically fused product under the HSQ family; Momentive's GE214 is a widely used electrically fused, direct-drawn tubing grade with a UV cutoff below 160 nanometres and no appreciable hydroxyl absorption.

Flame fused quartz melts natural quartz in an oxyhydrogen flame, so combustion water drives the hydroxyl content up to roughly 150 to 200 ppm. The high hydroxyl is actually beneficial for ultraviolet and visible transmission, so flame fused grades dominate UV and visible optics, lamp envelopes, and general labware. Heraeus natural-quartz optical grades such as Herasil, Homosil, and Optosil (good homogeneity, low fluorescence) fall in this high-OH branch, as do many of the GE natural grades. By contrast, Heraeus Infrasil is a low-OH natural grade made by electric fusion in vacuum, so it belongs with the electrically fused family above and is the benchmark choice for infrared optics, not a flame fused grade.

Synthetic fused silica is the purity champion. Flame hydrolysis or oxidation of silicon tetrachloride builds glass with metallic impurities at the parts-per-billion level, an order of magnitude or more cleaner than any natural grade. The trade-off is high hydroxyl, commonly 800 to 1000 ppm in standard grades, which causes strong infrared absorption near 2.2 and 2.7 micrometres but excellent deep-UV transmission. Corning HPFS 7980 is the classic high-OH synthetic grade for deep-UV; the same family includes 7979 (IR grade, hydroxyl below 1 ppm, metallics below 100 ppb) and 8655 (ArF grade, metallics below 10 ppb). Heraeus Suprasil and Spectrosil are the comparable synthetic optical grades.

A second, orthogonal classification is by optical clarity: transparent (clear) versus opaque (translucent) quartz. Clear quartz is fully fused and bubble-free for optics and lamps. Opaque quartz is sintered from high-purity grain so that controlled micro-bubbles remain, scattering infrared and giving far better thermal insulation; it is used for furnace liners, heat shields, and some diffusion process tubes where radiant heat retention matters more than clarity. Heraeus markets opaque grades under the OM designation. The chemistry and chemical resistance are essentially identical to clear quartz; only the optical and thermal-radiation behavior differs.

Optical, Electrical and Mechanical Behavior

Quartz is chosen as much for what it transmits and insulates as for what it withstands. Optically, fused silica is one of the most transparent bulk solids known, with a usable window from roughly 0.18 micrometres in the deep ultraviolet to about 3 micrometres in the near infrared. The refractive index is about 1.4585 at the 589 nanometre sodium line, the chromatic dispersion is low, and the nonlinear index is among the lowest of any optical glass, which is why fused silica dominates high-power and short-pulse laser optics. Because the material is amorphous it is isotropic and free of birefringence, unlike crystalline quartz.

The deep-UV edge and the infrared edge are both governed by grade chemistry. Deep-UV transmission below 200 nanometres is limited by metallic impurities, so only the cleanest synthetic grades (Corning 7980 ArF/KrF, 8655, Heraeus Suprasil) transmit usefully at 193 and 248 nanometres. Infrared transmission past 2 micrometres is limited by hydroxyl absorption, so IR optics use low-OH grades (Corning 7979, Heraeus Infrasil). This is the practical reason a single "fused silica" spec is never enough: the wavelength of interest selects the grade.

Electrically, quartz is an outstanding insulator. Volume resistivity at room temperature is on the order of 10^16 to 10^18 ohm-metres, the dielectric constant is about 3.7 to 3.8 across DC to microwave frequencies, the loss tangent is very low (on the order of 10^-4), and dielectric strength reaches 25 to 40 kV/mm in sections of 5 millimetres or more. These properties, combined with thermal stability, make quartz a preferred substrate for RF and microwave components, high-voltage standoffs, and precision capacitors. Note that resistivity falls sharply with temperature, dropping toward 10^5 ohm-metres near 1200 degrees Celsius, so quartz is not an insulator at furnace temperatures.

Mechanically, quartz is hard, stiff, and brittle. The table below collects the representative room-temperature mechanical and physical properties published on manufacturer datasheets. Values vary slightly between grades and suppliers, so always confirm against the specific datasheet for a critical design.

