Porous material porosity detection method based on low-field nuclear magnetism

Abstract

The invention belongs to the technical field of material detection, and particularly relates to a nuclear magnetic resonance detection method for porosity of a porous material. The method comprises the following steps: drying a porous material sample to constant weight; mixing the dried sample with excessive deionized water, and carrying out vacuum treatment; selecting a corresponding detection scheme according to sample characteristics; testing the saturated sample by adopting a low-field nuclear magnetic resonance analyzer to obtain a nuclear magnetic resonance semaphore of the sample; configuring a series of deionized water standard samples with known mass, performing nuclear magnetic resonance test under the same test parameters, establishing a standard curve of nuclear magnetic semaphore and water mass, and fitting an equation; according to the standard curve fitting equation, the nuclear magnetic semaphore of the to-be-detected sample is converted into the mass of saturated water, the porosity is calculated by combining the mass or volume of the sample, and correction is conducted. The method provided by the invention solves the problems that the traditional method is complicated to operate, damages the material and cannot effectively eliminate interference of free water, and realizes rapid, nondestructive and high-precision detection of the porosity of the negative electrode materials such as porous silicon-based and silicon-carbon composite materials.

Description

Technical Field

[0001] This invention relates to the field of materials testing technology, and in particular to a method for detecting the porosity of porous materials based on low-field nuclear magnetic resonance. Background Technology

[0002] Porosity is a key parameter for evaluating the structure and performance of porous materials, especially in the field of battery materials, where the porosity of porous anode materials directly affects ion transport efficiency, charge-discharge rate, and cycle stability. Currently used porosity detection methods include BET gas adsorption, mercury porosimetry, and gravimetric analysis, but these methods have significant drawbacks:

[0003] The BET gas adsorption method is complex and time-consuming to operate, and its accuracy in detecting large pores is limited.

[0004] Mercury intrusion porosimetry can cause irreversible damage to materials and is not suitable for materials with fine pores.

[0005] Traditional weighing methods are greatly affected by human operation and have low detection accuracy, making it difficult to meet the needs of rapid and accurate detection of porous materials.

[0006] Low-field nuclear magnetic resonance (NMR) technology has gained widespread attention in the field of materials testing due to its advantages such as non-destructive, rapid detection, and ease of operation. However, existing NMR detection methods lack specific solutions for different types of porous materials, the test parameters are not optimized, the sample pretreatment process is not standardized, and the ratio of free water between powder particles to bound water inside the pores is not clearly distinguished, resulting in poor accuracy and low repeatability of the test results, and the inability to effectively obtain pore distribution information.

[0007] Therefore, developing a nuclear magnetic resonance porosity detection method that is adaptable to a variety of porous materials, has high detection accuracy, is easy to operate, and can accurately distinguish between interparticle water and water within pores is of great practical significance. Summary of the Invention

[0008] The technical problem to be solved by this invention is to design a method for detecting the porosity of porous materials based on low-field nuclear magnetic resonance. By designing a targeted detection scheme, optimizing test parameters, and introducing a moisture separation correction mechanism based on low-field nuclear magnetic resonance signals, the method can achieve accurate detection of porosity, thereby solving the existing technical problems.

[0009] To address the aforementioned technical problems, this invention provides a method for detecting the porosity of porous materials based on low-field nuclear magnetic resonance, specifically comprising the following steps:

[0010] Step S1: Sample pretreatment: Dry the porous material sample to constant weight to remove moisture and volatile impurities, avoiding interference from impurities on the NMR signal. After drying, place the sample to be tested in a desiccator to cool to room temperature for later use.

[0011] Step S2: Sample saturation treatment: Mix the dried sample with excess deionized water and perform vacuum treatment to fully saturate the sample pores with water.

[0012] Step S3: Select a detection method: Select the corresponding detection method according to the sample characteristics, including the sample material and tap density. The detection method includes low temperature freeze-thaw method, room temperature filtration method and room temperature centrifugation method.

[0013] The low-temperature freeze-thaw method is suitable for porous powder materials with low tap density (e.g., around 0.247 g / cm³). It separates free water from bound water through low-temperature freezing, and combines this with T2 relaxation signal correction to improve detection accuracy. The room-temperature filtration method is suitable for silicon-based porous materials (e.g., silicon materials with a tap density of 0.39 g / cm³). Filtration removes surface free water, preventing residual moisture from affecting test results. The room-temperature centrifugation method is suitable for silicon-carbon composite porous materials (e.g., silicon-carbon materials with a tap density of 0.41 g / cm³). Centrifugation at different speeds removes excess water, and corrections based on the difference in water volume before and after centrifugation or the T2 relaxation signal are used to obtain pore radius distribution information.

[0014] Step S4: Nuclear Magnetic Resonance Test: The water-saturated sample is tested using a low-field nuclear magnetic resonance analyzer. The CPMG sequence parameters are set according to the detection scheme, and the T2 relaxation time is tested to obtain the nuclear magnetic resonance signal of the sample.

[0015] Step S5: Standard Curve Calibration: Prepare a series of deionized water standard samples with known masses (e.g., 0.00999 g, 0.03 g, 0.06 g, 0.09 g, 0.12 g, 0.15 g, etc.), and perform nuclear magnetic resonance (NMR) tests under the same test parameters. Establish a standard curve with water mass as the x-axis and NMR signal intensity as the y-axis, obtain the fitting equation, and use linear fitting to obtain the fitting equation. (in ), used for converting the mass of saturated water in the sample.

