Novel photovoltaic glass formula and production process thereof

Abstract

The invention discloses a novel photovoltaic glass formula and a production process thereof, and belongs to the technical field of photovoltaic glass, the formula innovatively introduces spodumene tailings as a part of raw materials, and the spodumene tailings comprise quartz sand, soda ash, aluminum hydroxide, limestone, dolomite, mirabilite, sodium pyroantimonate and a specific proportion of spodumene tailings. The core of the production process is that an electric field assisted smelting technology is adopted, and a direct-current electric field is applied in the melting process, so that the melting, clarifying and homogenizing processes of the glass are completed at a relatively low temperature. According to the technical scheme, the dependence on high-purity quartz sand is effectively reduced, the raw material cost is reduced by utilizing industrial solid waste, and the production energy consumption is reduced by optimizing the melting process. The prepared photovoltaic glass has excellent performances of low density, high transmittance, good thermal stability and the like.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic glass technology, and in particular to a novel photovoltaic glass formulation and its manufacturing process. Background Technology

[0002] As a key encapsulation material for photovoltaic modules, the market demand for photovoltaic glass is closely related to the global photovoltaic installed capacity, and has been growing rapidly in recent years. However, the industry is facing severe challenges. On the one hand, the surge in demand for core raw materials such as high-purity quartz sand and soda ash has led to poor supply chain stability and volatile prices, significantly increasing production costs. On the other hand, existing mainstream photovoltaic glass formulations are highly dependent on these traditional raw materials, lacking innovation in material systems and making it difficult to address supply chain risks at the source.

[0003] Furthermore, in terms of production processes, traditional photovoltaic glass manufacturing generally employs high-temperature melting technology (typically above 1550℃), which suffers from high energy consumption, long melting cycles, and significant furnace wear. During the melting process, the clarification and homogenization efficiency of the molten glass is limited, easily leading to microbubbles or uneven composition in the product, affecting the glass's transmittance and mechanical properties. Although industry standards have clear requirements for the performance of photovoltaic glass, existing technological approaches have encountered bottlenecks in further reducing costs and improving the overall performance of the product.

[0004] Therefore, there is an urgent need in this field for a new technical solution that can effectively reduce dependence on scarce raw materials such as high-purity quartz sand and achieve energy saving and consumption reduction in the production process while ensuring or even improving the optical and mechanical properties of glass. Summary of the Invention

[0005] The main objective of this invention is to provide a novel photovoltaic glass formulation and its manufacturing process, which can effectively solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A novel photovoltaic glass formulation and its manufacturing process are disclosed, comprising a raw material formulation melted according to a mass percentage, wherein the raw material formulation includes: Quartz sand, 55.56%~58.38% Soda ash with a purity of 18.63% to 18.92% 0.50%~0.97% aluminum hydroxide, 4.33%~4.34% limestone, Dolomite content: 15.62%~15.67% 0.64% Glauber's salt, 0.26% sodium pyroantimonate, And 0.88% to 4.41% spodumene tailings.

[0007] The core innovation lies in the precise introduction of a specific proportion of spodumene tailings as a functional alternative raw material. The proportions of each component are set based on a synergistic window determined through extensive experimentation, aiming to maximize the utilization of beneficial components in the tailings (such as K2O and Al2O3) while ensuring the stability of the final glass oxide composition. The amount of spodumene tailings added is strictly limited to 0.88%~4.41% to ensure that while effectively reducing raw material costs and dependence on high-purity quartz sand, it does not introduce excessive impurities or negatively impact the glass melting process.

[0008] Preferably, the content of ferric oxide (Fe2O3) in the spodumene tailings is no more than 150 ppm. Fe2O3 is a major coloring impurity affecting the transmittance of photovoltaic glass, and strictly controlling its content to ≤150 ppm is a prerequisite for ensuring that the final product meets the high transmittance requirements of the "Glass for Solar Cells" standard. This limit is derived from the screening and testing of various tailings samples, ensuring that the resource utilization of waste does not come at the expense of core optical performance.

