Preparation method of fiber sample for terahertz time-domain spectroscopy test

Abstract

The invention relates to the technical field of pretreatment of terahertz time-domain spectrum analysis testing, in particular to a preparation method of a fiber sample for terahertz time-domain spectrum testing, a cake-shaped sample is prepared by using a sleeve and a sleeve roller and then is pressed and tabletted, and the obtained sample has relatively flat and uniform texture and is not easy to deform. The method can be applied to the fields of natural fiber conformation research, chemical fiber crystallization observation and the like.

Description

Technical Field

[0001] The invention relates to the technical field of pre-processing of terahertz time-domain spectroscopy analysis and testing, and in particular to a method for preparing a fiber sample for terahertz time-domain spectroscopy testing. Background Art

[0002] Terahertz time-domain spectroscopy is one of the most widely used analytical and testing methods. By analyzing the rich skeleton motion information of the sample in the terahertz spectrum, not only can the microscopic molecular structure and intermolecular interaction of the sample be determined or inferred, but also a specific substance can be quantitatively detected. Therefore, this technology is of great significance for basic research in the fields of chemistry, materials, environmental protection, biology, etc. When conducting terahertz time-domain spectroscopy testing of samples, it is necessary to control the optical time delay, then sample the equivalent time of the terahertz pulse signal, and then extract the terahertz electric field time domain signal of the sample. Finally, by performing fast Fourier transform (FFT) on the obtained time domain signal, the terahertz frequency domain information of the sample is obtained. Since the terahertz wave involves reflection and transmission in air-medium-air during the test process, samples of different thicknesses absorb the terahertz spectral signal differently. Therefore, when conducting terahertz time-domain spectroscopy, high requirements are placed on the flatness and thickness uniformity of the fiber sample.

[0003] In the previous spectrum test of fiber or other samples with good ductility, most of them adopt the method of tableting in which the sample is chopped and mixed with a diluent. In order to perform infrared spectrum test on polymer samples with good ductility in Chinese patent (CN 106814044 A), the sample is cut into pieces during sample preparation and then mixed with potassium bromide for tableting. In order to make bamboo and hemp fiber samples evenly dispersed in the tablet, Chinese patent (CN103308473 A) uses a Hastelloy slicer to cut the fiber sample into powder during sample preparation, and then mixes the fiber with high-density polyethylene, grinds and presses to form a thin sheet sample for terahertz time-domain spectrum detection. However, due to the characteristics of the fiber sample, it is difficult to use a Hastelloy slicer to cut it into a uniform granular sample. At the same time, for some small or expensive fiber samples, slicing will cause irreversible damage to them, and the fiber in the prepared sample cannot be recovered. Therefore, the spectrum sample preparation methods of the above-mentioned fiber samples all have certain limitations, and it is necessary to find a fiber sample testing method that is safer, non-destructive and suitable for terahertz spectrum testing operations. Summary of the invention

[0004] 1. Technical issues

[0005] The present invention aims to at least solve the problems existing in the prior art.

[0006] (II) Technical content

[0007] This solution provides a mold for fabricating fiber samples for terahertz time-domain spectroscopy testing, which is achieved by the following specific technical means, including a sleeve and a sleeve rod;

[0008] The sleeve is a cylindrical structure with an open top, and a number of groups of needle holes are distributed in a matrix on the bottom wall of the sleeve;

[0009] The outer diameter of the sleeve rod is consistent with the inner diameter of the sleeve, and needles corresponding in number and position one by one to the needle holes are also distributed in a matrix on the bottom surface of the sleeve rod;

[0010] When the sleeve rod is inserted into the sleeve, each group of needles passes through the corresponding group of needle holes.