[0016] Step S6: Convert the NMR signal of the sample to saturated water mass according to the standard curve fitting equation, and calculate the porosity based on the sample mass or volume. The porosity calculation formula is as follows:

[0017] ;

[0018] or, ,in, .

[0019] Furthermore, in step S6 of this invention, the calculated porosity is further corrected by the following formula: Corrected porosity = porosity × correction coefficient, wherein the correction coefficient is the proportion of the signal integration area corresponding to the bound water in the pores to the total signal integration area, or the proportion of the mass of the bound water in the pores to the total water mass.

[0020] Furthermore, in this invention, when using the low-temperature freeze-thaw method for detection in step S3, the specific operation steps include the following:

[0021] Step A: Place the water-saturated sample in a low-temperature freeze-thaw nuclear magnetic resonance nanopore analyzer and gradually cool it from 0 ℃ to -12 ℃ to determine the freezing temperature point where the signal amount drops significantly.

[0022] Step B: At the freezing point, gradually increase the temperature from -8℃ to 0℃ to conduct the test, and record the initial signal quantity at different temperatures.

[0023] Step C: Convert the initial signal at the test temperature into an equivalent signal at 0 ℃, and substitute it into the standard curve fitting equation to calculate the saturated water mass.

[0024] Furthermore, in step C of this invention, the conversion formula is:

[0025]

[0026] Where, absolute temperature = test temperature + 273.15, coil correction factor K = 273.15.

[0027] Furthermore, this invention further distinguishes between interparticle frozen water and pore-bound frozen water, and, based on the T2 relaxation signal separation results, takes the signal integration area corresponding to the bound water in the pores. Area of ​​total signal integration The proportional correction calculation results This represents the signal integral area corresponding to the free water between particles.

[0028] Furthermore, in this invention, when using the room temperature filtration method for detection in step S3, the specific operation steps include the following:

[0029] Step A: Shake the water-saturated sample well and pour it into a funnel lined with filter paper. Filter until the filter paper stops dripping water.

[0030] Step B: Scrape the water-saturated sample off the filter paper, put it into a chromatographic bottle, weigh it, and then perform nuclear magnetic resonance testing.

[0031] Step C: After the test is completed, dry the sample to constant weight and record the drying mass and saturation mass.

[0032] Furthermore, in this invention, when using room temperature centrifugation for detection in step S3, the specific operation steps include:

[0033] Step A: Load the saturated sample into centrifuge tubes and place them together with the balanced centrifuge tubes into a centrifuge, centrifuging at different speeds.

[0034] Step B: After centrifugation, pour out the excess liquid water, perform nuclear magnetic resonance testing on the remaining water-saturated sample, and record the de-substrate saturation signal quantity.

[0035] Step C: Dry the centrifuge tubes and samples to constant weight, cool them, weigh them, and record the dried mass and saturated mass.

[0036] Furthermore, this invention also includes step D: optimizing the result using one of the following two correction methods based on signal or quality analysis:

[0037] Method 1: Based on the difference in water volume before and after centrifugation, the mass of bound water in the pores is obtained by the difference between the mass of the water-saturated sample after centrifugation and the mass of the dried sample. The total water content is obtained by measuring the difference between the mass of the water-saturated sample before centrifugation and the mass of the sample after drying. , For the mass of free water between particles, take The proportion of the total water volume is used to adjust the initial porosity.

[0038] Method 2: Combining T2 relaxation signal separation and correction, the signal integration area corresponding to the bound water in the pores is separated by the T2 spectrum. The signal integral area corresponding to the free water between particles ,Pick Area of ​​total signal integration The initial porosity is corrected proportionally.

[0039] Furthermore, in step S4 of this invention, the CPMG sequence parameters of the low-field nuclear magnetic resonance analyzer are as follows: sampling frequency SW = 250 kHz, waiting time TW = 350~5000 ms, radio frequency delay RFD = 0.08 ms, analog gain RG1 = 20 dB, digital gain DRG1 = 3, accumulation count NS = 8~128, preamplifier range PRG = 3, TE echo time = 0.06~0.08 ms, and NECH echo count = 5000~10000.

[0040] Furthermore, in this invention, the porous material includes silicon-based porous materials for battery negative electrodes and silicon-carbon composite porous materials, and the tap density of the samples ranges from 0.247 to 0.41 g / cm³.

[0041] Compared with existing technologies, the porosity detection method for porous materials based on low-field NMR of the present invention has the following specific advantages:

[0042] (1) This invention designs exclusive detection schemes for different types of porous materials. The low temperature freeze-thaw method, the room temperature filtration method and the room temperature centrifugation method are respectively adapted to low tap density porous materials, silicon-based porous materials and silicon-carbon composite porous materials. Combined with the conversion coefficient correction mechanism, the targeting and accuracy of the detection are greatly improved.

[0043] (2) The present invention optimizes the CPMG sequence parameters of nuclear magnetic resonance testing and combines them with a standardized sample pretreatment process (drying, water saturation, vacuum treatment) to ensure the repeatability and reliability of the test results. The relative deviation of three parallel tests is less than 5%.

[0044] (3) The present invention introduces a signal correction mechanism based on the ratio of water between powder particles to water inside pores, and provides two correction methods: weighing method and signal separation method. The corrector can be flexibly selected according to the detection scenario, and effectively eliminates the interference of free water on the detection results.

[0045] (4) The detection process of this invention does not require complex sample preparation, does not damage the material, has a short testing cycle (the testing time for a single sample is less than 1 hour), and can simultaneously obtain pore radius distribution information, providing more comprehensive data support for material structure analysis.