[0009] A novel photovoltaic glass manufacturing process includes the following steps performed sequentially: S1. Ingredients and mixing: Weigh each raw material according to the stated mass percentage, place them in a mixer and mix for 20-40 minutes to obtain a uniform compound. S2. Charging and melting: The batch material is charged into a corundum crucible and placed in a high-temperature lifting furnace. The furnace temperature is raised to 1500℃±10℃ at a heating rate of 5~10℃ / min, and held at this temperature for 2.5~3.5 hours to fully melt the raw material into molten glass. S3. Clarification and Homogenization: Reduce the furnace temperature to 1450℃±10℃ and continue to hold at this temperature for 0.5~1.5 hours to allow the bubbles in the glass melt to escape fully and achieve chemical homogenization. S4. Forming and Annealing: The clarified and homogenized molten glass is quickly poured into a metal mold that has been preheated at 500℃±20℃ for no less than 30 minutes, and a glass sheet sample is made by pressing. Then the mold with the glass sheet is immediately transferred into an annealing furnace set at 560℃ and slowly cooled to room temperature in the furnace to eliminate the internal stress of the glass.

[0010] The above steps define a reproducible, standardized laboratory preparation process that matches the new formulation. The specific heating rate, holding temperature, and time (e.g., 1500℃ for 3 hours, 1450℃ for 1 hour) are the result of extensive experimental optimization, aiming to ensure that the spodumene tailings and other raw materials react fully, melt uniformly, and effectively degas. The specific annealing regime (starting at 560℃, slow cooling ≤2℃ / min) is designed to effectively eliminate internal stress in the glass samples, ensuring the accuracy of their physicochemical property tests and providing a reliable basis for formulation validation.

[0011] Preferably, during the melting stage described in S2 and the clarification and homogenization stage described in S3, electrodes are inserted into the glass melt within the corundum crucible, and a DC electric field with a strength of 50~200V / cm is applied. This introduces the innovative process of electric field-assisted melting (EAM). After applying the DC electric field, the electric field drives alkali metal ions (Na+) in the glass melt. + ,K + Directional migration of ions, specifically the "electro-induced migration" effect, significantly accelerates mass transfer processes in the glass-forming network, thereby reducing the apparent viscosity of the melt, promoting homogenization, and facilitating bubble removal. An electric field range of 50-200 V / cm is an effective window verified through simulation and experiments; too low a voltage has little effect, while too high a voltage may lead to localized overheating or electrode corrosion.

[0012] Preferably, after applying the DC electric field, the melting temperature of S2 can be reduced to 1450℃~1480℃, and the holding time at this temperature can be shortened to 2.0~2.5 hours. Through electric field activation, the melting process that originally required a high temperature of 1500℃ can be achieved at a lower temperature of 1450℃~1480℃, while the holding time can be shortened by about 17%-33%. This significantly reduces energy consumption and mitigates the erosion of the refractory material of the melting furnace, demonstrating the outstanding advantages of this invention in terms of environmental protection and economic benefits.

[0013] Preferably, the pressing pressure in step S4 is 5-15 MPa, and the holding time is 10-30 seconds. A pressure of 5-15 MPa is sufficient to ensure the molten glass is densely formed in the preheated mold, preventing internal defects and preventing excessive pressure from causing the glass to be too thin or sticking to the mold. The holding time of 10-30 seconds ensures that the glass sheet shape is stable before demolding. These specific parameters provide crucial guarantees for the feasibility and reproducibility of this process.

[0014] Preferably, the annealing process described in S4 employs programmed temperature control, specifically: starting from 560°C, cooling to 300°C at a rate not exceeding 2°C / min, and then allowing the furnace to cool naturally to room temperature. This annealing is specifically designed for this system of glasses containing spodumene tailings. The selection of 560°C as the annealing starting temperature is based on its measured glass transition temperature (Tg) range. Strictly controlling the cooling rate to no more than 2°C / min ensures that the internal stress of the glass can be fully relaxed and released, preventing the formation of permanent stress due to excessively rapid cooling, and ensuring that the prepared glass sample possesses excellent mechanical strength and thermal stability.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention creatively introduces a specific proportion of spodumene tailings to replace some traditional raw materials such as quartz sand and soda ash, and adopts an electric field-assisted low-temperature melting and precise time-controlled heat preservation process. While successfully disposing of industrial solid waste, significantly reducing raw material costs and dependence on high-purity quartz sand, it utilizes the "mixed alkali effect" generated by the potassium and sodium components in the tailings to improve the chemical stability and transmittance of the glass (≥91.6%). Its optimized melting process effectively reduces melting temperature and energy consumption under the action of the electric field, and promotes the clarification and homogenization of the glass melt, thereby producing photovoltaic glass products with low density, high transmittance, excellent thermal stability and mechanical strength. Overall, it achieves multiple goals of cost reduction, efficiency improvement and environmental protection. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the production process of the present invention. Detailed Implementation

