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Introduction
This article presents an in depth look at quartz glass. Read
further and learn more about the following:
What is quartz glass?
Production of quartz glass
Properties of quartz glass
Applications of quartz glass
Handling of quartz glass
And much more...
Chapter One – What is Quartz Glass?
Quartz is one of the most abundant and widely distributed
minerals in nature and is the only stable polymorph of
crystalline silica on the Earth‘s surface. It is found in
all forms of rocks: igneous, metamorphic and sedimentary. It
becomes concentrated in soils, bodies of water and sand when a
quartz-bearing rock is weathered or eroded.
The chemical formula of quartz is SiO2. The silicon-oxygen
(Si-O) bond is polar and covalent. Elemental silicon contains
four valence electrons making the silicon atom bonded to four
oxygen atoms. One oxygen atom is bonded to two silicon atoms,
making the body-centered tetrahedral crystal system of quartz.
The tetrahedral crystal system is composed of four oxygen atoms
at the corners and a central silicon atom. In one tetrahedron,
the O-Si-O bond makes a 109° angle. In a network of SiO4
tetrahedra, the corner oxygen atoms link the central silicon
atom. The Si-O-Si bond makes a 144°. The structure of the
networked SiO4 is open with wide spaces, hence giving quartz a
hexagonal crystalline form.
Silica sand is a common material used for the production of
quartz glass. It is an inert hard mineral that has been broken
down over time into sand. High purity silica sand allows for
more control of the final product’s strength, clarity, and
color. Through the use of consistent chemical processing, each
batch of quartz glass is of high quality, uniform, and
exceptionally reliable.
Quartz can be manufactured into quartz glass, which is valued
for its exceptional purity and serves a wide range of
applications. Quartz glass does not contain additives. It is
sometimes referred to as fused quartz or fused silica; the
difference between the two is that fused quartz is made from
pure silicon dioxide (SiO2) while fused silica is made from
synthetic precursor.
The majority of quartz comes from silica sand that is used to
produce high purity quartz that has extra strength, clarity, and
color. Extensive sand processing produces quartz with
exceptional purity since the use of product chemistry removes
any impurities.
Quartz glass is valued due to its distinct and high value
characteristics. Among these are because of its low coefficient
of thermal expansion, high gas permeability, and extensive
optical transmission.
Chapter Two - Production of Quartz Glass
This chapter presents the steps in transforming the raw quartz
into a formed, fused quartz glass.
Washing and Drying
Dirt, moisture and contaminants present in the natural quartz
are removed in the early stages of processing which may affect
the quality and performance of the quartz glass to be produced.
This is only applicable for mined quartz.
Comminution
The objective of this step is to reduce the raw quartz into a
size suitable for the fusion method and machinery to be
utilized. Natural quartz undergoes a series of size reduction
steps such as crushing and milling (ball milling or roll
milling). Quartz is very brittle in nature, which makes
comminution quite easy. Afterwards, the particle size is
analyzed and larger grains are separated.
Fusion
In this stage, thermal energy is used to break the strong
silicon-oxygen bond. With increasing temperature, more bonds are
broken and result in the less viscous flow of quartz. After
shaping and cooling to its final form, the ordered crystalline
structure of SiO2 molecules is converted into a vitreous,
amorphous structure and metastable form of quartz.
Depending on the desired purity level and end use application,
the natural quartz may be homogenized and formed through the
following fusion methods:
Electric fusion
This method produces an industrially known Type I quartz glass.
As drawn the material will contain 100 ppm to 130 ppm OH
content. Electric fusion is used if a high level of purity and
low hydroxyl (OH) content (> 1 ppm – 30 ppm) is to be obtained.
Using the vacuum annealing process, the OH content can be
reduced to necessary levels as needed for certain applications.
Lower OH levels allow for greater transmission of UV in the IR
range (2750nm), which could be critical for use in certain
applications. The starting material is natural quartz grains,
and may be subject to the following production modes:
Continuous Mode: The quartz sand is continuously fed on
top of a refractory metal crucible column which contains an
electric heating device. The internal chamber of the crucible
is maintained at a dry and vacuum-sealed atmosphere to keep
the melted quartz from reacting with the refractory material.