[0046] (5) This invention is applicable to the porosity detection of various porous powder materials such as silicon-based and silicon-carbon-based materials used in battery negative electrodes. It can also be extended to the detection of porous materials in the fields of coal, rock, and petroleum, with a wide range of applications. Attached Figure Description

[0047] The specific embodiments of the present invention will be further explained below with reference to the accompanying drawings.

[0048] Figure 1 This is a flowchart of the porosity detection method for porous materials based on low-field nuclear magnetic resonance according to the present invention. Detailed Implementation

[0049] Combination Figure 1 The porosity detection method for porous materials based on low-field NMR in this embodiment specifically includes the following steps:

[0050] Step S1: Sample Pretreatment: Dry the porous material sample to constant weight to remove moisture and volatile impurities, avoiding interference from impurities on the NMR signal. After drying, place the sample to be tested in a desiccator to cool to room temperature for later use. Specifically, in this embodiment, the porous material sample is placed in an oven and dried at 80~105 ℃ for 24 h.

[0051] Step S2: Sample saturation treatment: Mix the dried sample with excess deionized water and perform vacuum treatment to fully saturate the sample pores. Specifically, in this embodiment, the dried sample is accurately weighed, placed in a suitable container (measuring tube, chromatographic bottle, or centrifuge tube), excess deionized water is added, and the sample is placed in a vacuum chamber and evacuated for 4 hours to ensure that the water fully penetrates into all the pores of the sample, achieving complete saturation.

[0052] Step S3: Select a detection method: Select the corresponding detection method according to the sample characteristics, including the sample material and tap density. The detection method includes low temperature freeze-thaw method, room temperature filtration method and room temperature centrifugation method.

[0053] The low-temperature freeze-thaw method is suitable for porous powder materials with low tap density (e.g., around 0.247 g / cm³). It separates free water from bound water through low-temperature freezing, and combines this with T2 relaxation signal correction to improve detection accuracy. The room-temperature filtration method is suitable for silicon-based porous materials (e.g., silicon materials with a tap density of 0.39 g / cm³). Filtration removes surface free water, preventing residual moisture from affecting test results. The room-temperature centrifugation method is suitable for silicon-carbon composite porous materials (e.g., silicon-carbon materials with a tap density of 0.41 g / cm³). Centrifugation at different speeds removes excess water, and corrections based on the difference in water volume before and after centrifugation or the T2 relaxation signal are used to obtain pore radius distribution information.

[0054] Step S4: Nuclear Magnetic Resonance Test: The water-saturated sample is tested using a low-field nuclear magnetic resonance analyzer. The CPMG sequence parameters are set according to the detection scheme, and the T2 relaxation time is tested to obtain the nuclear magnetic resonance signal of the sample.

[0055] Step S5: Standard Curve Calibration: Prepare a series of deionized water standard samples with known masses (e.g., 0.00999 g, 0.03 g, 0.06 g, 0.09 g, 0.12 g, 0.15 g, etc.), and perform nuclear magnetic resonance (NMR) tests under the same test parameters. Establish a standard curve with water mass as the x-axis and NMR signal intensity as the y-axis, obtain the fitting equation, and use linear fitting to obtain the fitting equation. (in ), used for converting the mass of saturated water in the sample.

[0056] Step S6: Convert the NMR signal of the sample to saturated water mass according to the standard curve fitting equation, and calculate the porosity based on the sample mass or volume. The porosity calculation formula is as follows:

[0057] ;

[0058] or, ,in, .

[0059] In this embodiment, preferably, step S6 further includes a correction calculation of the calculated porosity. The specific calculation formula is: Corrected porosity = Porosity × Correction coefficient, where the correction coefficient is the proportion of the signal integration area corresponding to the bound water in the pore to the total signal integration area, or the proportion of the mass of the bound water in the pore to the total water mass.

[0060] In this embodiment, when the low-temperature freeze-thaw method is used for detection in step S3, the specific operation steps are as follows:

[0061] Step A: Place the water-saturated sample in a low-temperature freeze-thaw nuclear magnetic resonance nanopore analyzer and gradually cool it from 0 ℃ to -12 ℃ to determine the freezing temperature point where the signal amount drops significantly.

[0062] Step B: At the freezing point, gradually increase the temperature from -8℃ to 0℃ to conduct the test, and record the initial signal quantity at different temperatures.

[0063] Step C: Convert the initial signal at the test temperature into an equivalent signal at 0 ℃, and substitute it into the standard curve fitting equation to calculate the saturated water mass.

[0064] In this embodiment, preferably, in step C, the conversion formula is:

[0065]

[0066] Where, absolute temperature = test temperature + 273.15, coil correction factor K = 273.15.

[0067] In this embodiment, preferably, the frozen water between particles and frozen water within pores is further distinguished, and the signal integration area corresponding to the bound water within the pores is taken based on the T2 relaxation signal separation results. Area of ​​total signal integration The proportional correction calculation results This represents the signal integral area corresponding to the free water between particles.

[0068] Specifically, in this embodiment, the signal integration area corresponding to the bound water within the pores is separated based on the T2 relaxation spectrum. The signal integral area corresponding to the free water between particles ,Pick Area of ​​total signal integration The saturated water mass was corrected proportionally, and the porosity was calculated using the formula "porosity = (corrected saturated water mass / sample mass) × 100%". The average value of three tests was taken as the final result.

[0069] In this embodiment, when the room temperature filtration method is used for detection in step S3, the specific operation steps are as follows:

[0070] Step A: Shake the water-saturated sample well and pour it into a funnel lined with filter paper. Filter until the filter paper stops dripping water.

