Li-li ZHU, Bin ZHANG

(1Chongqing Institute of Engineering, Chongqing 402260, China) (2Chongqing Aerospace Polytechnic, Chongqing 400021, China)

Abstract: A portable NMR sensor for rapidly nondestructive measurement of the aging status of rubber material was presented. The NMR sensor consists of a magnet structure which is made up of eight cylindrical magnets and a figure’8’ RF coil. Rubber material samples with different aging status were measured, the result demonstrates that: this sensor can accurately measure the equivalent transverse relaxation time T2eff of the samples, and distinguish the aging degree of different samples based on their T2eff values.

Key words: Single-sided NMR sensor, Rubber aging testing, Transverse relaxation time

1 Introduction

The state of aging of rubber is directly related to its service life. How to accurately evaluate tires, electrical insulation, and other aging state of rubber materials has always been a concern in the industry. NMR technology can measure the content of H protons in rubber materials and the chemical structure of H-containing groups. It is an accurate method for analyzing rubber materials. However, the conventional NMR measurement method needs to cut the sample into an appropriate shape and put it into the measurement cavity of the NMR apparatus, which is a lossy measurement method. In addition, the traditional MRI apparatus is bulky and expensive, and it cannot meet the requirements of non-destructive and rapid measurement of the project site. In view of this situation, this paper studies a portable single-sided nuclear magnetic resonance sensor. In recent years, single-sided NMR instruments [1-4] have received more and more attention. Compared with traditional closed nuclear magnetic resonance apparatus, its advantages are: small size, easy to carry, and open structure, making it possible to measure large and irregularly shaped samples. Therefore, single-sided NMR techniques should be widely applied in many biomedical diagnosis [4,6], geophysics [4,7], materials fields such as food analysis, quality control [4-5], science [4,8] and other fields.

The single-sided NMR sensor is mainly composed of a single-sided magnet structure that generates a main magnetic field, and an RF coil that generates a RF excitation magnetic field. Typical single-sided magnet structures are mainly barrel-shaped [4], U-shaped [9], Halbach [10] and so on. This dissertation improves on the basis of the structure of Halbach magnets, and selects planar 8-shaped coils as RF coils. A portable single-sided nuclear magnetic resonance sensor for detecting the aging of rubber materials is designed.

2 Single-sided NMR sensor

The single-sided nuclear magnetic resonance sensor is mainly composed of a main magnet, a radio frequency coil and a tuning matching circuit. The main magnet generates a main magnetic field B0. The radio frequency coil generates a radio frequency field B1. The B0 field and the B1 field are orthogonal in the target area, and the tuning and matching circuit realizes the radio frequency energy in order to maximize power output.

(1) Single-sided NMR sensor structure

The traditional Halbach magnet structure is closed and it is not convenient to measure irregular samples with large volume and shape. Based on the Halbach structure, this article designs a portable single-sided magnet structure with 8 cylindrical magnetic rods. The 3D structure model is shown in Fig.1(a). Each unit magnetic bar has a diameter of 20 mm, a height of 34 mm, and a remanence of 1.12 t. The red area in Fig.1(a) is a sensitive area for measurement with a diameter of 10mm and a height of 2 mm, and is located above the upper surface of the magnet. Fig.1(b) is a top view of the magnet structure, where the arrows indicate the magnetization direction of each unit magnet bar.

Fig.1 Magnet structure

The static magnetic field distribution in the sensitive area is shown in Fig.2. Fig.2(a) is a static magnetic field distribution diagram inXOYplane. The magnetic field intensity at the center point is 196 mT, and the corresponding NMR Larmor frequency is 8.35 MHz. Fig.2(b) shows the distribution of the magnetic field in theYOZplane. The static magnetic field exhibits a gradient change on theY=0 axis. The value of the magnetic field on this axis is shown in Fig.2(c). This gradient of the magnetic field distribution can be used to achieve a layered sample measurement in the height direction.

Fig.2 Magnet field distribution in different planes

(2) RF coil

Because the main magnetic field is perpendicular to the surface of the magnet structure, the 8-shaped coil that generates the horizontal RF magnetic field is selected as the RF excitation and detection coil. The size of the RF coil is 18 mm×16 mm, and the line width and line spacing are 0.5mm for a total of 4 turns. The RF coil at 8.35 MHz has a resistanceR=640 mΩ and an inductanceL=525.1 nH.

(3) Tuning matching circuit

The RF coil is equivalent to a series circuit of resistors and inductors, and a π-shaped tuning matching circuit (shown in Fig.3) constitutes a resonant circuit. By adjusting the capacitance value, the resonant frequency is adjusted to the nuclear Larmor frequency, and the RF coil’s matching impedance is 50 Ω, ensuring no energy reflection.