[0017] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0018] like Figure 1 The diagram shows the photovoltaic glass manufacturing process, which will be explained in detail below with reference to a specific embodiment.

[0019] I. Determination of Raw Material Specifications and Formula (I) Raw material selection and quality requirements All raw materials are selected from qualified industrial-grade products, and key indicators must meet the following requirements to ensure stable glass composition and performance standards: Quartz sand (silica sand): SiO2 purity ≥ 99.8%, particle size passing through a 100-mesh sieve (sieve residue ≤ 0.5%), Fe2O3 content ≤ 50ppm, to avoid introducing additional coloring impurities; Soda ash (sodium carbonate): Light soda ash, Na2CO3 purity ≥99.5%, moisture content ≤0.5%, to prevent bubbles from forming due to moisture evaporation during melting; Aluminum hydroxide: Al(OH)3 purity ≥98%, particle size ≤50μm, ensuring rapid decomposition into Al2O3 during the melting stage, participating in the formation of the glass network structure; Limestone: CaCO3 purity ≥ 95%, MgO content ≤ 1.0%, to avoid excessive MgO causing glass crystallization; Dolomite: CaCO3 content ≥50%, MgCO3 content ≥45%, with a stable ratio of the two (CaCO3:MgCO3≈1.1:1), ensuring that the CaO and MgO content in the glass is controllable; Sodium sulfate (Na₂SO₄): Na₂SO₄ purity ≥ 98%. As a clarifying agent, the moisture content must be controlled to ≤ 0.3%. Sodium pyroantimonate: NaSbO3 purity ≥99%, Fe2O3 content ≤30ppm, used as a decolorizing agent to inhibit Fe³+ staining; Spodumene tailings: taken from the waste slag after lithium extraction from lithium mines. The composition analysis shows that SiO2 is about 81.0%, Al2O3 is about 11.4%, Na2O+K2O is about 7.1% (of which K2O is about 3.2%), and Fe2O3 content is 125ppm (meeting the limit requirement of ≤150ppm). It needs to be dried in an oven at 105℃ for 2 hours (moisture content ≤0.2%) and passed through an 80-mesh sieve to remove large impurities.

[0020] (II) Specific formula ratio (taking 400g / auxiliary material as an example) Calculate the amount of each raw material used according to its weight percentage to ensure accurate control of the total weight (error ≤ ±0.1g), as follows: Quartz sand: 400g × 56.97% = 227.88g; Soda ash: 400g × 18.77% = 75.08g; Aluminum hydroxide: 400g × 0.74% = 2.96g; Limestone: 400g × 4.33% = 17.32g; Dolomite: 400g × 15.64% = 62.56g; Glauber's salt: 400g × 0.64% = 2.56g; Sodium pyroantimonate: 400g × 0.26% = 1.04g; Lithium tailings: 400g × 2.64% = 10.56g.

[0021] II. Detailed Production Process Steps (I) S1: Ingredients and Mixing (Core Objective: To achieve uniform composition of the batching materials) Raw material pretreatment: Pour the weighed quartz sand and spodumene tailings into 100-mesh and 80-mesh standard sieves respectively, and screen them for 2 minutes using a vibrating screener to remove impurities that exceed the particle size limit; Aluminum hydroxide and sodium pyroantimonate are easily hygroscopic due to their fine particle size, so they need to be re-dried in a 105℃ forced-air drying oven for 1 hour, cooled to room temperature and then weighed to avoid weighing deviations due to moisture.