[0071] Step B: Scrape the water-saturated sample off the filter paper, put it into a chromatographic bottle, weigh it, and then perform nuclear magnetic resonance testing.

[0072] Step C: After the test is completed, dry the sample to constant weight and record the drying mass and saturation mass.

[0073] In this embodiment, the saturated water sample is filtered, weighed, and then subjected to NMR testing. The mass of saturated water is calculated based on the standard curve, and the porosity is calculated in combination with the sample volume. The formula is "Porosity = (Saturated water volume / Total sample volume) × 100%".

[0074] In this embodiment, when using room temperature centrifugation for detection in step S3, the specific operation steps include the following:

[0075] Step A: Load the saturated sample into centrifuge tubes and place them together with the balanced centrifuge tubes into a centrifuge, centrifuging at different speeds.

[0076] Step B: After centrifugation, pour out the excess liquid water, perform nuclear magnetic resonance testing on the remaining water-saturated sample, and record the de-substrate saturation signal quantity.

[0077] Step C: Dry the centrifuge tubes and samples to constant weight, cool them, weigh them, and record the dried mass and saturated mass.

[0078] In this embodiment, step D is preferably included: optimizing the result using one of the following two correction methods based on signal or quality analysis:

[0079] Method 1: Based on the difference in water volume before and after centrifugation, the mass of bound water in the pores is obtained by the difference between the mass of the water-saturated sample after centrifugation and the mass of the dried sample. The total water content is obtained by measuring the difference between the mass of the water-saturated sample before centrifugation and the mass of the sample after drying. , For the mass of free water between particles, take The proportion of the total water volume is used to adjust the initial porosity.

[0080] Method 2: Combining T2 relaxation signal separation and correction, the signal integration area corresponding to the bound water in the pores is separated by the T2 spectrum. The signal integral area corresponding to the free water between particles ,Pick Area of ​​total signal integration The initial porosity is corrected proportionally.

[0081] In this embodiment, the core of eliminating the interference between free water between particles and bound water inside pores is to quantify the proportion of the two through physical separation or signal separation and then correct the result. The design logic and applicable scenarios of the two correction methods are as follows:

[0082] 1. Correction method based on the difference in water volume before and after centrifugation (Method 1)

[0083] This method physically separates free water between particles and bound water within pores through centrifugation, and quantifies their masses using a weighing method to directly establish a proportional relationship. During centrifugation, free water between particles is removed due to its weak adhesion, while bound water within pores is retained due to its strong adsorption. The total water volume can be obtained by measuring the mass difference between the saturated sample and the dried sample before and after centrifugation. and the amount of bound water in the pores ,Pick The corrected result of the proportion of total water volume can intuitively reflect the actual water content in the pores. It is suitable for scenarios that require high data intuitiveness and need to be verified by physical separation. It is highly compatible with the operation process of the room temperature centrifugation method in this application.

[0084] 2. Correction method combining T2 relaxation signal separation (Method 2)

[0085] This method utilizes the physical characteristics of the T2 relaxation time in low-field NMR to achieve water separation at the signal level: bound water within pores is strongly influenced by the solid surface, restricting molecular motion and resulting in a short T2 relaxation time, corresponding to signal peaks in the short relaxation interval of the spectrum; free water molecules between particles move freely, resulting in a long T2 relaxation time, corresponding to signal peaks in the long relaxation interval of the spectrum. Water is separated by integrating the signal peaks in different intervals. (Bound water signal within pores) and (Interparticle free water signal), take The proportion of the total signal is corrected without additional weighing, making it suitable for scenarios that require rapid and synchronous acquisition of pore distribution and porosity, and reduction of human weighing errors. It can be used in conjunction with the low-temperature freeze-thaw method and centrifugation method.

[0086] In this embodiment, preferably, in step S4, the CPMG sequence parameters of the low-field NMR analyzer are: sampling frequency SW = 250 kHz, waiting time TW = 350~5000 ms, radio frequency delay RFD = 0.08 ms, analog gain RG1 = 20 dB, digital gain DRG1 = 3, number of accumulations NS = 8~128, preamplifier setting PRG = 3, TE echo time = 0.06~0.08 ms, and number of NECH echoes = 5000~10000.

[0087] In this embodiment, preferably, the porous material includes silicon-based porous material for battery negative electrode and silicon-carbon composite porous material, and the tap density of the sample ranges from 0.247 to 0.41 g / cm³.

[0088] Example 1

[0089] In this embodiment, the low-temperature freeze-thaw method is used for testing, specifically as follows:

[0090] 1. Test Sample

[0091] The porous material SBX241219-1 of the battery negative electrode powder was selected as the test sample. The tap density of the sample was 0.247 g / cm³, and the sample mass was 0.4077 g.

[0092] 2. Testing equipment

[0093] The NMRC12-010V cryogenic freeze-thaw nuclear magnetic resonance nanopore analyzer was used. This equipment is equipped with a cryogenic control system and a signal acquisition and processing module, which can achieve precise control over a wide temperature range and high-sensitivity detection of nuclear magnetic resonance signals.

[0094] 3. Test Parameter Settings

[0095] The CPMG test sequence was used, with the following specific parameters:

[0096] Sampling frequency SW: 250 kHz;

[0097] Waiting time (TW): 500 ms;

[0098] Radio frequency delay (RFD): 0.08 ms;

[0099] Analog gain RG1: 20 dB;

[0100] Digital gain DRG1: 3;

[0101] Accumulation count NS: 128;

[0102] Forward gear position PRG: 3;

[0103] TE echo time: 0.06 ms;

[0104] NECH echo count: 10000.

