How to further process quartz stone raw materials into optical lenses?

Processing quartz stone raw materials into optical lenses requires many precise processes, involving material purification, precise processing and optical performance optimization. The following are the detailed processes and technical points:

1. Pretreatment of raw materials: purification and blank preparation

1. Quartz stone selection and crushing

Selection of raw materials: select high-purity quartz stone (SiO₂ content ≥99.95%, impurity FeO < 50 ppm, few defects such as bubbles and cracks), which is common in crystal Shi Ying or molten Shi Ying ore.

Coarse crushing and medium crushing: Quartz stone is crushed to 5-10mm particles by jaw crusher, and then ground to 1-2mm by cone crusher or ball mill to remove surface impurities.

2. Chemical purification (key step)

Acid leaching treatment: put Shi Ying particles into mixed acid solution (such as HF: HNO: HSO = 1: 2: 3), soak at 50-80℃ for 2-4 hours, dissolve metal oxide impurities (such as Fe³⁺ and Al), and wash with deionized water to neutrality after the reaction.

High-temperature melting (for fused Shi Ying lens): If a fused Shi Ying lens is prepared, the purified quartz sand needs to be melted at a high temperature of 1700-2000℃, the bubbles are removed by vacuum degassing, and then it is cast or drawn into a Shi Ying glass blank (density 2.2g/cm³, light transmittance ≥90%@200-2500nm).

3. Blank forming

Hot-pressing molding: quartz sand is mixed with a small amount of flux (such as Na₂CO₃) and hot-pressed into lens blank at 1100-1300℃ and 5-10MPa, and the dimensional accuracy is controlled within 0.5 mm..

Cold forming: For rock crystal Shi Ying, a diamond saw blade is used to cut it into a block blank, and then a CNC grinder is used to process it into a rough blank close to the lens shape, with a machining allowance of 0.5-1 mm..

Second, precision machining: from blank to optical surface

1. Rough grinding: contour forming

Equipment: Using high-speed rotating cast iron grinding disc (80-120# diamond abrasive), the blank is ground by numerical control grinder according to the radius of curvature (R value) of the lens to form a spherical or aspherical profile.

Parameter control: the rotating speed of the grinding disc is 1500-3000rpm, the feed rate is 0.01-0.05mm/min, and the cooling liquid (kerosene or special cutting fluid) is continuously washed to avoid Shi Ying overheating and cracking.

2. Fine grinding: surface refinement

Abrasive upgrading: replace it with 200-800# diamond micropowder, and cooperate with resin or ceramic grinding disc to remove rough grinding marks, so as to reduce the surface roughness Ra to 1-5 μ m.

Process optimization: planetary refiner is adopted to realize uniform grinding through multi-axis linkage, and the temperature is controlled at 20 5℃ to prevent thermal deformation.

3. Polishing: optical surface formation

Polishing solution and tools: cerium oxide (CeO₂) polishing powder (particle size 0.5-2μm) is mixed with water to form slurry, which is polished with asphalt or polyurethane polishing die at 800-1500rpm on a polishing machine.

Key parameters: polishing pressure 0.1-0.3MPa, slurry pH 8-10, polishing time adjusted according to lens size (it takes about 30-60 minutes for a lens with a diameter of 50mm), final surface roughness Ra < 0.1 μ m, and surface shape accuracy PV value (peak-valley difference) < λ/10 (λ = 633 nm).

4. Centering and edge processing

Centering edge grinding: grinding the lens edge by centering edge grinding machine, based on the optical surface, to ensure that the coaxiality between the optical axis and the geometric axis is less than ≤5arcmin and the edge roughness Ra is less than 10μ m..

Chamfering: Chamfering the lens edge with diamond grinding wheel (angle 45, width 0.1-0.3mm) to prevent edge collapse and stress concentration.

Third, post-processing: performance optimization and testing

1. Cleaning and drying

Ultrasonic cleaning: put the lens into acetone-alcohol mixed solution, ultrasonic cleaning for 10-15 minutes, remove the CeO₂ particles and oil stains left after polishing, and then rinse it with deionized water for 3 times.

Drying: drying in a vacuum drying oven (temperature 50-80℃, vacuum degree < ＜10Pa) for 2-4 hours to avoid the influence of water vapor residue on light transmittance.