After passing through the hot crucible column, melted quartz
is collected in an orifice located at the bottom of the column
in which it is shaped and cut into plates, tubes and rods.
This method is suitable for high volume manufacturing.
Batch or Boule Mode: Large quantity of quartz is placed
inside a refractory-lined vacuum chamber which also contains
an electric heating device. After the quartz is fused, the
viscous melt is collected and shaped into its final form. A
single boule may be 72” x 24” thick and weigh up to 8000 lbs.
From a boule, parts are cut, sliced, and diced to make plates,
discs, flanges, and other components. Most items are then
mechanically polished or fire polished to make clear.
Flame Fusion
In this method, a natural quartz or a synthetic precursor can be
a starting material. Natural quartz passes through a chamber
with a high temperature hydrogen/oxygen (H2/O2) flame until the
starting material is fused. If silicon tetrachloride (SiCl4), a
gaseous synthetic precursor, is to be used, it is made to react
with the H2/O2 flame.
The viscous melt is deposited in a refractory lined vacuum
chamber, collected slowly by a die at the bottom of the
container, and shaped to its final form. Due to its direct
contact with H2/O2 flame, this method produces quartz glass with
150-200 ppm OH content from natural quartz and up to 1000 ppm
for synthetic silica. Material produced in this way has a high
stable OH content that cannot be vacuum annealed. The quartz
produced will have a lower temperature softening point and lower
operating temperature.
Quartz glass made using this method has a higher and more stable
OH content that cannot be vacuum annealed out. The quartz
material will have a lower temperature softening point and lower
operating temperature.
Glass produced from crystal quartz through flame fusion is
classified as Type II, and from synthetic precursors as Type
III. Type III synthetic silica glass is a product of a chemical
reaction. The combustion of silicon tetrachloride gives
synthetic quartz and leaves environmentally toxic byproducts,
chlorine, and hydrochloric acid.
Plasma Fusion
This process is similar to flame fusion with water-vapor free
plasma flame being used as a source of heat. Plasma fused quartz
glass has high purity level, low OH content, minimal bubble
content and no drawing lines.
Natural quartz or a synthetic precursor may be the starting
material for this method. Quartz glass produced from the
combustion of a synthetic precursor in plasma flame is known as
Type IV.
Electric Arc Fusion
The quartz sand is melted in an electric arc furnace. The
heating of the sand produces a vitreous material with gaseous
micro-bubbles that diffract light giving the material its
opacity. The resulting glass ingots are crushed and molded into
parts that are dried and sintered. The quartz glass that is
produced is white and opaque and does not generally belong to
any types of quartz glass and contains a 100 ppm to 130 ppm OH
content.
Shaping and Finishing Processes
The manufacture and shaping of quartz glass is unlike processes
that are used to manufacture typical glass. The higher
temperatures are necessary due to the fact that quartz glass
does not flow but softens and becomes viscous.
Mechanical Forming
Shaping and forming of quartz glass may require diamond cutting
tools due to its hardness. Also, such operating parameters must
be optimized since the quartz glass is also brittle and there is
a limited force that can be applied before cracking or fracture
occurs. Some of the mechanical processes include:
Cutting: Band and wire saws, chop saws, CO2 lasers, and
water jet cutters are used to cut the quartz glass. Using a
laser cutter can leave a glazed and smooth cut. Saw cutting
can leave a rough cut. Thick quartz glass slabs may require
multiple consecutive cuts if a single cut is not sufficient.
Annealing may be required to relieve the thermally-induced
stress. With vacuum annealing processes, the OH content can be
reduced to fit the needs of certain applications. Lower OH
levels allow for greater transmission of ultraviolet rays in
the IR range of 2750 nm, which may be critical for certain
applications.
Drilling: As detailed in the fused quartz glass
product, holes may be produced using a diamond driller. A
laser driller may be used to cut thin, small plates. Proper
cooling must be ensured in order to prevent the tools from
premature wear-out.