[0105] 4. Testing Procedures

[0106] 4.1 Sample Pretreatment

[0107] The test sample was dried in an 80 ℃ oven for 24 h to completely remove the adsorbed moisture and volatile impurities in the sample pores. An appropriate amount of the dried sample was weighed and placed into a special measuring tube, and a vacuum treatment was performed and maintained for 4 h to make the sample pores a vacuum state. Then, deionized water was added to the measuring tube to ensure that the moisture fully penetrated into the sample pores and the sample was saturated with water.

[0108] 4.2 Determination of Freezing Temperature Point

[0109] A measuring tube containing a saturated water-absorbing sample was placed into the sample chamber of a cryogenic freeze-thaw nuclear magnetic resonance nanopore analyzer. The temperature was gradually reduced from 0 °C to -12 °C, with nuclear magnetic resonance signal acquisition performed every 1 °C during the cooling process, recording the initial signal intensity at different temperatures. By analyzing the temperature-initial signal intensity curve, the temperature point where the signal intensity drops significantly was determined as the freezing temperature of the sample. In this embodiment, this temperature point was -5 °C (at which point the external moisture of the sample begins to freeze, while the moisture inside the pores remains liquid).

[0110] 4.3 Establishment of Standard Curve

[0111] A standard curve calibration experiment was conducted at the determined freezing temperature (-5 °C). Different masses of deionized water (0.00999 g, 0.03 g, 0.06 g, 0.09 g, 0.12 g, and 0.15 g) were weighed and placed in identical measuring tubes. Nuclear magnetic resonance signal testing was performed using the CPMG parameters set above. The initial signal intensity for each water mass was recorded, and the data are shown in the table below.

[0112] Water mass (g) 0.00999 0.03 0.06 0.09 0.12 0.15 First point semaphore (au) 1463.483 3632.19 6803.246 10092.358 13057.052 15999.186

[0113] A linear fit was performed with water quality as the x-axis and the first point signal quantity as the y-axis to obtain the standard curve equation. Correlation coefficient R 2 =0.9965, indicating a good linear relationship between water quality and the first point signal quantity.

[0114] 4.4 Sample porosity test

[0115] Samples pretreated and with a determined freezing point were subjected to NMR signal testing at -5 °C using the aforementioned CPMG parameters. The test was repeated three times, and the initial signal intensity was recorded for each test. The initial signal intensity at the test temperature was then converted to the initial signal intensity at 0 °C using the following formula:

[0116] Corrected 0℃ signal quantity = Initial point signal quantity × Absolute temperature × (Absolute temperature + Coil correction coefficient) / (273.15 × (273.15 + Coil correction coefficient));

[0117] Wherein, absolute temperature = test temperature + 273.15, in this embodiment the test temperature is -5 ℃, the absolute temperature is 268.15K, and the coil correction coefficient K = 273.15.

[0118] Based on the established standard curve equation Substituting the converted 0 ℃ initial point signal into the equation, the mass of adsorbed water in the sample is calculated, and then the porosity of the sample is calculated using the following formula:

[0119] Porosity = (mass of water in the sample / mass of the sample) × 100%.

[0120] 5. Test Results

[0121] The specific data from the three repeated tests are shown in the table below. The average value of the three test results is taken, and the porosity of sample SBX241219-1 is 11.55%.

[0122] Number of tests 1 2 3 Test temperature (°C) -5 -5 -5 Absolute temperature (K) 268.15 268.15 268.15 First point semaphore (au) 4693.309 4799.223 6333.035 Convert the 0℃ initial point signal (au) 4565.229 4668.253 6160.207 Water mass (g) 0.042 0.043 0.057 Porosity (%) 10.272 10.504 13.861

[0123] Comparative Example 1

[0124] In this comparative example, the porosity of the silicon material was tested using the nitrogen adsorption-desorption method. A sample of the same material as in Example 1 was taken and weighed 0.6401 g.

[0125] The test sample is placed in a sample tube and placed in the degassing station of the fully automated gas adsorption analyzer. The degassing temperature is set to 120 ℃ and the degassing time is 6 h to thoroughly remove moisture, air and other volatile impurities adsorbed on the sample surface, ensuring that nitrogen only adsorbs into the sample pores during the test.

[0126] Adsorption-desorption isotherm test: The degassed sample tube is transferred to the analysis station and immersed in a liquid nitrogen bath (ensuring the temperature is maintained at -196 ℃). High-purity nitrogen gas (purity ≥99.999%) is introduced as the adsorbate. The relative pressure (P / P0) test range is set to 0.01~0.995. A multi-point adsorption-desorption mode is used to automatically collect the amount of nitrogen adsorbed by the sample under different relative pressures, generating a complete adsorption-desorption isotherm.

[0127] Data processing and porosity calculation: The adsorption isotherm data were fitted according to the BET theoretical model to calculate the total BJH pore volume of the sample; combined with the true density range of silicon-carbon materials (2.2~2.5 g / cm³), the porosity values ​​corresponding to different true densities were calculated using the above porosity calculation formula.

[0128] The test results are as follows:

[0129] (1) Hole structure parameters

[0130] The total BJH pore volume of the sample was measured to be 0.0462 cm³ / g. Based on the sample mass and actual density, the calculated pore structure parameters are shown in the table below.