Fig.3 Equivalent circuit of tuning and matching circuit

The final physical model is shown in Fig.4. The RF coil is placed on the upper surface of the magnet structure. The matching circuit is placed in the cavity of the magnet structure below the RF coil. The sample is placed on the surface of the RF coil during measurement. The overall structure of the sensor is cylindrical, with a diameter of 100 mm, a height of 68mm and a weight of about 2kg. The screw on the outside of the cylinder in the figure is used to fix the cylindrical permanent magnet.

Fig.4 Single-sided NMR sensor

3 Single-sided NMR experiment

This experiment uses the CPMG pulse sequence (as shown in Figure 5), measured the echo signals of different rubber samples, and plotted the peak value of the echo signal as shown by the broken line in the Fig.5. For different aged samples, the decay rate of the peak value of the echo signal is different. We extract the equivalent transverse relaxation time parameter that reflects the decay rate, which is used to characterize the aging of rubber samples.

Fig.5 CPMG pulse sequence

(1) Equivalent transverse relaxation time measurement of rubber samples

We measured the new silicone rubber and the 5-year-old silicone rubber material. Fig.6 shows the peak attenuation curve of echo signals for both samples. It can be seen that the red curve (5 years) decays faster than the black curve (0 year).

Fig.6 The equivalent transverse relaxation time attenuation curves of different silicone rubber composite insulators

By exponentially decaying the curve shown in Fig.6, we obtain a equivalent lateral relaxation timeT2effof new (0 year) silicone rubber material of 110.5±0.4 ms, and the equivalent lateral transverse relaxation timeT2effof a 5-year silicone rubber composite insulator is 94.2±0.9 ms. The results show that the equivalent transverse relaxation time of the aged silicone rubber material is obviously smaller than that of the new material, which proves that the single-sided NMR sensor designed in this paper can distinguish the different aging degrees of the silicone rubber material.

(2) One-dimensional hierarchical model recognition experiment

The magnet structure of this paper has a natural gradient and can be used as a one-dimensional layered model recognition experiments. The magnetic field gradient in the magnetic field sensitivity region is not constant. Analysis of magnetic field distribution trends revealed that the magnetic field was approximately constant gradient 1.5 t/m attenuation (frequency change rate 63.87 kHz/mm) in the range of 0～5 mm, and approximately constant gradient 2 t/m attenuation (frequency change rate 85.16 kHz/mm) in the range of 5～1 mm. Three rubber pieces with a diameter of 10 mm and a thickness of 0.2 mm and three glass pieces with a thickness of 0.14 mm were selected. The rubber and the glass pieces were alternately placed, as shown in Fig.7(a).

Fig.7 Diagram and result of three layered samples

The layered sample was placed on the RF coil surface and excited with a CPMG pulse. The excitation frequency was the frequency corresponding to the center point of the stratified sample. The identification of the three-layer sample is shown in Fig.7(b). It can be clearly seen from the figure that three rubbers correspond to 3 peaks in the recognition result, and the middle peak is the largest because the excitation frequency is closest to the middle layer rubber. When the main magnetic fieldGis known, the spatial distance Δdand the resonance frequency width Δfhave the following relationship:

Δf=Δd·G·γ

The1Hgyromagnetic ratioγis 42.58 MHz. The theoretical values of the frequency widths of the first and third rubber layers are 12.8 kHz and 17.0 kHz, respectively. The rubber of the second layer is at the turning point where the gradient value changes. Taking the average of the two gradient values as the gradient of the magnetic field where the second layer of rubber is located, the theoretical value of the rubber frequency width of the second layer is 14.9 kHz. Fig.7(b) calculates the experimental value of the rubber frequency width of each layer and finds that the frequency widths of the three layers are 14.8 kHz, 16.6 kHz, and 19.4 kHz, respectively, and the theoretical error is 15.9%, 11.7%, and 14.1%, respectively. Taking into account the influence of the main magnetic field in the horizontal plane and the inhomogeneity of the RF field, it is considered that the sensor can better identify the spatial position of the stratified sample, which provides the possibility of measuring aging degrees of different depth in the sample.

4 Conclusion

In this dissertation (I), a portable single-sided magnet structure was designed to measure the columnar area of the sensitive area above the upper surface of the magnet structure with a diameter of 10 mm and a height of 2 mm. The magnet and the 8-shaped RF coil form a unilateral nuclear magnetic resonance sensor. The total weight of the sensor is about 2 kg. (II) The sensor was used to measure two different degrees of aging of the silicone rubber material. The results show that the equivalent transverse relaxation timeT2effof the 5-year silicone rubber material is obviously smaller than that of the new silicone rubber material. The degree of aging of the rubber material can be quantitatively evaluated in the project. (III) The natural gradient of the sensor is used to measure the one-dimensional layered sample. The sensor can better identify the spatial location of the layered sample, which provides the possibility to measure aging degree of different depth in the sample.

In summary, the single-sided NMR sensor has an open structure, is easy to carry, and can be used to quantitatively analyze different degrees of aging state of rubber materials.

Acknowledgements

This work supported by the Science and Technology Major Special Project of Guangxi, China (GKAA No.17129002).