Grinding: The quartz glass surface may be smoothened
and its thickness may be reduced, depending on the end-use
application.
Hot Forming
The quartz glass is quite complex to thermoform due to its high
melting point and steep viscosity, allowing it to be formed on a
very narrow temperature range. If the temperature is too low,
the glass is solid; if the temperature is too high, the glass is
less viscous and volatile resulting in evaporation of the parts.
In addition to this, single or multiple annealing steps are
required to relieve the thermal stress and prevent fracture
induced by hot forming. The following are some hot forming
methods which a manufacturer can use in order to enhance the
glass product:
Welding: Two components of quartz glass are joined
together through a weld. The ends of each component are
heated, and a piece of quartz glass is melted to fill the gap
in the seam or joint. It is critical to keep the temperature
just high enough in order to avoid thermal stress.
Collapsing: A tube is locally heated to its softening
temperature. Pressure is applied to the inside to reduce the
tube's diameter.
Elongation and Compression: A positive or negative
radial force is applied to elongate or compress the quartz
glass rod to its final diameter. This is performed at the
softening temperature of the quartz glass, and an optimal
force must be applied in order to prevent fracture and
cracking.
Glass Blowing: A piece of molten quartz glass is
inflated with the aid of a blowpipe in order to acquire a
hollow shape.
Chapter Three: Leading Manufacturers of Machines for Processing Quartz Glass
Many machines are available to produce quartz glass items, and they are important in today's society because quartz glass is a critical material used in various applications, such as optical and lighting devices, and as chemical apparatuses, and these machines enable the precise shaping, fabrication, and processing of such quartz glass components. Below, we provide you with a general overview of some notable manufacturers known for providing machinery and equipment used in quartz glass processing.
Manufacturer: Haas Automation
Model: Haas CNC Machining Center
Haas CNC machining centers are highly precise machines that can perform a wide range of operations on quartz glass, including cutting, drilling, milling, and grinding. These machines use computer-controlled movements to ensure accuracy and repeatability.
Manufacturer: Meyer Burger
Model: Diamond Wire Saw
Meyer Burger's diamond wire saws are renowned for precision cutting of quartz glass. They utilize a thin wire embedded with industrial diamonds to cut through the material with minimal damage or chipping. Diamond wire saws are especially useful for cutting intricate shapes and profiles.
Manufacturer: Schiatti Angelo
Model: Glass Lathe
Schiatti Angelo's glass lathes are specialized machines designed for precision turning and shaping of quartz glass. They excel at creating cylindrical shapes, threads, and other intricate geometries. Glass lathes often come equipped with diamond or carbide tools for cutting and shaping the glass.
Manufacturer: BENTELER Maschinenbau GmbH
Model: Glass Grinding Machine
BENTELER's glass grinding machines are ideal for precision grinding of quartz glass. These machines use abrasive belts or wheels to remove material and achieve the desired shape, smoothness, and surface finish of the glass.
Manufacturer: DMG Mori
Model: DMU 50
The DMG Mori DMU 50 is a versatile CNC machining center that is well-suited for working with quartz glass. This machine offers high precision and can perform various operations such as cutting, drilling, milling, and grinding on quartz glass. It incorporates advanced technologies and features to ensure accuracy and efficiency in quartz glass
Please note that the specific models, features, or components may vary, and it's recommended to consult the respective manufacturers or industry resources for the most up-to-date information on their quartz glass processing equipment available in the United States and Canada.
Leading Manufacturers and Suppliers
Chapter Four - Properties of Quartz Glass
This chapter presents the notable properties and characteristics
of quartz glass.
Chemical Purity
Purity is one of the most important aspects in quartz glass
manufacturing. Contaminants, even in very low levels, influence
the thermal, electrical and optical properties of the resulting
quartz glass and material in contact in their final application.
Strict handling precautions must be taken at the starting
material source and all stages of production to ensure high
purity. The most common impurities are metal oxides (Al2O3,
Fe2O3, MgO, etc.), water, and chlorine.