2. Coating (according to optical requirements)

Anti-reflection film: in the ultraviolet band (such as 193nm), plating MgF₂ or Al₂O₃ multilayer film to make the reflectivity < 0.5%; In the visible light band (400-700nm), SiO₂-TiO₂ multilayer film is plated, and the light transmittance is increased to over 99%.

High reflection film: If it is used for reflector, it is plated with metal film (such as Al and Ag) or dielectric film (such as TiO₂/SiO₂ alternating layer), and the reflectivity is ≥99.5%.

Coating process: electron beam evaporation or magnetron sputtering technology is used to deposit the thin film in the vacuum < ＜10⁻³Pa environment, and the film thickness control accuracy is 1 nm.

3. Optical performance testing

Surface shape detection: use an interferometer (such as ZYGO GPI XP) to measure the surface shape error, and the PV value should be < λ/20 and the RMS value should be < λ/100.

Light transmittance test: use an ultraviolet-visible-near infrared spectrophotometer (such as PerkinElmer Lambda 1050) to test the light transmittance of all bands, and the light transmittance of Shi Ying lens should be ≥85% in the range of 220-2500nm.

Defect detection: observing the surface through a microscope with a magnification of 50-100 times, bubbles, scratches or impurity particles with a diameter of > 50μ m are not allowed.

Fourth, special technology: aspheric surface and special-shaped lens processing

1. Aspheric lens processing

CNC Single Point Diamond Turning (SPDT): For high-precision aspheric surfaces (such as paraboloids and ellipsoids), the surface with Ra < 20nm can be directly obtained by using a diamond cutter on an ultra-precision lathe with a cutting speed of 100-200m/min and a feed of 0.5-2μ m/rev..

Magnetorheological Polishing (MRF): Using the viscosity change of magnetorheological fluid in the magnetic field, the aspheric surface is definitely polished, and the removal accuracy reaches 0.1μm, which is suitable for correcting complex surface shape errors.

2. Machining special-shaped lens

Laser machining: for the special-shaped structures such as aperture and groove, femtosecond laser (wavelength 1030nm, pulse width < ＜500fs) is used for micro-machining, with heat affected zone < 1 μ m and edge roughness Ra < 1 μ m..

Chemical etching: use HF solution (concentration 5%-10%) to etch the surface of Shi Ying to form microstructure (such as microlens array), the etching rate is about 1-5μm/min, and the etching time and temperature (20-30℃) should be strictly controlled.

V. Key process difficulties and solutions

Solutions to difficult causes

Shi Ying has high brittle fracture hardness (Mohs hardness 7) and low tensile strength. Diamond micro-powder is used for low-speed grinding, and the cutting fluid is fully cooled to avoid stress concentration. After rough grinding, annealing treatment (500℃×2h) is carried out to eliminate internal stress.

Shi Ying has low polishing efficiency and high chemical stability, so it is difficult to chemically decompose to improve the concentration of CeO₂ polishing powder (5%-10%), optimize the polishing mold material (such as porous polyurethane), and adopt high-pressure polishing (pressure ≥0.3MPa).

The coating adhesion is poor, the Shi Ying surface energy is low, and the film is easy to fall off. Before coating, plasma cleaning (Ar plasma, power 100-200W, treatment time 5-10min) is carried out to increase the surface roughness to Ra 0.2-0.5 μ m..

The precision of aspheric surface is insufficient, and it is difficult to control the complex curvature in traditional machining. Computer controlled optical surface forming (CCOS) technology is adopted, and the high-precision surface with PV value < λ/50 is achieved through discrete point grinding and MRF polishing.

VI. Application Scenarios and Material Selection

Ultraviolet lens: Fused Shi Ying (high ultraviolet transmittance, transmittance ≥75% at 193nm) is used for mask aligner and ultraviolet laser.

High temperature resistant lens: crystal Shi Ying (melting point 1713℃) is used for high temperature spectrometer and infrared thermometer.

Precision instrument lens: ultra-high purity Shi Ying with impurity less than ＜10ppm is used in astronomical telescopes and laser interferometers.

Through the above process, quartz stone is transformed from raw material into high-precision optical lens, and the key is to control the impurity content, surface accuracy and optical performance in the whole process. Each step needs to combine the material characteristics and precision processing technology to ensure that the lens meets the optical application requirements of ultraviolet, visible or infrared bands.