Transparent Materials Comparison

What’s the difference between glass & crystalline material?

What is the difference between glass and crystalline material? Glass is an amorphous solid, which means its atoms are randomly positioned with respect to each other. Whereas, a crystalline material may contain the exact same type and quantity of atoms as glass, but the atoms are ordered in rigid, well-defined patterns. Some examples:

  • Fused quartz glass and crystalline quartz have the same type of atoms (SiO2), but they are structured in such a way that one forms a glass and the other forms a crystal.
  • A more exotic example is common water (H2O) where the exact same atomic chemistry can be amorphous or crystalline. Almost all solid (frozen) water on earth is crystalline ice, but the most common form of water in the universe is actually amorphous ice.

Crystalline materials are solid and keep their shape until they reach a very specific temperature, at which point (the melting point), they become fluid. In contrast, all glasses, by definition, are viscous fluids, even at very low temperatures. The viscosity of glass is determined by the temperature of the glass. Even at room temperature some types of glass are moving, albeit very slowly.

For production purposes, when tolerances such as flatness are very tight and temperatures are high, the choice of a crystalline material is superior to glass because it will maintain its shape at higher temperatures. I.e. glass may change shape over time, regardless of the melting point and especially at elevated temperatures.

Both crystalline solids and glasses are normally colorless in the pure state. Impurities are actually what give natural sapphire and common glass their color. The impurities also change the mechanical, thermal and optical properties of both material types, but especially for glass.

For example: Chromium in sapphire makes ruby crystal; gold in soda lime glass makes ruby red (cranberry) glass.

Fused Quartz & Fused Silica
Borosilicate (Pyrex™)
Soda Lime
Transmission Curves

Sapphire is a single crystal - Aluminum Oxide (Al2O3) - which is colorless and optically clear. Synthetic sapphire is grown in boules (bulk pieces) no larger than 300mm in diameter. It can also be grown into shapes such as sheets, ribbons, domes and tubes with very smooth surface qualities, high purity and optical translucence. When used "as grown" there is very little need for grinding and polishing.

Sapphire Benefits:

  • Mechanically second only to diamond. One of the hardest and most scratch resistant materials available. The high modulus of elasticity and high tensile strength make it extremely wear, abrasion and impact resistant.
  • Colorless optical characteristics are superior to any standard glass, with up to a 98.5% transmission and a transmission window from 190 nanometers in the UV to 5 microns in the IR.
  • No solarization in high-radiation systems (high purity sapphire only).
  • Very high dielectric constant and low loss tangent.
  • Thermally very stable. Does not lose its mechanical and optical qualities from cryogenic to over 2000C.
  • Thermal conductivity greater than other optical materials and most dielectrics.
  • No surface damage and devitrification due to extreme thermal cycling.
  • Does not sag or slump at very elevated temperatures.
  • Highly corrosion resistant. More resistant to corrosive chemicals than most standard hard materials.

Sapphire Drawbacks:

  • Cost of material. Sapphire often costs more than other refractory materials. But not always: quantity and geometry play a major role in the cost of the final product, especially with smaller products where the labor is the primary cost.
  • Cannot be bent, molded, drawn or melt-fused like glasses and metals. Sapphire can only be ground and mechanically polished.
  • Sapphire has a higher dielectric constant, but fused quartz
    has a slightly lower loss tangent.
  • Size limitations. The maximum size of a sapphire product cannot exceed the largest boule that can be grown. Therefore, maximum part size cannot exceed 300mm for two of the dimensions.
  • If thermal insulating is required, fused quartz or other glasses are superior.
  • Large pieces can be thermally shocked and broken if not heated uniformly.
  • Birefringent unless the optical axis is very precisely aligned with the C-axis.

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Fused quartz and fused silica are the amorphous form of quartz. Fused quartz is made from purifying and melting natural crystalline quartz (usually natural quartz sand). Fused silica, a purer version of fused quartz, is a commercial glass usually made from a silicon-rich chemical precursor that is then oxidized to silicon dioxide. Chemically known as SiO2, silica is "pure" glass. All other commercial glasses are SiO2 with dopants added to lower the melting temperature and modify optical, thermal and mechanical characteristics.

Fused Silica & Fused Quartz Glass Benefits:

  • Extremely low coefficient of expansion, making it far more shock resistant than any other refractory material.
  • Best transmission characteristics of any standard glass:
    o 220 nanometers to 3 microns for standard semiconductor-grade fused quartz
    o 175 nanometers to 3 microns for many types of fused silica
  • Highest temperature characteristics of any glass. A continuous maximum continuous use temperature of 900C to 1100C, depending on the size and shape of the part. Can be used up to 1400C for very short periods of time.
  • High dielectric constant and the lowest loss tangent of almost all known materials.
  • Very low thermal conductivity.
  • Can be fused, drawn and welded into tube and rod forms.
  • For some geometries can be slump molded.
  • Can be ground and polished and fire polished.
  • Comes in boules as large as 72” diameter x 26” high, allowing for very large workpieces.
  • Harder than most glasses.
  • High resistance to non-fluorinated acids, solvents and plasmas.
  • Excellent for containing many high-purity chemicals.
  • Less expensive than sapphire for larger parts.