[0011] This solution also provides a method for preparing fiber samples for terahertz time-domain spectroscopy testing, including the following steps:

[0012] (1) Pretreatment of fibrous samples

[0013] Sort and clean the fiber samples to be tested, carefully remove any possible doped foreign fibers or other impurities, place the processed samples in an oven for low-temperature drying, and thoroughly dry them to obtain non-oriented pure fiber samples that can be used for terahertz time-domain spectroscopy detection. Put them into a sealed bag for later use;

[0014] (2) Preparation for sample pressing

[0015] Take the processed fiber samples and gather them into small groups, then stuff them into the sleeve;

[0016] Place the sleeve with the fiber samples on a polyurethane sponge sheet, use the sleeve rod with needles to press and pierce the fiber samples in the sleeve and repeat until the fiber samples form a relatively uniform cake shape;

[0017] (3) Preparation of sample pressings

[0018] Gently take out the fiber samples pressed into a cake shape from the sleeve, then place them in a pressing mold, apply pressure, hold it, and take it out after releasing the pressure; The fiber sample thin slices with good properties should have a flat surface, uniform thickness and no holes; If they do not meet the requirements, re-disperse the pressings, and then repeat the operations in steps (1) to (3) until thin slices that meet the requirements are obtained.

[0019] In the present invention, in the step (1), the process of removing impurities includes: removing other possible doped fibers by manual picking, then ultrasonic cleaning in a neutral cleaning solution for 15 minutes, and then cleaning with flowing deionized water for 5 minutes.

[0020] In the present invention, in the step (1), the low-temperature drying parameters of the oven are: the temperature range is 40°C to 80°C, and the time is 48h to 72h.

[0021] In the present invention, in the step (2), the thickness of the polyurethane sponge should be 2-3 times longer than that of the lancet, and the size should be larger than the cross-section of the sleeve.

[0022] In the present invention, in the step (2), the weight of the taken-out fiber is 20±0.5 mg.

[0023] In the present invention, in the step (2), the number of repetitions of the fiber sample in the sleeve rod pressing and stabbing sleeve is 10-15 times.

[0024] In the present invention, in the step (3), the parameter conditions of the tablet pressing die are: the pressure is 15 MPa, and the pressing time is 5 min.

[0025] In the present invention, in the step (3), the prerequisite for re-pressing the sample by repeating the operations in the steps (1)-(3) is that the weight of the fiber sample ≥19.5 mg.

[0026] In the present invention, in the steps (2) and (3), the environmental humidity is less than 30%, and the temperature is not higher than 25°C.

[0027] Description of the invention principle:

[0028] The present invention utilizes the characteristics that the fiber itself has a certain length, strength, deformation ability, elasticity, flexibility and mutual entanglement and interlocking, and combines the sleeve and the sleeve rod to optimize and adjust the size and uniformity of the fiber mass to be measured. Among them, the limiting module in the sleeve restricts the penetration depth of the lancets on the sleeve rod into the fiber sample; the stabbing grooves on the sleeve uniformly reinforce the fiber mass from point to surface; the lancet replacement module on the sleeve rod can adjust the lancet model according to different fineness fiber samples. In the process of processing the fiber sample into a relatively uniform initial standard sample, when multiple lancets on the sleeve rod penetrate the fiber mass, the barbs on the lancets will drive the surface and local inner layer fibers to move along the penetration direction, so that some fibers are forced to penetrate into the fiber mass. When the needle penetration reaches a certain depth, the lancet begins to rise. Due to the forward direction of the barbs, the displaced fibers break away from the barbs and remain in the fiber mass in an almost vertical state. As the number of needle penetrations increases, the fiber mass compresses, and the fibers are intertwined with each other through the friction and mutual entanglement and interlocking characteristics, thus having a certain strength and forming a flaky sample.

[0029] The present invention has the characteristics of not damaging the fibrous sample, not changing its shape (non-destructive), and having low requirements for the amount of the fibrous sample.

[0030] (III) Technical effects

[0031] Adopting the above structure enables the present solution to have the following beneficial effects:

[0032] 1. The samples prepared by this solution can be recycled after terahertz time-domain spectroscopy detection. The fibrous sample tablets directly prepared by the sleeve mold are not sliced and not contaminated by diluents, and their surface structures can still remain intact after detection, which can improve the repeatability of precious samples;

[0033] 2. The samples prepared by this solution require less amount. During the terahertz time-domain spectroscopy test, the terahertz signal will refract and reflect multiple times in the sample to be measured, and the sample thickness has a great influence on the test results. Therefore, it is necessary to control the sample amount and tablet thickness to ensure the accuracy of the results;