[0131] True density of solid (g / cm³) 2.5 Solid particle volume (cm³) 0.6401 / 2.5≈0.2560 Total pore volume (cm³) 0.6401×0.0462≈0.0296 Total volume (cm³) 0.2560+0.0296=0.2856 Porosity （0.0296 / 0.2856）×100%≈10.36%

[0132] In Example 1, the average porosity of sample SBX241219-1 was measured to be 11.55% using the low-temperature freeze-thaw nuclear magnetic resonance method; in Comparative Example 1, the average porosity of the sample was measured to be 10.36% using the nitrogen adsorption-desorption (BET) method. The results of the two methods are close, which fully verifies the accuracy of the low-field nuclear magnetic resonance method in measuring porosity, and its data reliability is comparable to that of the traditional BET method.

[0133] Example 2

[0134] In this embodiment, the sample SBX20250104 is tested using a room temperature filtration method, specifically as follows:

[0135] 1. Sample pretreatment: Weigh 0.1735 g of dried sample and put it into a chromatographic bottle. Add excess deionized water and vacuum for 4 hours to ensure that the pores are fully saturated with water.

[0136] 2. Removal of free water: Use a disposable pipette to remove the water floating on the sample surface. Shake the remaining sample well and pour it into a filter paper funnel. Filter until the filter paper no longer drips water. Scrape the water-saturated sample from the filter paper and put it into a clean chromatography bottle. Weigh and record the weight.

[0137] 3. Sample drying treatment: Place the above saturated sample in an oven at 105 ℃ and dry it to constant weight. After cooling to room temperature, weigh and record the weight.

[0138] 4. Establishment of standard curve: Select standard water samples of different masses (0.05 g, 0.1 g, 0.3 g, 0.6 g), perform nuclear magnetic resonance testing using the set CPMG sequence parameters, record the nuclear magnetic signal quantity corresponding to each water mass, and establish a linear regression equation between water mass and signal quantity.

[0139] 5. Sample signal test: Place the chromatographic vial of the water-saturated sample into the sample tube of the nuclear magnetic resonance analyzer, perform T2 relaxation signal test, and record the amount of de-basin saturation signal.

[0140] 6. Porosity Calculation: The water mass in the pores is calculated based on the standard curve equation, and the porosity of a single test is calculated in combination with the sample mass.

[0141] The test results of sample SBX20250104 are shown in the table below. The measured porosity is 52.40%.

[0142] Debasal-saturated semaphore (au) 2787.586 Nuclear magnetic conversion water volume 0.23 Sample volume (cm³) 0.44 Drying weight (g) 6.23 Saturated mass (g) 6.49 Mass of saturated water (g) 0.26 Weigh the water volume (mL) 0.26 Porosity (%) 52.40

[0143] Comparative Example 2

[0144] In this comparative example, the porosity of sample SBX20250104 was tested using the nitrogen adsorption-desorption method, specifically as follows:

[0145] 1. Place the test sample into a special sample tube and put it into the degassing station of the fully automatic gas adsorption analyzer. Set the degassing temperature to 120 ℃ and the degassing time to 6 h to thoroughly remove the moisture, air and volatile impurities adsorbed on the sample surface and in the pores, and ensure that nitrogen only specifically adsorbs into the sample pores during the test.

[0146] 2. Adsorption-desorption isotherm test:

[0147] The degassed sample tubes were transferred to the analysis station and immersed in a liquid nitrogen bath to maintain a low-temperature environment (temperature maintained at -196 ℃). High-purity nitrogen gas (purity ≥99.999%) was introduced as the adsorbate. The relative pressure (P / P0) test range was set to 0.01~0.995, and a multi-point adsorption-desorption test mode was adopted to automatically collect nitrogen adsorption data at different pressures and generate complete adsorption-desorption isotherms.

[0148] 3. Data processing and porosity calculation:

[0149] The adsorption isotherm data were fitted using the BET theoretical model, and the total pore volume of the sample was calculated using the BJH model. Based on the true density range of silicon material (2.329~2.33 g / cm³), the porosity values ​​corresponding to different true densities were calculated using the above porosity calculation formula, and the average value was taken as the final test result.

[0150] The test results are as follows:

[0151] The total BJH pore volume of the sample was measured to be 0.0239 cm³ / g. Based on the sample mass and the actual density of silicon, the calculated pore structure parameters are shown in the table below.

[0152] True density of solids 2.329 g / cm³ 2.33 g / cm³ Solid particle volume 0.1735 / 2.329≈0.0745 cm³ 0.1735 / 2.33≈0.07446 cm³ Total pore volume 0.0239 × 0.1735 ≈ 0.00415 cm³ 0.0239 × 0.1735 ≈ 0.00415 cm³ Total volume 0.0745 + 0.00415 = 0.07865 cm³ 0.07446 + 0.00435 = 0.07881 cm³ Porosity （0.00415 / 0.07865）×100%≈52.77% （0.00415 / 0.07881）×100%≈52.66%

[0153] Considering the slight fluctuations in the actual density of silicon material and the errors in the testing system, the average of the two calculation results is taken as (52.77% + 52.66%) / 2 ≈ 52.72%. This value is close to the average porosity measured by the room temperature filtration nuclear magnetic resonance method in Example 2.

[0154] Example 3

[0155] In this embodiment, the sample PS@C-250117 was tested using the room temperature centrifugation method, specifically as follows:

[0156] 1. Sample pretreatment: Weigh the corresponding mass of dried sample and put it into a 5mL centrifuge tube. Add excess deionized water and vacuum for 4 hours to ensure that the pores are fully saturated with water.