Fused Silica & Fused Quartz Glass Drawbacks:

  • Much more costly than other standard glasses.
  • Can sag and slump over time at elevated temperatures (>1000C).
  • Surface devitrifies over time when temperature is cycled at high temperatures (>1150C).
  • Breaks down with some caustics, fluorinated acids and plasmas.
  • Can solarize in high radiation environments.
  • If thermal dissipation is required sapphire works much better for conducting away heat.
  • Due to the high melting point, fabrication costs for melting and blowing are much higher than other standard glasses.
  • Standard shapes are tubes and boules. Does not come in standard sheets like borosilicate and soda lime glasses. In other words, other than tubes, all fused quartz and silica products must be ground and polished from a large block.

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Borosilicate glass is an "engineered" glass developed specifically for use in laboratories and applications where thermal, mechanical and chemical conditions are too harsh for standard, household-type glass. Some common names of borosilicate are Pyrex™ by Corning and Duran™ or Borofloat™ by Schott Glass. Like most glasses, the main component of borosilicate glass is SiO2 with boron and various other elements added to give it its excellent qualities.

Borosilicate Glass Benefits:

  • Much easier to hot-work than quartz, making fabrication less costly.
  • Material cost is considerably less than fused quartz.
  • High dielectric constant (higher than fused quartz), but high loss tangent (not nearly as good as fused quartz or sapphire).
  • Compared to all glasses, except fused quartz, it has a low coefficient of expansion (three times less than soda lime glass). The risk of breakage due to thermal shock is very low making it useful for cooking, heating and other thermal environments.
  • Like soda lime glass, the float process is used to make relatively low-cost, optical-quality borosilicate glass sheet in a variety of thicknesses (less than 1mm to over 25mm thick)
  • Easily moldable (compared to quartz).
  • Minimal devitrification when molding and flame working. High-quality surfaces can be maintained when molding and slumping
  • Thermally stable up to:
    o 450C for continuous use
    o 600C for short periods
(but must be re-annealed if over 430C)
  • More resistant to non-fluorinated chemicals than household soda lime glass.
  • Mechanically stronger and harder than soda lime glass.

Borosilicate Glass Drawbacks

  • Material will not maintain its shape if exposed to thermal conditions greater than 450C for long periods of time.
  • High loss tangent.
  • Borosilicate is usually 2 to 3 times more expensive than soda lime material.
  • Not as thermal shock resistant as fused quartz (FQ has a CTE ~60x less).
  • For high purity chemicals, some minor leaching can occur over time, especially if exposed to certain acidic or basic chemicals.
  • Cannot be fully tempered like soda lime glass.
  • Cannot be fully chemical strengthened like soda lime glass.

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Soda lime glass is the "original" glass, appearing in its most basic form thousands of years ago. Commonly called float glass, it is often formed by floating soda lime glass on a bed of molten tin. It is also knows as crown glass, a high-silica form of soda lime that was historically used for windows. Soda lime glass is composed of SiO2: sodium oxide (soda) and calcium oxide (lime). About 90% of the glass used in the world - including most windows, dinnerware, art and lighting products - is one of 50,000 variations of soda lime glass

Although soda lime glass typically has a green or blue-green tint to it, the iron content can be reduced to the point where the glass becomes crystal clear, also known as "water white."

Soda Lime Glass Benefits:

  • Inexpensive and easy to mass produce.
  • Low melting temperature; maintains softness for a long time. This allows for long working times and faster production rates.
  • Easily "floated" because of its low softening temperature, making it a very low-cost, flat (float), optically clear sheet glass.
  • Softer than borosilicate and quartz, making scribe cutting easier and faster.
  • Because of its high coefficient of expansion, it is easily tempered. Tempered glass is up to 3 times stronger than non-tempered glass and it crumbles when broken; a good (and often required) safety feature.
  • Because of its high sodium content, it chemical strengthens very well; allowing for a very hard, scratch resistant surface.

Soda Lime Drawbacks:

  • High coefficient of expansion, thus very poor thermal shock resistance. Only good in thermal environments where heating is uniform and gradual.
  • Will sag easily at relatively low temperatures.
  • Does not come in as many stock thicknesses as borosilicate.
  • Many chemicals will leach the glass over time, making it unsuitable for pure chemical applications.
  • Not as scratch resistant as borosilicate and quartz.
  • Because of its massive use in commercial applications where perfection is not important, raw material processes are not as tightly controlled. Therefore, purity and quality varies more than its high tech siblings.

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Diamond is such an extraordinary material that it deserves mention even though RSW only makes a few diamond sight windows per year. Not only has diamond been noted for millennia for its beauty and hardness, it has a number of other unique, qualities that make it an excellent sight window material.

Diamond Benefits:

  • Hardest known material.
  • 2x-3x stronger than silicon carbide and sapphire, 4x stiffer.
  • Optically clear from 230nm to over 100microns with a small reduction in transmission between 2.5 and 6.2 microns. This is the widest transmission spectrum of all materials.
  • Thermally conducts better than all other optical materials and most metals. Thermally conducts 5x better than copper.
  • Coefficient of thermal expansion is extremely low (almost as low as fused silica). This low CTE combined with high thermal conductivity makes diamond the most thermal shock resistant of all optical and refractory materials.
  • An excellent insulator with a very high dielectric constant and low loss tangent
  • Chemically inert, as inert as sapphire if not better.

Diamond Drawbacks:

  • Maximum size for single crystal diamond is 8mm diameter.
  • Maximum size for polycrystalline diamond is 150mm.
  • Costs between 2x and 4x sapphire depending on size.

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