Heraeus Quarzglas

Basic MaterialsDivision

Tomorrow’s innovationsspring from 100 yearsof experience

Heraeus

Heraeus is a family-held group of compa-nies founded in 1851 with operations allover the world. The 150-year history ofsuccess has been marked by innovationand technological breakthroughs.

Wilhelm Carl Heraeus developed the firstlarge-scale platinum melting process andfounded the first German platinum smelt-er. In 1899 the Heraeus company suc-ceeded in fusing rock crystal into a highgrade vitreous silica or “quartz glass”using a high temperature oxygen/hydro-gen flame. This technology formed thecornerstone of Heraeus Quarzglas.

Heraeus Quarzglas

Today Heraeus Quarzglas is a technolog-ical and market leader in the productionand processing of high-purity quartz glasswith the experience of one century.

A number of unique optical, mechanicaland thermal properties have made quartzglass an indispensable material in thefabrication of high-tech products.

Heraeus Quarzglas supplies a great vari-ety of industries including semiconductor,fiber optic and chemical processing witha wide range of products made of naturalor synthetic raw material.

Basic Materials Division

The basic materials division ofHeraeus Quarzglas produces and sup-plies semi-finished quartz glass products.The organization is headed in Kleinost-heim (Germany) with subsidiaries and

affiliates in the USA and Japan. Productioncapacities in the basic material divisionhave been greatly expanded worldwide.This reflects a commitment to the ever-increasing demand for quartz glass pro-ducts in high-tech industries.

Responsiveness and flexibility are thetrademarks of Heraeus Quarzglas.Anticipation of the constantly changingdemands and expectations of customersis essential. The continuous pursuit ofzero defects, controlled processes andimproved product reliability is a fun-damental aspect of business at HeraeusQuarzglas.

By focusing on customer satisfaction viaboth experience and technical innovation,Heraeus Quarzglas strives to be a 'com-pany of excellence' emphasizing researchand development with great appeal forhighly qualified and motivated employees.

Heraeus QuarzglasA Brief History

Heraeus Quarzglas Basic Materials Division - Kleinostheim / Germany

Subsidiary Buford / USA

Subsidiary Koriyama / Japan

Heraeus Quarzglas - Hanau / Germany

Fused Quartz and Fused Silica

Fused quartz and silica are among themost valuable materials available to indus-try and science.

Semi-finished products made of fusedquartz or fused silica possess a uniquerange of properties:

• High purity level• Low OH Content • High homogeneity• Low thermal expansion • High chemical resistance • Excellent thermal shock resistance • Low dielectric losses• Low bubble content• High optical transmission in the

IR & UV domain• Low thermal conductivity• High use temperatures

Originally the term “fused quartz” wasused for transparent quartz glass prod-ucts manufactured from quartz crystals.“Fused silica” was the name for opaquequartz ware produced from fused sand.

The distinguishing feature today is thederivation of the raw material and not theappearance of the finished product.

• The raw materials for fused quartz - both transparent and opaque - are quartz crystal or quartz sand of natural origin.

• Synthetic fused silica refers to quartz glass prepared from synthetic chemicalprecursors.

Manufacturing Process

There are three different processes formanufacturing quartz glass:

• Electric Fusion• Flame Fusion• Soot Process

Electric Fusion is the most commonly used melting pro-cess for manufacturing quartz glass. Twodifferent methods of electric fusion canbe used: continuous and batch (boule)fusion.

In the continuous method, quartz sand ispoured into the top of a vertical melterwhich consists of a refractory metal cru-cible surrounded by electric heating ele-ments. The interior of the crucible ismaintained in a neutral or slightly reduc-ing atmosphere that keeps the silica fromreacting with the refractory metal. Themelted material exits the bottom orifice ofthe crucible which is shaped to producerods, tubes, plates or other products ofvarious dimensions.

In the batch fusion method, a large quan-tity of raw material is placed inside arefractory lined vacuum chamber whichalso contains graphite heating elements.Although this method has historicallybeen used to produce large single bou-les of material, it can also be adapted toproduce much smaller, near-net shapes.

Flame Fusion was originally known as the Verneuilleprocess.

Heraeus chemist Richard Küch firstbegan fusing quartz rock crystal in anhydrogen/oxygen (H2/O2) flame morethan 100 years ago. Heraeus has beenproducing quartz glass on an industrialscale with this process ever since.