[0157] 2. Removal of free water: Place the sample centrifuge tube and the balanced centrifuge tube into a centrifuge and centrifuge at 5000 rpm, 9000 rpm and 13000 rpm for 1 h respectively. After centrifugation, pour out the excess liquid water in the tube and weigh and record the mass of the remaining water-saturated sample.

[0158] 3. Sample drying treatment: After centrifugation, place the centrifuge tubes of the sample in an oven at 105 ℃ to dry to constant weight, cool to room temperature, weigh and record the mass of the dried sample.

[0159] 4. Establishment of standard curve: Select standard water samples of different masses (0.05 g, 0.1 g, 0.3 g, 0.6 g), perform nuclear magnetic resonance testing using the set CPMG sequence parameters, record the nuclear magnetic signal quantity corresponding to each water mass, and establish a linear regression equation between water mass and signal quantity.

[0160] 5. Sample signal test: Nuclear magnetic resonance T2 relaxation signal test was performed on saturated water sample and dry sample respectively, and the signal amount after substrate saturation was recorded; the test was repeated 3 times at each rotation speed and the average signal amount was taken.

[0161] 6. Porosity Calculation: The water mass within the pores is calculated based on the standard curve equation, and the porosity of a single test is calculated in combination with the sample mass; at the same time, the theoretical porosity is referenced: theoretical porosity = (1 - tap density / true density) × 100%, the true density of silicon is 2.329~2.33 g / cm³, and the true density of silicon-carbon is 2.2~2.5 g / cm³.

[0162] Conversion factor test: The conversion factor was calculated to be 0.55 by using the proportion of the short relaxation integral area S1 to the total integral area (S1+S2) in the low-field NMR T2.

[0163] 7. Correct the porosity using the conversion factor to obtain the final result:

[0164] Number of tests 1 2 3 Debasal-saturated semaphore (au) 6701.792 6111.461 5683.145 Nuclear magnetic conversion water volume 0.56 0.51 0.48 Sample volume (cm³) 0.78 0.78 0.78 Drying weight (g) 0.32 0.32 0.32 Saturated mass (g) 0.98 0.98 0.98 Mass of saturated water (g) 0.67 0.67 0.67 Weigh the water volume (mL) 0.67 0.67 0.67 Nuclear magnetic resonance porosity (%) 71.93 65.6 61 Weighing porosity (%) 85.81 85.67 85.64 Theoretical porosity (%) 81.55 81.55 81.55 Conversion factor 0.55 0.55 0.55 Corrected porosity (%) 39.56 36.08 33.55

[0165] Comparative Example 3

[0166] This comparative example uses nitrogen adsorption-desorption method to test sample PS@C-250117, specifically as follows:

[0167] 1. Place the test sample into a special sample tube and put it into the degassing station of the fully automatic gas adsorption analyzer. Set the degassing temperature to 120 ℃ and the degassing time to 6 h to thoroughly remove the moisture, air and volatile impurities adsorbed on the sample surface and in the pores, and ensure that nitrogen only specifically adsorbs into the sample pores during the test.

[0168] 2. Adsorption-desorption isotherm test:

[0169] The degassed sample tubes were transferred to the analysis station and immersed in a liquid nitrogen bath to maintain a low-temperature environment (temperature maintained at -196℃). High-purity nitrogen gas (purity ≥99.999%) was introduced as the adsorbate. The relative pressure (P / P0) test range was set to 0.01~0.995, and a multi-point adsorption-desorption test mode was adopted to automatically collect nitrogen adsorption data at different pressures and generate complete adsorption-desorption isotherms.

[0170] 3. Data processing and porosity calculation:

[0171] The adsorption isotherm data were fitted using the BET theoretical model, and the total pore volume of the sample was calculated using the BJH model. Based on the true density range of silicon-carbon materials (2.2~2.5 g / cm³), the porosity values ​​corresponding to different true densities were calculated using the above porosity calculation formula, and the average value was taken as the final test result.

[0172] 4. Test Results:

[0173] The total pore volume of the BJH sample was measured to be 0.2412 cm³ / g. Based on the sample mass and the actual density of the silicon-carbon material, the pore structure parameters were calculated as shown in the table below, with the porosity taken as an average of 36.15%.

[0174] True density of solid (g / cm³) 2.2 2.35 2.5 Solid particle volume (cm³) 0.32 / 2.2≈0.1455 0.32 / 2.35≈0.1362 0.32 / 2.5=0.1280 Total pore volume (cm³) 0.2412×0.32=0.077184 0.2412×0.32=0.077184 0.2412×0.32=0.077184 Total volume (cm³) 0.1455+0.077184=0.222684 0.1362+0.077184=0.213384 0.1280+0.077184=0.205184 Porosity （0.077184 / 0.222684）×100%≈34.66% （0.077184 / 0.213384）×100%≈36.17% （0.077184 / 0.205184）×100%≈37.62%

[0175] For the PS@C-250117 sample, the average porosity after conversion factor correction by room temperature centrifugal nuclear magnetic resonance was 36.15%, and the average porosity measured by BET method was 35.96%. The data are highly consistent, which fully demonstrates the reliability of low-field nuclear magnetic resonance in the porosity testing of silicon-carbon composite materials.

