The multiple properties and application characteristics of quartz glass

Quartz glass is key in areas like semiconductor processing, optics, and scientific research. People know it for its excellent performance. Its usage and processing methods are unique. Even though its name differs from quartz by just one character, it has key differences. Quartz is a SiO2 (silicon dioxide) crystal. It differs from quartz glass, which is glassy, amorphous, and non-crystalline. Manufacturers create quartz glass, or fused quartz, in different ways. One method is melting quartz sand at high temperatures. Another way is hydrolyzing silicon tetrachloride in a high-temperature hydrogen-oxygen flame.

1. Mechanical Properties

The mechanical properties of quartz glass are crucial for its processing and application. Understanding how changes in the density of quartz glass affect costs is crucial. Density is key to calculating the material cost of quartz products. Transparent quartz glass has a density of 2.2 g/cm³. Opaque quartz glass, however, has a density that changes. This change depends on the type of dopants or bubbles inside. For specific values, please refer to relevant material handbooks.

Quartz glass has a lower Mohs hardness than quartz crystal. But its surface can still get scratched by quartz sand in dust. The theoretical tensile strength of quartz glass is high. But surface defects lower its actual measured strength. During mechanical processing, it’s important to avoid applying tensile force. This helps prevent cracks in the quartz glass.

Quartz glass has high compressive strength. This is why it is usually processed by pressure grinding in mechanical processing. We must also be aware that quartz glass fragments can be sharp. So, we need to take safety measures when handling and processing them to avoid cuts.

Also, like quartz glass, obsidian is a volcanic glass that is very sharp. Its edge can reach the molecular level, far sharper than polycrystalline materials. Obsidian is too fragile for uses like human surgery, which needs strong materials.

2. Chemical Purity and Chemical Properties

Quartz glass, in its form as pure silicon dioxide, is inherently pure but inevitably contains trace amounts of impurities. Even small amounts of these impurities can greatly affect the performance and use of quartz glass.

Key factors that affect purity include:

• The quality of raw materials.

• The choice of manufacturing methods.

• The processing steps that follow are.

Common impurities include metal elements such as aluminum, sodium, and iron. They also consist of hydroxyl groups and chlorine. Common impurities can change the thickness and light absorption of quartz glass. They might also contaminate other materials that touch it. The hydroxyl content affects the viscosity and infrared light absorption of quartz glass. Quartz glass made by electric or plasma melting in vacuum or dry conditions has low hydroxyl content. It can be as low as 1-30 ppm due to its careful dehydroxylation process. The hydroxyl content in hydrogen-oxygen flame-fused quartz is quite high. It ranges from 150 to 200 ppm. This is because moisture forms during the hydrogen-oxygen flame reaction. Synthetic fused quartz has a high hydroxyl content, up to 1000 ppm. Yet, a two-step synthesis

Quartz glass shows great resistance to elements and compounds. This trait is a big plus for its high-end uses. It can withstand the erosion of water, salt solutions, and various acids, except for hydrofluoric acid and phosphoric acid. Thus, hydrofluoric acid is usually used to clean quartz glass. The reaction rate of hydrofluoric acid with quartz glass depends on both temperature and concentration. But, it reacts so quickly that precise control is hard to achieve. So, we usually pick low concentration and room temperature. Then, we control how much etching happens by adjusting the reaction time. The etching rate of quartz glass with hydrofluoric acid doesn’t change in a straight line as the concentration varies.

Most metal elements do not react with quartz glass or synthetic fused quartz. This includes all metals except alkali and alkaline earth metals. Quartz glass reacts easily with alkali and alkaline earth metals when heated. Even small amounts of these metals can speed up crystallization. Before heating quartz glass, wipe the surface with alcohol. This removes fingerprints from alkali metals. Doing this ensures stable performance.

3. Thermal Properties

The viscosity of quartz glass shows how well it performs. Key temperatures to know are the softening point, annealing point, and strain point. The softening point is the temperature at which quartz glass deforms due to its own weight. The annealing point is the temperature at which internal stress releases over time. The strain point is the temperature at which most internal stress releases over time.

The hydroxyl content differs in fused quartz, air-fused quartz, and synthetic quartz. This variation affects their viscosities. Synthetic quartz has the most hydroxyl content. Air-fused quartz comes next, while fused quartz has the least. The softening, annealing, and strain temperatures of these three quartz glass types decrease from high to low.

Quartz glass has low thermal conductivity. This lets people hold one end while using a flame on the other end during manufacturing. Quartz glass does not conduct heat as well as materials like asbestos, refractory bricks, stainless steel, graphite, and copper.