[0034] 3. The samples prepared by this solution have good uniformity. An adjustable limiting device is designed in the sample sleeve mold for different types of fibrous samples, which can adjust the depth of the needle piercing according to the fiber type to ensure the uniformity of the tablet samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation to the present invention. In the drawings:

[0036] Figure 1 is a schematic structural diagram of the sleeve of this solution;

[0037] Figure 2 is a schematic structural diagram of the sleeve rod of this solution;

[0038] Figure 3 is a comparative diagram of the terahertz absorption spectra of the wool fiber samples prepared in Example 1 and Comparative Example 1;

[0039] Figure 4 is a physical comparison diagram of the wool and cashmere fiber samples prepared in Example 1, 2 and Comparative Example 1, 2.

[0040] Among them, 1. Sleeve, 11. Needle hole, 12. Limiting groove, 2. Sleeve rod, 21. Limiting rib, 22. Needle, 3. Lug. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the scope of protection of the present invention.

[0042] Please refer to Figure 1 - Figure 2, A mold for fabricating fiber samples for terahertz time-domain spectroscopy testing is achieved by the following specific technical means, including a sleeve 1 and a sleeve rod 2;

[0043] The sleeve 1 is a cylindrical structure with an open top, and a number of groups of needle holes 11 are distributed in a matrix on the bottom wall of the sleeve 1. A limiting groove 12 is opened along the length direction on the inner wall of the sleeve 1, and the top of the limiting groove 12 extends to the top opening of the sleeve 1. At least one group of limiting grooves 12 is distributed on the circumferential side of the sleeve 1;

[0044] The outer diameter of the sleeve rod 2 is the same as the inner diameter of the sleeve 1. On the outer wall of the sleeve rod 2, there are limiting ridges 21 along the length direction. A group of limiting ridges 21 slides into a corresponding group of limiting grooves 12 to achieve the vertical sliding of the sleeve rod 2 in the sleeve 1, avoiding relative rotation between the two. Also, on the bottom surface of the sleeve rod 2, there are needle points 22 distributed in a matrix and corresponding one-to-one in position with the needle holes 11 in terms of quantity;

[0045] When inserting the sleeve rod 2 into the sleeve 1, ensure that each group of needle points 22 passes through the corresponding group of needle holes 11.

[0046] Please refer to Figure 1 - Figure 2 , A mold for fabricating fiber samples for terahertz time-domain spectroscopy testing. There are lugs 3 on both sides of the top opening of the sleeve 1 for convenient gripping; at the same time, there are also lugs 3 on both sides of the top of the sleeve rod 2 for convenient gripping and applying force.

[0047] Please refer to Figure 1 - Figure 2 , A mold for fabricating fiber samples for terahertz time-domain spectroscopy testing. Both the sleeve 1 and the sleeve rod 2 are made of transparent materials to facilitate observing the internal situation.

[0048] This solution also provides a method for preparing fiber samples for terahertz time-domain spectroscopy testing, including the following steps:

[0049] (1) Pretreatment of fibrous samples

[0050] Sort and clean the fiber samples to be tested, carefully remove any possible doped foreign fibers or other impurities. After cleaning, place the samples in an oven and dry them at a low temperature to obtain an undirected pure fiber sample that can be used for terahertz time-domain spectroscopy detection, and store it in a sealed bag for later use;

[0051] (2) Preparation for sample pressing

[0052] Take the treated fiber samples (the sampling amount of the test fiber samples is 15 - 30 mg) and gather them into small groups, and stuff them into Figure 2 the sleeve 1 shown;

[0053] Place the sleeve 1 with the fiber samples on a polyurethane sponge sheet and use Figure 1The thimble 2 with the lancet 22 presses the fiber sample in the pressing thimble 1 and repeats until the fiber sample forms a relatively uniform cake;

[0054] (3) Prepare the sample tablet

[0055] Gently take out the fiber sample pressed into a cake from the thimble 1, then place it in a tablet mold, apply pressure (pressurize to 10 - 15 MPa), hold for (3 - 5 min), and take it out after pressure relief;

[0056] The thin slice of the fiber sample with good properties should have a flat surface, uniform thickness and no holes; if it does not meet the requirements, re - disperse the tablet, and then repeat the operations in steps (1) - (3) until a thin slice that meets the requirements is obtained.