The basic process consists of tricklinghigh purity silica sand (made of crushedquartz crystal or natural quartz sand) at acontrolled rate into a high temperature H2/O2-flame. There it melts and collectson a bait rod that is slowly withdrawnfrom the flame thus forming a solid roundingot. This ingot can then be shaped intoany desired dimension.

Illustration of flame fusion processRaw MaterialQuartz Sand

Raw Material

Quartz CrystalRaw Material

SiCl4

Method

Electric FusionMethod

Flame FusionMethod

Soot Process

FusedQuartz

FusedSilica

Quartz Glass

Synthetic Fused Silica

is produced by the oxidation/hydrolysis of silicon tetrachloride (SiCl4) vapor in anH2/O2 flame. This process is called thesoot process.

Because silicon tetrachloride is a synthet-ic chemical with exceptional starting puri-ty, alkali and metal contamination in thefinal product is reduced to ppb levels. Toput this in perspective, one part per bil-lion is equivalent to a 40 mm section of theearth´s equator (total length >40,000 km).

Physical Chemistry

At first glance, quartz glass appears verysimple, both chemically and structurally,since it is made from a single oxide com-ponent (silicon dioxide – SiO2).

Silica, as it is also known, is foundthroughout the earth's crust. However,only a small fraction has sufficient purity(> 99.98 %) to be suitable as raw materi-al for quartz glass.

The generalized atomic structure consistsof tetrahedral (four-faced) units construct-ed of four oxygen atoms surrounding acentral silicon atom. These tetrahedra linktogether at the corners to form a three-dimensional network.

Their relative arrangement is very disor-dered compared to that of crystallinequartz which is melted to form quartzglass. This is what gives the material itsglassy nature, which accounts for its melt-ing behavior and formability. Quartz crystalon the other hand cannot withstand being heated more than a few hundred

degrees. This is because abrupt changesoccur in the crystal structure and resultsin cracking. However, the glassy form ofsilica (fused quartz) stands up to themost extreme temperature shocks.

Note that the structure is completelyconnected: meaning that all atoms arebonded to at least two others.

This connectivity together with thestrength of the silicon-oxygen (Si-O)chemical bond gives quartz glass its hightemperature stability and chemical resis-tance. Note also that the structure is rath-er open with wide spaces (interstices)between the structural units. This accountsfor the higher gas permeability and muchlower thermal expansion coefficient ofquartz glass relative to other materials.

This is because the open spaces, be-sides allowing gas molecules to diffusemore easily, gives the structure room toexpand as its atoms become excited byincreasing temperature and oscillate withgreater amplitude about their positions.

Fused Quartz and Fused SilicaPhysical Chemistry

SiCl4 H2 / O2 SiO2 + 4HCl

Natural Quartz Crystal

Quartz Glass

Crystalline SiO2 structure

Glassy SiO2 structure

NaCl

SiCl4

H2

O2

NaOHHCl

SyntheticQuartz Glass

Chemical Purity

Despite existing at very low levels, con-taminants have subtle yet significant ef-fects. Purity is mostly determined by theraw material, the manufacturing methodand subsequent handling procedures.Special precautions must be taken at allstages of manufacture to maintain highpurity.

The most common impurities are metals(such as Al, Na and Fe among others),water (present as OH groups) and chlo-rine. These contaminants not only affectthe viscosity, optical absorption and elec-trical properties of the quartz glass; theycan also influence the properties of mate-rial processed in contact with the quartzglass during the final use application.

The purities of fused quartz and fusedsilica are outstandingly high. Syntheticfused silica from Heraeus contains totalmetallic contamination below 1 ppm. Forfused quartz the amount is approximately20 ppm and consists primarily of Al2O3with much smaller amounts of alkalis,Fe2O3, TiO2, MgO and ZrO2.

Metallic impurities come mostly fromnatural quartz. Very carefully controlledprocesses are used to greatly reduceimpurities in raw material from 200 ppmto less than 20 ppm (SiO2-purity of99.998%).

Aluminum is the most prevalent and littlecan be done to reduce it beyond the levelpresent in the raw sand. This is becausealuminum bonds directly into the quartzglass structure through direct substitutionfor silicon. Thus it has very low mobilityeven at high temperature which makes italmost impossible to remove. However,this also prevents migration to materialsbeing processed in contact with thequartz glass (such as semiconductors).