One of the most remarkable features of quartz glass is its very low coefficient of thermal expansion (CTE). Its average CTE value is approximately 5.0E-7/°C, which is much lower than that of many other common materials. When you heat a 1m³ stainless steel block from room temperature to 500°C, it expands. Its volume increases by over 28 liters. Borosilicate glass expands by 5 liters. In contrast, quartz expands by less than 1 liter. Quartz glass has a very low thermal expansion rate. This gives it great thermal stability. It can handle extreme thermal shocks. For example, it won’t crack if you quench a hot quartz glass sheet in cold water. No matter if it is electrically fused, air fused, or synthetic quartz glass, the CTE values stay the same. This consistency lets you join them easily. It also reduces the chance of cracking from thermal shock.

4. Permeability

Most gases can’t go through quartz glass. But hydrogen, deuterium, helium, neon, and a few others can. They move through at a certain speed. Temperature and pressure affect this penetration rate. The gas permeability at 700 °C is a key indicator.

In high temperatures, ions can enter quartz glass. The permeability at 1000°C is especially important. Metals can also penetrate quartz glass when heated. This may lead to glass contamination. To keep quartz glass pure, use hydrofluoric acid to etch the surface. This method removes contaminants effectively.

5. Crystallization Phenomenon

As quartz cools and solidifies from molten rock, its density changes several times. It begins with β-tridymite, which has a density like quartz glass. Then, it changes to β-cristobalite, which is a bit denser. Next, it moves to β-quartz, which has an even greater density. Finally, it transforms into α-quartz, with a density of 2.65 g/cm³. As density goes up and volume goes down, quartz glass can peel locally. This weakens the quartz glass. In semiconductor applications, it can also contaminate wafers with particles from peeling.

Researchers can subdivide the crystallization process into two stages: nucleation and growth. Nucleation starts when alkali metals or other metals contaminate the quartz glass surface. Then, growth depends mainly on temperature and viscosity. It is also influenced by oxygen content and water vapor. As temperature increases, hydroxyl content rises. This lowers viscosity and speeds up the crystallization rate.

During electric melting of quartz, you can suppress crystallization. We keep the crucible environment neutral to reduction. It’s also important to maintain a slightly oxygen-deficient state. Before thermal processing or annealing, clean the quartz glass well. This step is crucial to avoid surface contamination that can cause crystallization. Choose low-hydroxyl electrically fused quartz for quartz glass products in semiconductor high-temperature processes. Avoid high-hydroxyl air-fused quartz.

6. Optical Properties

Fused silica lets light pass through well. It covers the full visible spectrum and goes into the infrared and ultraviolet areas. The ultraviolet cutoff wavelength is about 0.180 micrometers. The infrared cutoff wavelength goes up to 3.5 micrometers. When photon energy exceeds the silicon-oxygen band gap, it leads to electron transitions. This action sets the location of the ultraviolet cutoff wavelength. The vibration of the silicon-oxygen network lattice creates the infrared cutoff wavelength. The material’s purity and the manufacturing process greatly impact transmittance. Metal impurities make the ultraviolet cutoff wavelength move to longer wavelengths. The hydroxyl absorption band also absorbs in the infrared. It peaks at 2.73 micrometers.

7. Light-induced damage

Synthetic quartz glass absorbs UV light poorly. For a thickness of 1 mm, it cuts off around 160 nm. This makes it great for lenses in high-energy laser applications. It is also useful in UV light sources like excimer and deuterium lamps. The laser’s wavelength, energy density, and peak intensity all impact how quartz glass gets damaged.

High laser intensity can cause quartz glass to undergo photoionization and plasma effects. This may result in damage to the front and back surfaces. Additionally, tiny channels can form along the direction of the laser in the quartz glass. Quartz glass also reacts to radiation. This creates color centers and leads to absorption at certain wavelengths. The E’ center absorbs light at 215 nm. In contrast, the non-bridging oxygen hole (NBOH) center has an absorption peak at 265 nm. NBOH absorbs light in its absorption band. Then, it emits red fluorescence at about 650 nm. These defects can react with hydrogen in the glass. This reaction changes the glass’s optical properties.

Quartz glass can change its structure when exposed to radiation. This can cause the refractive index to change too. These changes can either raise or lower the refractive index. It depends on the type of quartz glass and the irradiation conditions.

8. Electrical Properties

Quartz glass has special electrical properties. This is because it has a wide band gap and pure silicon-oxygen bonds. It offers great electrical insulation. It keeps high resistivity and strong high-frequency traits, even at high temperatures. Quartz glass behaves differently than metals. Its resistivity goes down as the temperature goes up. The dielectric constant of quartz glass is about 4. This is much lower than that of other glass types. Also, it stays stable across a wide range of frequencies. The low dielectric constant comes from the strong silicon-oxygen network. This makes the structure’s polarization rate very low.