[0176] Many specific details have been set forth in the foregoing description to provide a thorough understanding of the present invention. However, the above description is merely a preferred embodiment of the present invention, and the present invention can be implemented in many other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed above. Furthermore, any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, using the methods and techniques disclosed above, without departing from the scope of the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A method for detecting porosity of a porous material based on low-field nuclear magnetic resonance, characterized in that: The method comprises the following steps: Step S1: sample pretreatment: dry the porous material sample to constant weight; Step S2: sample saturation treatment: mix the dried sample with excess deionized water and perform vacuum treatment to fully saturate the sample pores with water; Step S3: select a detection scheme: select a corresponding detection scheme according to the sample characteristics, including sample material and tap density, the detection scheme includes low-temperature freeze-thaw method, normal-temperature filtration method and normal-temperature centrifugation method; Step S4: nuclear magnetic resonance test: use a low-field nuclear magnetic resonance analyzer to test the saturated sample, set the CPMG sequence parameters according to the detection scheme, perform T2 relaxation time test, and obtain the nuclear magnetic resonance signal amount of the sample; Step S5: standard curve calibration: configure a series of deionized water samples with known mass, perform nuclear magnetic resonance test under the same test parameters, and establish a standard curve with water mass as the abscissa and nuclear magnetic signal amount as the ordinate to obtain a fitting equation; Step S6: convert the nuclear magnetic signal amount of the sample to be tested into saturated water mass according to the standard curve fitting equation, and calculate the porosity combined with the sample mass or volume, the porosity calculation formula is: ； or wherein .

2. The low-field nuclear magnetic based porous material porosity detection method according to claim 1, characterized in that: In step S6, the calculated porosity is also subjected to correction calculation, and the specific calculation formula is: corrected porosity = porosity × correction coefficient, wherein the correction coefficient is the proportion of the signal integral area of the bound water in the pores to the total signal integral area, or the proportion of the mass of the bound water in the pores to the total water mass.

3. The low-field nuclear magnetic based porous material porosity detection method according to claim 1, characterized in that: When the low-temperature freeze-thaw method is used for detection in step S3, the following operation steps are specifically included: Step A: place the saturated sample in a low-temperature freeze-thaw nuclear magnetic resonance nanopore analyzer, gradually reduce the temperature from 0 ℃ to – 12 ℃, and determine the freezing temperature point at which the signal amount obviously decreases; Step B: test at the freezing temperature point by gradually increasing the temperature from – 8 ℃ to 0 ℃, and record the initial signal amount at different temperatures; Step C: convert the initial signal amount at the test temperature to the 0 ℃ equivalent signal amount, and substitute it into the standard curve fitting equation to calculate the saturated water mass.

4. The low-field nuclear magnetic based porous material porosity detection method according to claim 3, characterized in that: In step C, the conversion formula is: ; wherein absolute temperature = test temperature + 273.15, and coil correction factor K = 273.

15.

5. The low-field nuclear magnetic based porous material porosity detection method according to claim 4, characterized in that: Further distinguish the inter-particle frozen water and the intra-pore frozen water, combine the T2 relaxation signal separation result, take the signal integral area corresponding to the intra-pore bound water The proportion of the total signal integral area The calculation result is corrected, The signal integral area corresponding to the inter-particle free water.

6. The low-field nuclear magnetic based porous material porosity detection method according to claim 1, characterized in that: When the normal-temperature filtration method is used for detection in step S3, the following operation steps are specifically included: Step A: shake the saturated sample and pour it into a funnel with filter paper, filter until no water drips from the filter paper; Step B: scrape the saturated sample on the filter paper into a chromatographic bottle, weigh it, and then perform nuclear magnetic resonance test; Step C: after the test, dry the sample to be tested to constant weight, and record the dried mass and saturated mass.

7. The low-field nuclear magnetic based porous material porosity detection method according to claim 1, characterized in that: When the normal-temperature centrifugation method is used for detection in step S3, the following operation steps are specifically included: Step A: place the saturated sample in a centrifuge tube, and place it in a centrifuge with a counterbalanced centrifuge tube, and centrifuge at different speeds; Step B: after centrifugation, pour out the excess liquid water, and perform nuclear magnetic resonance test on the remaining saturated sample, and record the base-saturated signal amount; Step C: dry the centrifuge tube and sample to constant weight, and weigh them after cooling, and record the dried mass and saturated mass.

8. The low-field nuclear magnetic based porous material porosity detection method according to claim 7, characterized in that: It also includes step D: one of the following two correction methods based on signal or mass analysis is used to optimize the results: Method one: based on the difference between the water content before and after centrifugation, the mass of bound water in the pores is obtained by the difference between the mass of the saturated sample after centrifugation and the mass of the dried sample , the total water content is obtained by the difference between the mass of the saturated sample before centrifugation and the mass of the dried sample , is the mass of free water between particles, and is taken The preliminary porosity is corrected by the ratio of to the total water content Method two: combined with T2 relaxation signal separation correction, through T2 spectrum separation pore bound water corresponding signal integral area and free water between particles corresponding to the signal integral area , take The proportion of the total signal integral area Correct the preliminary porosity.

9. The low-field nuclear magnetic based porous material porosity detection method according to claim 1, characterized in that: In step S4, the CPMG sequence parameters of the low-field nuclear magnetic resonance analyzer are as follows: sampling frequency SW=250 KHz, waiting time TW=350-5000 ms, radio frequency delay RFD=0.08 ms, analog gain RG1=20 db, digital gain DRG1=3, accumulation number NS=8-128, preamplifier gear PRG=3, TE echo time=0.06-0.08 ms, and NECH echo number=5000-10000.

10. The low-field nuclear magnetic based method for porosity detection of porous materials according to any one of claims 1-9, wherein: The porous material includes a silicon-based porous material for a battery negative electrode and a silicon-carbon composite porous material, and the tap density of the sample ranges from 0.247 to 0.41 g / cm3.

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