A small amount of aluminum is actuallybeneficial because it increases quartzglass viscosity, thus raising the maximumuse temperature.

Hydroxyl Content

In addition to metallic impurities, fusedquartz and fused silica also contain waterpresent as OH units. The approximatevalues are given in the following table.

Hydroxyl is present in amounts deter-mined by the level of moisture to whichthe silica is exposed during the formingprocess. The presence of hydroxyl resultsin lower viscosity and optical absorptionbands in the mid-IR.

Electrically fused quartz has the lowesthydroxyl content (0 - 30 ppm) since it isnormally made in vacuum or a dry atmos-phere. Hydroxyl content in this range isnot fixed in the glass structure. It can go up or down depending on the thermaltreatment and amount of moisture towhich the quartz glass is exposed at elevated temperature.

Flame fused quartz has significantly morehydroxyl (150 - 200 ppm) since fusionoccurs in a hydrogen/oxygen flame.

Typical Trace Elements and OH Content in Quartz Glass (ppm by weight oxide)

Elements Al Ca Cr Cu Fe K Li Mg Mn Na Ti Zr

CFQ 099 15 0.8 < 0.05 < 0.1 0.4 0.8 1.2 0.1 0.1 0.9 1.5 0.8

HSQ 100 15 0.5 < 0.05 < 0.05 0.1 0.4 0.6 0.05 < 0.05 0.3 1.1 0.7

HSQ 300 3) 15 0.5 < 0.05 < 0.05 0.1 0.4 0.6 0.05 < 0.05 0.3 1.1 0.7

HSQ 700 3) 15 0.5 < 0.05 < 0.05 0.1 0.1 0.05 0.05 < 0.05 0.05 1.1 0.7

HSQ 3513) 15 0.6 < 0.05 0.07 0.2 0.7 0.4 0.1 0.05 0.8 1.1 1.1

HSQ 7513) 8 0.5 < 0.05 < 0.06 0.2 < 0.1 0.2 < 0.05 0.05 < 0.05 1.4 0.1

HSQ 9004) < 0.04 < 0.02 < 0.001 < 0.001 < 0.03 < 0.01 < 0.002 < 0.01

Synthetic quartz prepared by flamehydrolysis of silicon tetrachloride canhave high (> 1000 ppm) or very lowhydroxyl content depending on whether a hot chlorination step is employed toremove it. The hydroxyl level can be sohigh because the silica particles thatresult from the hydrolysis reaction areextremely fine and therefore have tremen-dous surface area capable of absorbingmoisture present in the flame.

Hot chlorination treatment will remove thehydroxyl but results in some residual chlo-rine in the material. Chlorine behavesmuch like hydroxyl in that it reduces vis-cosity by terminating the bridging oxygenbonds that connect the quartz glassstructure together.

The main attributes of electrically fusedmaterials are the low hydroxyl contentand reduced devitrification rates.

The low hydroxyl content increases infra-red transparency and viscosity. The higher viscosity results in an increasedmaximum use temperature as well ashelping to inhibit devitrification.Devitrification is also restrained by theneutral /reducing atmosphere used duringmelting. This causes the material to beslightly oxygen deficient, which helps torestrain devitrification.

Chemical Behavior of Various Elements and Compounds

Fused quartz is outstandingly resistant towater, salt solutions and acids. It is onlyattacked by hydrofluoric and phosphoricacid. Metals which are free from oxide,with the exception of alkalis and alkaline-earths, do not react with fused quartz orfused silica.

Quartz glass is sensitive to all alkali andalkaline-earth compounds because evenslight traces of them hasten devitrificationat high temperatures. It is always advisa-ble to remove fingerprints, which containtraces of alkalis, from quartz glass withalcohol before heating.

