Fourier transform infrared spectrometerDue to the wide spectrum characteristics, the selection of infrared window materials is relatively strict. At present, Yingsa Optics has more than ten different infrared window materials to choose from, each with its own characteristics.When choosing window panel materials that are suitable for your application needs, the following factors need to be considered:The spectral range to be studied, the chemical properties of the sample and window, and the physical properties of the window,And the relative cost for specific needs.

The factors that affect the energy transmittance of the transmission cell (excluding the sample) are:The transmission range of the window panel, the effective thickness of the window panel, the energy loss caused by surface reflection of the window panel, and the energy loss during use, such as scratches and fogging.The effective transmission range listed below is determined when the transmission level of a single window drops to 50%. Under the same material, thin window panels can transmit to lower wave numbers. When using a window with a high refractive index, there will be significant energy loss due to the reflection of the window, and strong sine waves will appear, known as interference fringes. These interference fringes are caused by the mutual interference of internal reflections from the parallel window material of the infrared light transmission sample cell. The intensity of stripes is determined by the refractive index of the material, and the higher the refractive index, the stronger the stripes. The air gap between the window panels in the pool determines the frequency of the stripes. Small gaps (such as 0.015mm optical path pool) will produce high-frequency stripes, while large gaps (such as 0.50mm optical path pool) will produce low-frequency stripes. Interference fringes for calculating the transmission celloptical pathVery useful.

An important factor to consider when selecting a suitable window for a specific application is the chemical properties of the sample and window.Samples that react with the window panels inside the pool will corrode the inner surface. Common samples or solvents that are relatively water-soluble and can corrode the surface of window panels are usually listed in the attribute list. The solubility data is the grams of the material that can be dissolved in 100 milliliters of water at room temperature and under equilibrium conditions. Additionally, the solubility in water should not be confused with the degree of moisture absorption of window material. Moisture absorbing materials are prone to absorbing moisture from the atmosphere, causing window panels to fog or become opaque.

The ideal window material must also be compatible with the physical properties of the sample,For example, hardness.For very hard samples, soft window material should not be used. Most methods of measuring hardness are based on using a predetermined shaped indenter and pressing the material with a certain pressure. The shape of the indenter determines the category of this method, which is generally divided into Brinell and Vickers（Vickers）Hardness testing methods such as Knoop. The hardness measurement value will vary with the applied pressure, duration of pressure, and speed of pressurization and release. This variability depends on the creep, cold flow, and stress relieving properties of the material with small cracks. Some softer materials, such as potassium bromide and sodium chloride, can be hand polished again after being corroded or scratched. However, very hard materials such as silicon and sapphire, although able to resist scratches, are extremely difficult to manually re polish.

Transmission cells are generally divided into two types: sealed fixed optical path cells and detachable cells.

Sealed poolWindows with precise alignment and fixed optical path length. It consists of a drilled window and an unperforated window sealed together. The window panels are separated by a metal shim, which determines the optical path length. The windows are parallel and the optical path length is fixed, making it easy to calibrate for quantitative measurements and other precise spectral analyses. Sealed pools can be used for highly volatile samples without the risk of leakage.

Detachable poolIt is a practical and economical alternative to sealed pools, allowing for easy replacement of window panels in the pool. These pools are also composed of a drilled window, an unperforated window, a spacer (usually made of Teflon material), and a sealing ring. The value of detachable pools is demonstrated when used to measure viscous or difficult to clean samples, as they can be easily disassembled, cleaned, and reassembled. When the required optical path length for some applications is unknown and requires experimentation to determine, a detachable pool is a very good choice. The detachable pool can change its optical path length by replacing the spacer.

Sulfur based glassIt is the best material for manufacturing mid infrared optical fibers. This sulfur based optical fiber is composed of AsSeTe glass, which has chemical inertness and mid infrared transmittance. Its applicable pH range is 1-9.