[0022] Step-by-step mixing: Using a planetary mixer, first add quartz sand, limestone, and dolomite (inorganic mineral raw materials), and stir at low speed (100 r / min) for 5 minutes to initially disperse large particles of raw materials; then add soda ash and aluminum hydroxide (hygroscopic raw materials), and stir at medium speed (200 r / min) for 10 minutes; finally add mirabilite, sodium pyroantimonate, and spodumene tailings (functional raw materials), and stir at high speed (300 r / min) for 15 minutes, for a total mixing time of 30 minutes.

[0023] Uniformity verification: After mixing, a 5g sample was taken from each of the three positions (top, middle, and bottom) of the mixer using the "three-point sampling method". The SiO2 content was detected by X-ray fluorescence spectrometry. The uniformity of the batch material met the standard after three tests, and the process can proceed to the next step.

[0024] (ii) S2: Charging and Melting (Core objective: to achieve complete melting of raw materials and reduce energy consumption) Crucible pretreatment: Select a 99% high-purity corundum crucible, first place it in a box-type resistance furnace, heat it to 800℃ at 5℃ / min, hold it for 1 hour to remove the moisture and residual impurities adsorbed on the inner wall of the crucible (to avoid the formation of stones by reacting with the molten glass during melting), after cooling to room temperature, wipe the inner wall with a lint-free cloth dipped in alcohol to ensure that there is no oil stains.

[0025] Loading and Furnace Insertion: Slowly pour the mixed batch material into the corundum crucible, with a loading volume of about 80 mL (occupying 80% of the crucible volume, leaving space for melting expansion). Gently tap the crucible wall 3 times to compact the batch material and avoid internal voids. Place the crucible in the center of the high-temperature lifting furnace, ensuring that the distance between the crucible and the furnace wall is ≥50 mm for uniform heating.

[0026] Electric field assisted melting: Temperature control: The temperature rise program is set to "rise from room temperature (25℃) to 1460℃ at 8℃ / minute". During the temperature rise process, the furnace door observation window is used for real-time monitoring. The batch material gradually dehydrates at 200~500℃, and the soda ash begins to decompose (releasing CO2). At 800~1200℃, limestone and dolomite decompose (releasing CO2). At 1400℃, it completely becomes a viscous melt. At 1460℃, the melt is transparent.

[0027] Electric field application: When the furnace temperature rises to 1200℃ (the initial stage of melt formation), insert two molybdenum electrodes (5mm in diameter and 200mm in length, made of molybdenum material that is resistant to high temperature and has good conductivity) from the top of the furnace. Insert the lower end of the electrodes into the melt to a depth of 15mm and space them 20mm apart. Apply a DC electric field of 120V / cm through a DC regulated power supply.

[0028] Thermal insulation optimization: The temperature was maintained at 1460℃ for 2.2 hours. The principle of the electric field effect is "electro-induced migration effect"—the electric field drives the Na in the melt. + K + Directional migration (from soda ash and spodumene tailings) accelerates mass transfer in the glass network structure, reduces the apparent viscosity of the melt, promotes the rapid dissolution of unmelted small particles, and achieves complete melting.

[0029] (III) S3: Clarification and Homogenization (Core objective: to eliminate bubbles and achieve uniform composition) Cooling and Clarification: After the melting and holding period, the DC power supply is turned off, the molybdenum electrode is removed, and the high-temperature furnace is set to cool down to 1450℃ at a rate of 5℃ / minute, and held for 1 hour. During this stage, the temperature decrease causes a slight increase in the viscosity of the melt, but it still maintains good fluidity. Tiny bubbles (≤0.1mm in diameter) in the melt slowly rise to the surface and rupture under the action of buoyancy. At the same time, Al2O3 in the spodumene tailings and SiO2 in the quartz sand further react to form a stable glass network, avoiding component segregation.

[0030] Homogenization verification: 2 mL of molten glass was drawn from the upper part and the lower part of the melt using a quartz glass tube (3 mm in diameter). After cooling, thin sections were prepared and observed under a metallographic microscope. No visible bubbles were observed in either sample (bubble density ≤ 0.5 bubbles / cm²). The Al2O3 content was then detected by XRF. The upper part was 0.94% and the lower part was 0.95%, with a deviation of ≤ 0.01%, indicating that the clarification and homogenization met the standards.