The symbols used in the table have thefollowing significance:

The element or compound does ï not react with fused quartz or

fused silica

ò It reacts only above the indicated temperature

õ Only the melt of the compound reacts with fused quartz or fused silica

å The element or compound reacts with fused quartz or fused silica

Elementsï Agò Al from 700 to 800 °C rapid reactionï Auï Brò C only above 1500 °Cò Ca only above 600 °Cï Cdò Ce only above 800 °Cï Cl also with heat and

humidity no reaction å F only in humid stateï Hgï Jå Li only above 250 °Cò Mg from 700 to 800 °C rapid reactionï Mnï Moï Na reacts only in vapor stateå Pï Pbï Ptò S above 1000 °C very weak reactionõ Siï Snï Tiï Wï Zn

Acidsï H2SO4ï HNO3ï HClå HF but weaker than with ordinary glasså H3PO4 but weaker than with ordinary glassï Organic acidsGases and Vaporsï HClï H2; N2; O2ï NO2; SO2ï COOxidesò Al2O3 only above 1200 °Cò BaO only above 900 °Cò CaO only above 1000 °Cò CuO only above 950 °Cò Fe-oxides only above 950 °Cò MgO only above 950 °Cõ PbOò ZnO only above 800 °Cò Basic oxides only above 800 °C

acceleration of divitrificationSaltsõ BaCl2ò BaSO4 only above 700 °Cõ Borateò BCl3 only above 900 °Cõ KCl promotes devitrificationõ KFõ NaClå Na-metaphosphateå Na-polyphosphateï Na2SO4å Na-tungstate promotes devitrificationõ Nitrateò Pt-ammoniumchloride only above 900 °Cõ ZnCl2ò Zn-phosphate slight at 200 °C

considerable at 1000 °Cò Zn-silicate only above 1000 °C

Fused Quartz and Fused SilicaChemical Behavior / Mechanical Properties

Electrically FlameTechnical Properties Fused Fused Fused

Quartz Quartz Silica

Mechanical Data

Density g/cm3 2.203 2.203 2.201Mohs Hardness 5.5 ... 6.5 5.5 ... 6.5 5.5 ... 6.5Micro Hardness N/mm2 8600 ... 9800 8600 ... 9800 8600 ... 9800Knoop Hardness N/mm2 5800 ... 6100 5800 ... 6100 5800 ... 6200Modulus of elasticitiy (at 20°C) N/mm2 7.25 x104 7.25 x104 7.0 x 104

Modulus of torsion N/mm2 3.0 x104 3.1 x104 3.0 x 104

Poisson´s ratio 0.17 0.17 0.17Compressive strength (approx.) N/mm2 1150 1150 1150Tensile strength (approx.) N/mm2 50 50 50Bending strength (approx.) N/mm2 67 67 67Torsional strength (approx.) N/mm2 30 30 30Sound velocity m/s 5720 5720 5720

Thermal Data

Softening temperature °C 1710 1660 1600Annealing temperature °C 1220 1160 1100Strain temperature °C 1125 1070 1000Max. working temperaturecontinuous °C 1160 1110 950short-term °C 1300 1250 1200Mean specific heat 0...100 °C 772 772 772J/kg·K 0...500 °C 964 964 964

0...900 °C 1052 1052 1052Heat conductivity 20 °C 1.38 1.38 1.38W/m·K 100 °C 1.47 1.47 1.46

200 °C 1.55 1.55 1.55300 °C 1.67 1.67 1.67400 °C 1.84 1.84 1.84950 °C 2.68 2.68 2.68

Mean expansion 0...100 °C 5.1 x10-7 5.1 x10-7 5.1 x 10-7

coefficient 0...200 °C 5.8 x10-7 5.8 x10-7 5.8 x 10-7

K-1 0...300 °C 5.9 x10-7 5.9 x10-7 5.9 x 10-7

0...600 °C 5.4 x10-7 5.4 x10-7 5.4 x 10-7

0...900 °C 4.8 x10-7 4.8 x10-7 4.8 x 10-7

-50...0 °C 2.7 x10-7 2.7 x10-7 2.7 x 10-7

Mechanical Properties Strength and Reliability

The theoretical tensile strength of silicaglass is greater than 1 million psi.Unfortunately, the strength observed inpractice is always far below this value.The reason is that the practical strengthof glass is extrinsically determined ratherthan being solely a result of chemistryand atomic structure as is an intrinsicproperty like density. It is the presence ofsurface flaws that concentrates stresscoupled with the chemical effects ofatmosphere (water vapor in particular)plus design considerations that ultimatelycontrol the strength and reliability of afinished piece of quartz glass. Becauseof this stress concentration effect, failuremost always occurs in tension rather thancompression.