Water is present in quartz glass as hydroxyl (OH) groups. The OH
content can change depending on the thermal treatment and amount
of moisture to which the quartz glass is exposed at an elevated
temperature. OH influences infrared transmission, viscosity and
attenuation. High levels of OH reduces infrared transmission. OH
also lowers thermal stability; higher OH content means that the
quartz glass is not suitable for high temperature end
applications. An annealing step may reduce the OH content of the
quartz glass in electric fused quartz glass.
Chemical Behavior
Quartz glass is chemically inert to most chemical compounds:
water, salt and acids, making it an advantageous material in
chemical laboratories and industries. It is essentially
impermeable to gases. Hydrofluoric acid and phosphoric acid are
the only agents that can etch and disintegrate quartz glass at
ambient temperatures. However, alkali and alkali earth agents
attack the surface, causing accelerated devitrification. 0.1 mg
of alkali per square centimeter of alkali compounds can amplify
to transform all of the semi-stable molecules. Even
fingerprints, which contains traces of alkali, can trigger
devitrification.
Thermal Properties
Quartz glass is known for its very low coefficient of thermal
expansion (CTE). Thermal expansion refers to the fractional
change in size of an object in response to the change of its
temperature. For most materials, CTE is directly proportional to
temperature change. Quartz glass also has excellent thermal
shock resistance, which can withstand sudden and extreme changes
in temperature. Quartz glass also has low thermal conductivity.
Quartz glass is softened starting at 1630°C and acts like a
viscous liquid at high temperatures like most glass types. This
state occurs at a wide range of temperature, and viscosity
decreases with increasing temperature. Viscosity is also
increased by the presence of impurities.
Mechanical Properties
Quartz glass has almost similar mechanical properties compared
to other glass types. Quartz glass has high compressive
strength, but also exhibits high brittleness. Surface defects
can also affect the overall strength of this material.
Machine-polished parts tend to be weaker than fire-polished
ones. Also, the age of the glass also affects reliability due to
exposure to the environment.
Optical Properties
Quartz glass has been a subject of research due to its extensive
optical transmission properties, covering the ultra-violet
regions, visible and infrared wavelengths. It can be further
enhanced through addition of doping materials. Transmission is
influenced by the quartz glass‘ purity and OH content. The
increase in metallic impurities and OH-molecular vibrational and
rotational excitations can lead to light absorption and hence
affect the consequent transmission.
Electrical Properties
Quartz glass is an excellent electrical insulator, retaining
high resistivity at elevated temperatures. It has a high
dielectric strength. This is due to the absence of charged
mobile ions in the molecular lattice and the strong
silicon-oxygen bond which imparts very low polarizability to the
structure.
The table below summarizes the some of the important property
coefficients absolute to quartz glass, which are discussed in
this article:
The following are the common applications of quartz glass:
Optical Devices
A majority of the applications of quartz glass utilize its
optical properties due to its wide transparency range and
superior light transmittance, ranging from the ultraviolet to
infrared regions. Quartz glass is not easily damaged by
ultra-violet and high energy radiation. Light can pass through a
quartz glass in a functionalized optical path with minimal
distortions. Examples of products with optical applications are:
prisms, lenses, beam splitters, polarizers, mirrors and windows.
Lighting Devices
High purity quartz glass is used in various lamps and lighting
systems, such as mercury lamps, halogen lamps, xenon lamps,
ultra-violet lamps and arc and filament lamps which provide
light source at high temperatures. These lamps are utilized in
several industries, among which are sterilization and cleaning
apparatuses in the food and medical industries and exposure
devices in the semiconductor industry.
Chemical Equipment and Apparatuses
Quartz glass material is a good but expensive alternative, since
it is chemically inert, to other glass types which cannot
withstand high temperature application for a specific use.
Common applications are glasswares, plates and tubes.
Refractory Materials
Fused silica has highly efficient wave transmission in the
ultraviolet spectrum and is commonly used to make ultraviolet
windows, lenses, and optics.