AMTIR (Amorphous Material Transmitting Infrared Radiation) GeAsSe GlassInsoluble in water, it has strong corrosion resistance in acidic solutions. If exposed for a long time (one week), it will dissolve in alkaline solution. This material can be used up to a maximum temperature of 300 ° C. It is a good attenuated total reflection（ATR）Materials.

Barium fluoride (BaF ₂)The transmission range is wider than that of calcium fluoride (CaF ₂). Due to the easy dissociation of barium fluoride caused by thermal or mechanical shock, caution should be taken when using it. Compared with calcium fluoride, barium fluoride has a narrower temperature range for use, reacting with air at 500 ° C and reaching up to 800 ° C (in dry air). In addition, barium fluoride should not be used together with ammonium salt solutions. Barium fluoride is commonly used for infrared transmission cell windows.

Calcium fluoride (CaF ₂)It is one of the hardest known crystalline materials, particularly suitable for high-pressure tanks. It can withstand the corrosion of most acids and alkalis. At room temperature, calcium fluoride does not absorb moisture, but it reacts with air at high temperatures. If kept dry, the crystal can be used at a high temperature of 900 ° C. It should be noted that calcium fluoride should not be used together with ammonium salt solutions. Calcium fluoride is commonly used for infrared transmission cell windows.

Cadmium tellurideEasy to be corroded by oxidants and slightly soluble in acids. Cadmium telluride has lower hardness than zinc sulfide (ZnS) and zinc selenide (ZnSe). Cadmium telluride is suitable for infrared transmission windows and attenuated total reflection (ATR) elements.

Optical CVD growth of diamondsBetween 4000 and 400 cm^-1The infrared band has good transparency, with clear spectra ranging from 2600 to 1600 cm^-1The intrinsic molecular vibration characteristic peaks and fingerprint regions. This type of diamond is particularly suitable for high-pressure and highly corrosive working environments.

germaniumIt is a hard and brittle substance. Due to high reflection loss, its transmittance is relatively low. Germanium is soluble in hot sulfuric acid and aqua regia, but insoluble in water, making it a good attenuated total reflection (ATR) material.

KRS-5Often used for attenuated total reflection (ATR) crystals, it has a wide spectral range, high refractive index, and is not easily broken. It is slightly soluble in water, soluble in alkali, and insoluble in acid. This material should not be used with ammonium salt solution. KRS-5 is toxic, but it only poses a hazard when inhaling dust or when the wound comes into contact with dust. KRS-5 window material and ATR material.

Potassium bromideIt is a very useful material because it covers a wider spectral range than sodium chloride (NaCl). Potassium bromide has good thermal shock resistance and mechanical shock resistance. althoughKBrWindow panels are easy to polish, but they are more susceptible to moisture compared to NaCl. Potassium bromide is commonly used in the production of infrared transmission cell windows.

quartzMainly used in the ultraviolet visible (UV Vis) and near-infrared (NIR) spectral regions. Infrared grade quartz has high purity and extremely low hydroxyl (OH) content. Quartz is insoluble in water.

sapphireSingle crystal alumina is a very hard material with stable chemical properties and birefringence. Sapphire will be corroded by concentrated acid and alkali.

Optical grade silicon (Si)Similar to germanium (Ge), it has better resistance to mechanical and thermal shock. Silicon is soluble in a mixed solution of hydrofluoric acid and nitric acid. Silicon is used as an attenuated total reflection (ATR) material and an infrared transmission cell window material.

Zinc sulfideThere are mainly two types of materials. The first type is zinc sulfide (CVD ZnS) prepared by chemical vapor deposition method, which has a thickness of 1640 cm^-1There is a water absorption band at this location, which can interfere with attenuated total reflection (ATR) testing. The second type is dedicated to attenuated total reflection (ATR) and diffuse reflection crystals, as it does not have a water absorption band. Zinc sulfide is insoluble in water, but is susceptible to corrosion by strong oxidants. Zinc sulfide has good thermal shock resistance and mechanical shock resistance.