[0031] (iv) S4: Shaping and Annealing (Core objective: to obtain a stable shape and eliminate internal stress) Mold pretreatment: Select a 45# steel metal mold, place it in another box-type resistance furnace, set the temperature to 500℃±10℃, and keep it warm for 40 minutes.

[0032] Compression Molding: Wearing high-temperature gloves, remove the corundum crucible from the high-temperature lifting furnace and quickly pour the molten glass into the preheated metal mold (approximately 150g, ensuring the cavity is completely filled). Immediately place the mold under the pressure head of the hydraulic press, set the pressure to 10MPa, increase the pressure to 10MPa at a rate of 2MPa / second, hold the pressure for 20 seconds, and then release the pressure at a rate of 1MPa / second. The core function of holding the pressure is to ensure that the molten glass fully conforms to the mold cavity, avoiding shape defects caused by decreased fluidity, resulting in a uniform glass sheet thickness (3mm ± 0.1mm) with no burrs on the edges after molding.

[0033] Procedural annealing: Annealing curve setting: Immediately transfer the mold with the glass sheet into the annealing furnace. The preset temperature of the annealing furnace is 560℃. Set the program temperature control: reduce the temperature from 560℃ to 300℃ at a rate of 1.5℃ / minute, which takes approximately (560-300) / 1.5 ≈ 173 minutes. Turn off the heating when the temperature drops below 300℃ and allow the furnace to cool naturally to room temperature (approximately 25℃). The total annealing time is approximately 6.5 hours.

[0034] Annealing principle: At 560℃, glass is in a "viscoelastic state". Slow cooling can gradually stabilize the atomic arrangement inside the glass and slowly relax the stress. If the cooling rate is too fast, the surface of the glass cools and shrinks faster than the inside, which can easily generate permanent internal stress, causing the glass to crack during subsequent cutting or use.

[0035] Demolding and trimming: After the mold has cooled to room temperature, gently tap the edge of the mold with a plastic hammer. The glass sheet will fall off naturally. Use a diamond glass cutter to cut off the excess part of the glass sheet at the edge to obtain a standard sample of 100mm×100mm×3mm.

[0036] III. Finished Product Performance Testing and Result Analysis (I) Testing Items and Methods In accordance with photovoltaic glass industry standards, six core performance tests were conducted on the finished glass sheets. The specific methods are as follows: Visual inspection: Under natural light (illuminance ≥ 500 lux), visually inspect the glass slide from a distance of 50 cm to check for defects such as bubbles, stones, and scratches; Density test: The Archimedes method was used. The mass of the glass slide in air (m1) and the mass in water (m2) were weighed using an electronic balance (accuracy 0.0001g). The density was calculated using the formula ρ=m1 / (m1-m2). Linear expansion coefficient test: Use a thermal expansion meter, sample size 50mm×5mm×3mm, heating rate 5℃ / min, test range 0~300℃, record the length change, and calculate the average linear expansion coefficient; Softening point test: The temperature at which the viscosity of a glass melt is measured using a rotational viscometer (definition of softening point). Transmittance test: Using a UV-Vis spectrophotometer, the glass slide is polished on both sides, and the wavelength is tested from 380 to 1100 nm (the main range of sunlight), which is converted into the transmittance of a standard thickness of 3 mm. Chemical stability test: According to GB / T6582-2021, the glass slide was immersed in distilled water at 98℃ for 24 hours, dried, and the mass loss was measured and the mass loss rate was calculated.

[0037] (II) Test Results and Compliance Analysis

[0038] (III) Analysis of Key Advantages Energy saving effect: Electric field-assisted melting reduces the temperature from 1500℃ to 1460℃ (a reduction of 40℃), and shortens the holding time from 3 hours to 2.2 hours. Based on the energy consumption of 500kWh / ton in the high-temperature furnace, the energy consumption per ton of glass is reduced by (40℃ / 1500℃ + 0.8 hours / 3 hours) × 500≈75kWh, with an energy saving rate of 15%.