The key design caution is brittleness. Quartz has high compressive strength near 1150 N/mm2 but a far lower practical tensile and bending strength, around 50 to 67 N/mm2, and like all brittle solids its real strength is governed by surface flaws and scratches rather than by the bulk. Components fail from surface defects, edge chips, and thermal stress concentrators, not from gross overload. Fire-polishing, flame-finishing edges, and avoiding hard mechanical clamps all raise effective strength. The flip side is the exceptional thermal-shock tolerance that the very low expansion delivers, which is what lets a glowing quartz tube survive a water quench that would shatter ordinary glass instantly.

Thermal Limits, Chemistry and Devitrification

The thermal behavior of quartz is its headline selling point, but the published "softening point" is not the real service limit. Quartz glass has no sharp melting point; instead it passes through a sequence of viscosity reference temperatures. Below the strain point it behaves as a rigid solid; between strain and annealing points internal stress relaxes over hours; above the softening point it deforms under its own weight. The continuous-service ceiling sits well below softening because two slow processes, sag and devitrification, set in long before the glass flows. The table compares the reference temperatures across the three grade families.

Two patterns stand out. First, lower hydroxyl content shifts every reference temperature higher, which is why electrically fused, low-OH grades tolerate the hottest sustained duty and high-OH synthetic grades the coolest. Second, the gap between the published softening point and the realistic continuous-service limit is several hundred degrees. A common engineering mistake is to size a furnace tube against the 1660 degree softening figure when the true sustainable limit, set by sag and devitrification, is closer to 1100 to 1150 degrees Celsius.

Devitrification is the dominant high-temperature failure mode and deserves its own attention. Above roughly 1150 degrees Celsius the amorphous surface slowly crystallizes into cristobalite, a crystalline form of silica. Cristobalite itself is stable at temperature, but on cooling it undergoes a sharp volume change near 200 to 270 degrees Celsius that cracks and spalls the surface, producing a frosty white skin and eventual structural failure of tubes and crucibles. The reaction is catalyzed by surface contamination: alkali and alkaline-earth ions from fingerprints, salts, dust, and metal vapors act as nucleation sites and accelerate crystallization dramatically. Practical countermeasures are cleanliness (handle with gloves, clean before firing), choosing low-OH electrically fused grades, and in tube applications deliberately growing a thin even cristobalite layer to stabilize the surface against further attack and sag.

Chemical resistance is nearly universal at moderate temperature, which is the other reason quartz is prized. It is inert to water, brines, and essentially all mineral and organic acids, with only two important exceptions: hydrofluoric acid, which etches quartz at any concentration and temperature, and phosphoric acid, which attacks it above about 150 degrees Celsius. Strong alkalis such as sodium and potassium hydroxide, and molten alkali salts and carbonates, also dissolve and devitrify the surface, with the attack rate climbing steeply with temperature. The table summarizes the chemical envelope for selection.

Key Specification Parameters

A quartz datasheet can list dozens of numbers, but only a handful drive a sound selection. Reading them correctly, and knowing which are grade-dependent, is the core skill. The parameters below are listed roughly in the order they should be resolved during specification.

Grade and OH content. The grade name fixes the manufacturing route, and with it the hydroxyl content and metallic purity. Hydroxyl is quoted in ppm and ranges from below 1 ppm (electrically fused, IR grades) to around 1000 ppm (standard synthetic). It sets both the infrared transmission and the high-temperature viscosity, so it should be matched to the wavelength and temperature of the duty, not left as a default.

Metallic purity. Quoted as total metallic impurities, in ppm for natural grades (around 20 ppm) and ppb for synthetic grades (below 1 ppm, down to 10 ppb for the cleanest Corning 8655). Purity governs deep-UV transmission, fluorescence, and contamination risk in semiconductor processing. It is the single property where synthetic silica decisively beats natural quartz.

Coefficient of thermal expansion. About 0.5 to 0.55 x 10^-6 per Kelvin near room temperature, rising modestly with temperature. This is the number behind quartz's thermal-shock immunity and its dimensional stability; it is essentially grade-independent and is the headline reason to choose quartz over borosilicate.

Temperature reference points. Strain, annealing, and softening points, plus the continuous and short-term service maxima from Chapter 4. For thermal designs, size against the continuous-service maximum (around 1100 to 1160 degrees Celsius), never against the softening point, and account for devitrification margin if alkali contamination is possible.