In other words: mechanical reliability isproportional to the chance of finding aflaw that will cause failure under the operating conditions.

This could also be stated as the proba-bility that the piece will experience a me-chanical stress greater than the strengthof any existing flaws. As a result of thisdependence on probability, reliabilitydecreases as the size of the glass articleincreases. Similarly, if the number of pieces in service increases, so does thechance of experiencing a failure.

Surface condition is very important. Forexample, machined surfaces tend to beweaker than fire polished ones. Also,older surfaces are usually weaker thanyounger ones due to exposure to dust,moisture or general wear and tear. Thesefactors have to be considered thoroughlywhen comparing the strengths of different„brands“ of quartz glass.

Fused Quartz and Fused SilicaMechanical & Thermal Properties

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Fiber diameter [µm]

Tensil strength of fused silica fibres as a function of fibre diameter

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This is because these tests in reality oftenturn out to be just comparisons of surfacequality resulting from sample preparation,small differences in which easily over-whelm any differences in intrinsic strength.

Because of these considerations, it isimportant to realize that strength datataken from a small number of similar sam-ples in a laboratory setting is usually notvery useful in predicting the observedstrength of a large and complex finishedpiece. Such tests usually give results for

tensile strength in the range of 30 to 60 kpsi but they generally do not test thesame flaw size distribution that exists inpractice. A very basic rule of thumb thatseems to have been proven out in practiceis that the design should be such that ordinary tensile stresses do not exceed1000 psi.

Elasticity modulus of quartz glass Internal damping of transparent fused silica

Temperature [0C]

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Tens

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Temperature [0C]

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Temperature [0C]

Tensile strength of fused silica fibers

Fiber diameter [µm]

Sound velocity in transparent fused silica for longitudinal waves

Thermal Properties

One of the most attractive features ofquartz glass is its very low thermal coeffi-cient of expansion (TCE). The averageTCE value for quartz glass at about 5.0 x 10-7/°C is many times lower thanthat of other common materials. To putthis in perspective, imagine if 1 m3 blocksof stainless steel, borosilicate glass andquartz glass were placed in a furnaceand heated by 500°C. The volume of thestainless steel block would increase bymore than 28 liters and that of the borosil-icate block by 5 liters. The quartz blockwould expand by less than one liter. Suchlow expansion makes it possible for thematerial to withstand very severe thermalshock.

It is possible to rapidly quench thinarticles of quartz glass from over 1000°Cby plunging them into cold water withoutbreakage. However, it is important to re-alize that the thermal shock resistancedepends on factors other than TCE suchas surface condition (which definesstrength) and geometry. The varioustypes of fused silica and fused quartzhave nearly identical TCE's and thus canbe joined together with no added risk ofthermally induced breakage.

Viscosity

Somewhere above 1000°C, the timerequired for quartz glass to change itsshape permanently in response to anapplied load becomes compatible withthe patience of someone trying to mea-sure the deformation. At this temperatureand above, the material exists as a super-cooled liquid. The term "super-cooledliquid" is sometimes used to describequartz glass at lower temperatures butthis is not strictly correct. Below 1000°Cthe material can be regarded as a truesolid.

The viscosity has an exponential depen-dence on inverse temperature and thusdecreases rapidly as temperature rises.Despite this fact, quartz glass at 1700°Cis still quite thick, having a consistencysimilar to that of tar on a cold day. Theviscosity is significantly affected by traceimpurities. The hydroxyl (OH) contentlowers the viscosity thus making themethod of manufacture an important con-sideration for defining maximum use tem-perature. Alkalis and halogens such assodium and chlorine lower viscosity whilesmall amounts of aluminum and refractorymetals like molybdenum increase viscosity.

Viscosity

Temperature [0C]

a 0,

t x 1

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Cha

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[mm

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Temperature [0C]

Coefficient of expansion of transparent fused silica Change in length of 1 m long rod of transparent fused silica

Devitrification

The term "super-cooled liquid" refers tothe fact that, at least from a thermo-dynamic equilibrium point of view, quartzglass should actually be a crystalline solid rather than a liquid. This fact is the key tounderstanding why quartz glass devitrifies.Although the thermodynamically preferredstate of quartz glass is crystalline, thehigh viscosity prevents the structural re-arrangement necessary to achieve it. In other words the molecules cannot arrange themselves quickly enough com-pared to the relatively fast rate of coolingthat quartz glass normally experiences.However, under certain conditions thisconstraint can be removed resulting inthe glass reverting to a crystalline state.This usually happens at elevated temper-ature in the presence of a contaminantthat drops the viscosity by breaking upthe highly connected silicon-oxygen net-work as well as acting as a nucleatingsource. Alkalis like sodium or potassiumare the most common contaminants thatcause devitrification.