[0057] Example 1

[0058] (1) Fiber sample treatment

[0059] Select the wool fiber with a fineness of 25 μm and a breaking strength of 4.8 cN / dtex. According to its characteristics of sharp scaly layer and low crimp, select a conical lancet 22 with a diameter of 0.6 mm (the tip curvature radius is 15 μm) to penetrate the gap between the fiber scales and avoid cutting the main body of the fiber;

[0060] (2) Mold selection

[0061] Select the A - type groove (depth 30 mm) in the mold to insert the thimble 2 to guide the parallel arrangement of the fibers. The lower layer of the thimble 1 is a micro - hole array to assist in fixing the fibers, so as to eliminate the natural crimp of the wool fibers;

[0062] (3) Sample preparation process

[0063] Fiber pretreatment: Soak the wool in warm water at 30 °C for 5 minutes, soften the scaly layer and then centrifuge to dehydrate (rotation speed 2000 rpm, duration 2 minutes);

[0064] Pressure gradient: The initial pressure of 1.0 MPa is maintained for 30 s (to avoid breaking of the scaly layer), increased step - by - step to 15 MPa, and the final sample thickness is 1.3 ± 0.1 mm (porosity < 5%);

[0065] (4) Terahertz test:

[0066] In an environment with a humidity of 30% (web page 1), the scanning range is 0.3 - 2.5 THz. The characteristic absorption peak of wool is measured at 0.85 THz (corresponding to the vibration of the keratin structure in the scaly layer), the absorption coefficient α = 9.8 cm - 1, and the signal - to - noise ratio > 800:1.

[0067] Example 2

[0068] (1) Fiber sample treatment

[0069] Select cashmere with a fineness of 15 μm and a breaking strength of 3.2 cN / dtex. For its medulla-free hollow structure and smooth scale layer characteristics, a flexible silicone needle 22 (diameter 0.3 mm, round head design) is selected for non-destructive puncture;

[0070] (2) Die selection

[0071] Select type A grooves (depth 30 mm) in the die to insert the sleeve rod 2 to guide the fibers to be arranged parallelly. The lower layer of the sleeve 1 is a microporous array to assist in fiber fixation to eliminate the natural curl of cashmere fibers;

[0072] (3) Sample preparation process

[0073] Fiber pretreatment: Soak the cashmere in warm water at 30 °C for 5 minutes, soften the scale layer and then centrifuge to dehydrate (rotation speed 2000 rpm, duration 2 minutes);

[0074] Pressure gradient: The initial pressure of 1.0 MPa is maintained for 30 s (to avoid scale layer breakage), and it is increased stepwise to 15 MPa. The final sample thickness is 1.1 ± 0.1 mm (porosity < 5%);

[0075] (4) Terahertz test:

[0076] In an environment with a humidity of 30% (web page 1), the scanning range is 0.3–2.5 THz. The characteristic peak of cashmere at 0.76 THz is detected, the absorption coefficient α = 15.2 cm-1, the signal-to-noise ratio > 1200:1, and the full width at half maximum of the peak is reduced by 18% compared to wool (due to more uniform fiber arrangement).

[0077] Example three

[0078] (1) Fiber sample treatment

[0079] Select carbon nanotube (CNT)-reinforced silk fibers (CFFRs) with a fineness of 18 μm and a breaking strength of 1.2 GPa. Its β-sheet crystallinity reaches 62%. The scale layer has a smooth surface due to sericin removal. A conical needle 22 with a diameter of 0.4 mm (tip curvature radius 10 μm), which is 2.2 times the fiber fineness, is used to penetrate the pores between fibroin fibers with the conical tip to avoid damaging the β-sheet crystal orientation;

[0080] (2) Die selection

[0081] Select type B grooves with a depth of 20 mm to guide the silk fibers to be arranged parallelly. A polyethersulfone (PES) porous membrane with a pore diameter of 5 μm is used as the lower layer fixing substrate to adsorb residual sericin droplets through capillary action to maintain the cleanliness of the fiber interface;