Chapter Six – Handling of Quartz Glass
This section gives the recommended practices when handling or
using quartz glass products, to preserve its valued
characteristics and maximize its service life:
Handling Instructions
Handling:
Quartz glass may be used for a long time if they are kept clean
before and after it is used. Even small amounts of impurities
can promote gradual devitrification. It is recommended to use
clean and lint- free, powder free, or cotton gloves when
handling the quartz glass to prevent further contamination.
Cleaning:
Quartz glass may be cleaned by immersing it in a >7% Ammonium
Bifluoride solution for no longer than ten minutes or a >10% by
volume Hydrofluoric Acid solution for no longer than five
minutes. After cleaning, it must be thoroughly rinsed by
deionized or distilled water and dried.
Storage:
Quartz glass must be stored in an enclosed container when not in
use to protect it from surface flaws and moisture that could
affect the quality and performance of the quartz glass. Ideally,
the glass must be wrapped. In case of a tube, the end openings
must be covered.
Operational Considerations
Rapid Temperature Changes:
Quartz glass can resist extreme heat and thermal shock better
than other glass types. However, heat and thermal shock
resistance is lower when the quartz glass is thicker. Also,
thick and opaque glass products can develop cracks with rapid
temperature change.
Operations Above the Distortion Point:
Before it is annealed, quartz reaches a distortion point, or the
strain point. When a quartz glass is cooled very rapidly after
the distortion temperature (approximately 1100°C),
distortions may again be developed.
Use Different Materials:
Quartz glass has a relatively low thermal expansion coefficient.
The fused quartz may crack if another material of significantly
higher coefficient is attached, fastened or clamped into it.
Caution When Placing in a Furnace:
Due to its low thermal conductivity, cracks may develop on the
surface of the glass when it is heated locally or when it comes
in contact with a flame, at a temperature above the distortion
point. Also, quartz glass becomes less viscous with increasing
temperature. It is advised to take this into consideration when
utilizing quartz glass as a finished product or as a component
of another equipment or device.
Devitrification
Devitrification can shorten the service life of quartz glass,
and drastically remove all the desirable characteristics of
quartz. Devitrification is the conversion of the metastable
quartz glass into a stable, crystalline cristobalite. This
occurs when quartz is heated at high temperatures into an
extended period of time, or when it is heated with impurities
attached to its surface, even in small amounts. With no
impurities present, devitrification normally starts at
1200°C, and hastens with increased temperature. Impurities
lower the devitrification threshold.
Conclusion
Quartz glass is valued for its superior optical properties
(i.e. light transmittance), low coefficient of thermal
expansion, and good chemical resistance.
The starting material for quartz glass production is natural
quartz, cultured quartz or a synthetic precursor such as
silicon tetrachloride.
The crystalline structure of quartz comprises strong, covalent
silicon-oxygen bonds; a single molecule forms a tetrahedral
geometry. A network of SiO4 molecules forms a body-centered
crystal in hexagonal prism form. When processed, the
crystalline structure is converted to metastable, amorphous
quartz glass.
Quartz crystals undergo particle size reduction to prepare it
for the fusion process.
Electric fusion produces Type I glass. The quartz sand or
crystals are fed into a refractory crucible and the melt is
collected to be formed into various parts. Flame fusion
utilizes hydrogen-oxygen flame. Type II glass is produced from
crystal quartz by passing through the hydrogen-oxygen flame;
Type III glass, on the other hand, is produced by combustion
of silicon tetrachloride with the flame. Plasma fusion, which
produces Type IV glass, is almost similar to flame fusion,
except in which plasma flame is used in this method.
Resulting quartz glass undergoes further processing to
transform into utilizable products.
Purity is an important aspect of quartz glass. Hydroxyl (OH)
groups can drastically affect the performance of the quartz
glass. Quartz glass is resistive to most chemical reagents,
but sensitive to alkali compounds.
Quartz glass is used in the manufacturing of optical devices,
lighting systems, refractory materials, and chemical
apparatuses.
One must observe proper precautions when handling quartz glass
to protect its reliability. The glass must be kept clean all
the time. Its thermal behavior must be taken into
consideration when designing its application.
Devitrification is the conversion of the metastable quartz
glass into a stable cristobalite quartz crystal.
Leading Manufacturers and Suppliers
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