Atmospheres high in water vapor or chlo-rine also exacerbate this process signifi-cantly. The growth of the devitrified layerusually starts on the surface and pro-gresses into the material at a rate thatdepends exponentially on temperature. The crystalline material formed is a hightemperature form of silica known as highcristobalite.

High cristobalite has nearly the samedensity as glassy silica and thus cannotbe seen on the surface. However, uponcooling, high cristobalite undergoes astructural change from a cubic to a tetragonal crystal structure at about275°C. This is accompanied by a largedecrease in density that can result insome cracking and spalling. Refractiveindex differences resulting from the bire-fringent tetragonal crystal structure alsocause the devitrified spots to turn white.

Electrical Properties

Fused quartz and silica are excellentelectrical insulators. The large band gapinherent in the electronic structure of thesilicon-oxygen bond results in electricalconduction being limited to current car-ried by mobile ionic impurities. Since thelevel of these impurities is very low, theelectrical resistivity is correspondinglyhigh.

Since ionic conduction is related to thediffusion coefficient of the ionic carriers,the resistivity also has a strong expo-nential temperature dependence. Hence,unlike typical conductors such as metals,the resistivity decreases with increasingtemperature.

The dielectric constant of quartz glasshas a value of about 4 which is signifi-cantly lower than that of other glasses.This value changes little over a widerange of frequencies. The reason for thelow dielectric constant is, once again, the lack of highly charged mobile ions but it also results from the stiffness of the silicon-oxygen network which imparts a very low polarizability to the structure.

Fused Quartz and Fused SilicaElectrical & Optical Properties

Electrical FlameElectrical Data Fused Fused Fused

Quartz Quartz Silica

Electrical resistivity in Ω x m

20 °C 1016 1016 1016

400 °C 1010 1010 106

800 °C 6.3 x106 6.3 x106 6.3 x104

1200 °C 1.3 x105 1.3 x105 1.3 x103

Dielectric strength in kV/mm (sample thickness ≥ 5 mm)

20 °C 25 ... 40 25 ... 40 25 ... 40

500 °C 4 ... 5 4 ... 5 4 ... 5

Dielectric loss angle (tg δ)

1 kHz 5.0 x 10-4 5.0 x 10-4 5.0 x 10-4

1 MHz 1.0 x 10-4 1.0 x 10-4 1.0 x 10-4

3 x 1010 Hz 4.0 x 10-4 4.0 x 10-4 4.0 x 10-4

Dielectric constant (ε)

20 °C 0...106 Hz 3.70 3.70 3.70

23 °C 9 x 108 Hz 3.77 3.77 3.7723 °C 3 x 1010 Hz 3.81 3.81 3.81Original Quartz Glass Layer

Devitrified Layer

Optical Properties

The intrinsic UV and IR absorption edgesin silica glass are located at roughly 3.5and 0.180 microns wavelengths, respec-tively. The intrinsic UV absorption edgeresults from the onset of electronic transi-tions within the Si-O network at the pointwhere the photon energy exceeds thenetwork bandgap energy. The intrinsic IRedge arises due to lattice (multi-phonon)vibrations of the Si-O network.

Various overtones of the fundamentalSiO4 tetrahedron vibrational modes arethe first to be observed. These intrinsicabsorption edges are then further modi-fied by the presence of impurities.Metallic impurities shift the UV edge tohigher wavelengths. Water (OH) intro-duces absorption bands just below the IR edge. The strongest of these is thefundamental O-H stretching band at 2.73 microns.

The transmission versus wavelengthspectrum for silica is shown in the figuresbelow. The significant features of thecurve are a broad transparent area cen-tered over the visible part of the spectrumthat extends partly into the infrared andultraviolet regions.

Transmission

Stresses in fused quartz can be easily seen in polarized light

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