[0082] (3) Sample preparation process

[0083] Fiber pretreatment: Rinse with deionized water at 40 °C for 10 minutes to remove residual sericin on the surface (retention rate < 2%), and centrifuge to dehydrate (2500 rpm, 3 minutes) to avoid fiber entanglement;

[0084] Pressure gradient process:

[0085] Preloading stage: Maintain at 1.5 MPa for 20 seconds, and utilize the natural elasticity of silk fibers (Young's modulus 24.4 GPa) to buffer the initial pressure shock;

[0086] Directional puncture: Increase the pressure step by step to 18 MPa, and drive the β-sheet to align axially through high pressure;

[0087] Heat setting: Cure at a constant temperature of 45 °C for 10 minutes to promote the rearrangement of silk fibroin molecular chain segments. The final sample thickness is 0.9 ± 0.05 mm, and the porosity < 3%;

[0088] (4) Terahertz test:

[0089] In an environment with a humidity of 30%, the scanning range is 0.3–2.5 THz. A strong absorption peak is observed at 1.12 THz, the absorption coefficient α = 22.5 cm-1, and the signal-to-noise ratio > 1200:1.

[0090] Comparative example 1

[0091] (1) Fiber sample treatment

[0092] Select the same fine wool fiber as in Example 1 (fineness 25 μm, breaking strength 4.8 cN / dtex), but adopt the traditional tablet pressing method:

[0093] Do not use the customized tapered needle 22, but use the standard metal flat needle 22 (diameter 0.8 mm). Due to the blunt tip, the tearing rate of the fiber scale layer increases during the puncture process (about 12%). Use an open mold with a non-grooved structure, and only press the upper and lower flat plates together. Without the guide rod 2 to align the fibers, the natural curl of the fibers is not eliminated;

[0094] (2) Sample preparation process

[0095] Soak in room temperature water for 3 minutes. The scale layer is not softened sufficiently, and the residual moisture after centrifugal dehydration causes the fibers to stick together. Apply a constant pressure of 15 MPa directly for 60 seconds without using stepwise pressure increase. The fibers break internally due to the instantaneous high pressure (the breaking elongation rate decreases by 18%). The final thickness is 1.5 ± 0.3 mm, the porosity is as high as 9.8%, and the fiber arrangement is disordered (the standard deviation of the orientation angle > 25°);

[0096] (3) Terahertz test:

[0097] A 0.85 THz peak was detected (consistent with the patent case), but the absorption coefficient α decreased to 7.2 cm-1 (a 26.5% decrease compared to the patent case). Due to the high porosity, the scattering of terahertz waves increased, the signal-to-noise ratio was only 450:1, and the full width at half maximum increased to 0.32 THz (37% worse than the patent case), making it impossible to clearly resolve the interfacial characteristics between the scale layer and the cortical layer.

[0098] Comparative Example 2

[0099] (1) Fiber sample treatment

[0100] The same cashmere fibers as in Example 2 were selected (fineness 15 μm, breaking strength 3.2 cN / dtex), but the polyethylene (PE) mixed matrix method was used:

[0101] The acupuncture needle 22 was missing: the flexible silicone acupuncture needle 22 was not used. After the fibers were chopped with a Haas microtome, they were directly mixed with polyethylene powder (mass ratio 1:10) and pressed into shape, resulting in the fibers being wrapped by PE and the loss of interfacial clarity;

[0102] The mold was simplified: a cylindrical single cavity mold (without a microporous array) was used, and the fiber distribution was uneven (the local agglomeration rate reached 30%);

[0103] (2) Sample preparation process

[0104] Warm water soaking was not carried out, and the cashmere scale layer remained rigid, with a weak interfacial bonding force with PE (the peel strength was only 0.8 MPa), a thickness of 1.2 ± 0.2 mm, a porosity of 7.5%, and the PE matrix produced additional absorption interference in the terahertz band (the baseline noise increased by 3 times in the range of 0.6 - 1.2 THz);

[0105] (3) Terahertz test:

[0106] The 0.76 THz peak of cashmere was covered by the broad absorption band of PE (0.5 - 1.5 THz), and the peak position could only be barely identified through the second derivative spectrum. The absorption coefficient α could not be accurately calculated (due to matrix interference), and the signal-to-noise ratio < 300:1, which could not meet the conformational analysis requirements.