[0039] Performance optimization: K₂O in spodumene tailings and Na₂O in soda ash form a "mixed alkali effect"—the two alkali metal ions have different ionic radii (K₂O and Na₂O) due to their different ionic radii (K₂O and Na₂O). + Radius 138 pm, Na + The mutual constraints between the 95pm radius and the glass network inhibit the relaxation of the glass network, thereby improving chemical stability and slightly increasing transmittance to meet the high light transmittance requirements of photovoltaic cells.

[0040] V. Process Precautions and Troubleshooting Abnormalities in the mixing stage: If the uniformity deviation of the sampled batch is >0.1%, it is necessary to check whether the mixing impeller of the mixer is worn (if the impeller gap is >5mm, it needs to be replaced), or whether there is agglomeration of the raw materials (the agglomerated raw materials need to be broken up and remixed).

[0041] Abnormalities during the melting stage: If black impurities are observed in the melt, it may be due to insufficient preheating of the corundum crucible. Heating should be stopped, the crucible replaced after cooling, and the melt remelted. If the current suddenly increases when the electric field is applied, it may be due to poor electrode contact. The electrode insertion depth should be adjusted to ensure complete immersion in the melt.

[0042] Abnormalities during annealing: If cracks appear in the glass sheet after annealing, the cooling rate should be reduced (e.g., from 1.5℃ / min to 1.0℃ / min), or the holding time at 560℃ should be extended (e.g., from 1 hour to 1.5 hours) to ensure that stress is fully released.

[0043] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A novel photovoltaic glass, characterized in that: It is produced by melting raw materials according to a mass percentage formula, wherein the raw material formula includes: Quartz sand, 55.56%~58.38% Soda ash with a purity of 18.63% to 18.92% 0.50%~0.97% aluminum hydroxide, 4.33%~4.34% limestone, Dolomite content: 15.62%~15.67% 0.64% Glauber's salt, 0.26% sodium pyroantimonate, And 0.88% to 4.41% spodumene tailings.

2. The novel photovoltaic glass according to claim 1, characterized in that: The content of ferric oxide (Fe2O3) in the spodumene tailings is no more than 150 ppm.

3. A process for preparing the novel photovoltaic glass as described in claim 1 or 2, characterized in that: The following steps are performed sequentially: S1. Ingredients and mixing: Weigh each raw material according to the stated mass percentage, place them in a mixer and mix for 20-40 minutes to obtain a uniform compound. S2. Charging and melting: The batch material is charged into a corundum crucible and placed in a high-temperature lifting furnace. The furnace temperature is raised to 1500℃±10℃ at a heating rate of 5~10℃ / min, and held at this temperature for 2.5~3.5 hours to fully melt the raw material into molten glass. S3. Clarification and Homogenization: Reduce the furnace temperature to 1450℃±10℃ and continue to hold at this temperature for 0.5~1.5 hours to allow the bubbles in the glass melt to escape fully and achieve chemical homogenization. S4. Forming and Annealing: The clarified and homogenized molten glass is quickly poured into a metal mold that has been preheated at 500℃±20℃ for no less than 30 minutes, and a glass sample is made by pressing. The mold with the glass sample is then immediately transferred into an annealing furnace set at 560℃ and slowly cooled to room temperature in the furnace.

4. The novel photovoltaic glass formulation and its manufacturing process according to claim 3, characterized in that: In the melting stage described in S2 and the clarification and homogenization stage described in S3, electrodes are inserted into the glass melt in the corundum crucible, and a DC electric field with an intensity of 50~200V / cm is applied.

5. The novel photovoltaic glass formulation and its manufacturing process according to claim 4, characterized in that: After applying the DC electric field, the melting temperature of S2 can be reduced to 1450℃~1480℃, and the holding time at this temperature can be shortened to 2.0~2.5 hours.

6. The novel photovoltaic glass formulation and its manufacturing process according to claim 3, characterized in that: The compression molding pressure described in S4 is 5~15MPa, and the holding time is 10~30 seconds.

7. The novel photovoltaic glass formulation and its manufacturing process according to claim 3, characterized in that: The annealing process described in S4 uses programmed temperature control, specifically: starting from 560°C, it is cooled to 300°C at a cooling rate of no more than 2°C / minute, and then it can be naturally cooled to room temperature with the furnace.

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