Optical transmission and refractive index. For optical parts, the transmission curve from 180 nanometres to 3 micrometres, the internal transmittance at the working wavelength, the refractive index (about 1.4585 at 589 nanometres), the homogeneity grade, and for laser duty the stress birefringence and laser-induced damage threshold. Corning and Heraeus certify homogeneity and inclusion classes for optical blanks; these classes, not just the grade, set the price.

Mechanical and electrical figures. Density near 2.20 g/cm3, Young's modulus about 72 GPa, compressive strength near 1150 N/mm2, and the much lower tensile and bending strength near 50 to 67 N/mm2 that governs real-world fracture. For electrical and RF use, volume resistivity (10^16 to 10^18 ohm-metres at 20 degrees C), dielectric constant (3.7 to 3.8), loss tangent, and dielectric strength (25 to 40 kV/mm).

• Clarity class: transparent (optics, lamps, viewports) versus opaque or translucent (furnace liners, heat shields, radiant insulation).
• Form and dimensions: tube outside diameter and wall, rod diameter, plate flatness, crucible volume, or polished optical surface figure and parallelism.
• Surface finish: as-drawn, ground, fire-polished, or optically polished, each with a different effective strength and contamination behavior.
• Bubble and inclusion class: bubble count and size limits, important for both optical scatter and high-temperature integrity.

Selection Decision Factors

Quartz selection is a sequence, not a single choice. Working through the steps below in order avoids the most common error, which is fixing the supplier or the form before the duty requirements (wavelength, temperature, purity, chemistry) are pinned down. These eight steps double as an RFQ template.

1. Define the governing duty: decide first whether the application is optical (wavelength-driven), thermal (temperature and shock-driven), chemical (corrosion-driven), or electrical (insulation and RF-driven). The governing duty selects the family before any other decision.
2. Set the wavelength window, if optical: deep-UV (193 and 248 nanometres) demands high-purity synthetic grades such as Corning 7980 ArF/KrF, 8655, or Heraeus Suprasil; infrared past 2 micrometres demands low-OH grades such as Corning 7979 or Heraeus Infrasil. Specify the working transmittance, not just the grade.
3. Set the temperature ceiling, if thermal: match the continuous-service maximum, not the softening point. For the hottest sustained duty choose low-OH electrically fused grades; reserve margin for devitrification if alkali contamination is plausible.
4. Confirm chemical compatibility: quartz suits almost everything except hydrofluoric acid, hot phosphoric acid, and hot alkalis or molten salts. If any of those appear, switch material rather than derate quartz.
5. Choose clarity class: transparent for optics, lamps, and viewports; opaque or translucent for furnace liners and heat shields where radiant insulation and lower cost outweigh clarity.
6. Specify purity grade: semiconductor and analytical duties need synthetic ppb-level purity (Corning 8655, Heraeus HSQ class); general furnace, lamp, and labware duties are well served by natural fused quartz at far lower cost.
7. Fix form, dimensions and finish: tube OD and wall, rod diameter, plate flatness, crucible volume, or optical surface figure, parallelism, and coating. Call out edge finish and surface polish, since these govern fracture strength and contamination.
8. Evaluate total cost of ownership: price scales steeply from natural fused quartz to standard synthetic to certified deep-UV optical blanks, often by an order of magnitude. Overspecifying purity or homogeneity wastes money; underspecifying causes field failure, devitrification, or optical loss. Buy to the duty, not to the catalog top grade.

One frequently overlooked dimension is serviceability and supply continuity: lead time and minimum order quantity for the chosen grade and form, the supplier's homogeneity and inclusion certification, lot-to-lot consistency, and the availability of fabrication services such as flame working, precision grinding, and optical polishing. For semiconductor and optical programs these factors often outweigh the unit price. Heraeus, Corning, Momentive (the former GE Quartz grades), Tosoh, Shin-Etsu, and Saint-Gobain anchor the high-purity and optical end, while a deep base of Chinese suppliers in the Lianyungang and Donghai quartz clusters serves lamp tubing, labware, and semiconductor consumables at competitive prices for non-critical duties.