[0107] The data of the above examples show that the use of the acupuncture needle 22 and the mold to prepare fiber samples for terahertz spectroscopy has a very important impact. Otherwise, even if the samples are prepared under the same conditions, it is impossible to obtain a high-resolution terahertz spectrum of the fiber samples.

[0108] Unless otherwise expressly stipulated and defined, the terms "arranged", "installed", "connected" and "coupled" shall be construed in a broad sense. For example, it may be a fixed connection, a detachable connection or an integral connection; it may be a mechanical connection or an electrical connection; it may be a direct connection or an indirect connection through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application may be understood according to specific circumstances.

[0109] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A mold for fabricating fiber samples for terahertz time-domain spectroscopy tests, characterized in that: It includes a sleeve 1 and a sleeve rod 2; The sleeve 1 is a cylindrical structure with an open top, and a number of groups of needle holes 11 are distributed in a matrix on the bottom wall of the sleeve 1; The outer diameter of the sleeve rod 2 is the same as the inner diameter of the sleeve 1, and a number of needle points 22 that are the same in number as the needle holes 11 and are in one-to-one correspondence in position are also distributed in a matrix on the bottom surface of the sleeve rod 2; When the sleeve rod 2 is inserted into the sleeve 1, each group of needle points 22 penetrates out of the corresponding group of needle holes 11.

2. The mold for fabricating fiber samples used in terahertz time-domain spectroscopy testing according to claim 1, characterized in that: A limiting groove 12 is opened along the length direction on the inner wall of the sleeve 1, and the top of the limiting groove 12 extends to the top opening of the sleeve 1; There are limiting ribs 21 along the length direction on the outer wall of the sleeve rod 2, and a group of limiting ribs 21 slide into the corresponding group of limiting grooves 12.

3. A mold for fabricating fiber samples for terahertz time-domain spectroscopy testing according to claim 1, characterized in that: Both the sleeve 1 and the sleeve rod 2 are made of transparent materials.

4. A mold for fabricating fiber samples for terahertz time-domain spectroscopy testing according to claim 1, characterized in that: There are lugs 3 on both sides of the top opening of the sleeve 1.

5. The mold for fabricating a fiber sample for terahertz time-domain spectroscopy testing according to claim 4, wherein: At the same time, there are also lugs 3 on both sides of the top of the sleeve rod 2.

6. A method for preparing a fiber sample for terahertz time-domain spectroscopy according to any one of claims 1-5, characterized in that, It includes the following steps: (1) Gather the fiber sample into a small mass, evenly stuff it into the sleeve 1 mold placed on the polyurethane sponge, and gently poke the fluffy part of the fiber sample with the needle points 22 on the sleeve rod 2 until it is evenly and flatly gathered into a cake shape, and then take it off; (2) Place the cake-shaped fiber sample into a tablet pressing mold, apply pressure, hold, and take it out after releasing the pressure; The fiber sample thin slice with good properties should have a flat surface, uniform thickness and no holes; If it does not meet the requirements, re-disperse the tablet, and then repeat the operation in step one until a thin slice that meets the requirements is obtained.

7. The preparation method of a fiber sample for terahertz time-domain spectroscopy testing according to claim 6, wherein: In the said step (1), the sampling amount of the test fiber sample is 15 - 30 mg.

8. The preparation method of a fiber sample for terahertz time-domain spectroscopy test according to claim 6, characterized in that: In the said step (2), applying pressure means applying pressure to 10 - 15 MPa.

9. The preparation method of a fiber sample for terahertz time-domain spectroscopy testing according to claim 8, characterized in that: In the said step (2), the holding time is 3 - 5 min.

10. The preparation method of a fiber sample for terahertz time-domain spectroscopy testing according to claim 6, characterized in that: The fiber sample tablet making process is carried out in an environment with a humidity less than 30% and a temperature not higher than 25°C.

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