Figure 12 shows the influence of the sample thickness in the measurement of the SE. To achieve this, a CF reinforcement sample (sample 4) with a thickness of *t* = 2 mm and 3w%CNT filling is compared with two stacked sheets of the sample (sample 6). On the other hand, the red trace shows the measurement result of the two overlapping sheets. These observations suggest that the sample thickness significantly affects the shielding capability of the material. Below the reference frequency, the behavior of the traces is similar up to approximately 3.5 GHz, where the two traces diverge. It can be observed how the CF sample (black trace) presents considerable attenuation, particularly from 4 GHz. From this point up to 7.125 GHz, the black trace (sample 2) shows a linear behavior with a slightly decreasing slope, whereas the red trace (sample 6) shows a more abrupt decrease until the reference frequency, where it flattens out. Comparing the results for the selected frequency of 7.125 GHz, it can be observed that the SE value of the single-layer CF sample is −60.03 dB, whereas the SE value of the double-layer CF sample shows an increase of 21.29 dB, reaching a SE value of −81.32 dB. Moreover, it is noted that the red trace shows an increase in slope starting at approximately 16 GHz. This phenomenon may be attributed to multiple reflections that occur between the two sheets of material. 3w%CNT filling is compared with two stacked sheets of the sample (sample 6). On the other hand, the red trace shows the measurement result of the two overlapping sheets. These observations suggest that the sample thickness significantly affects the shielding capability of the material. Below the reference frequency, the behavior of the traces is similar up to approximately 3.5 GHz, where the two traces diverge. It can be observed how the CF sample (black trace) presents considerable attenuation, particularly from 4 GHz. From this point up to 7.125 GHz, the black trace (sample 2) shows a linear behavior with a slightly decreasing slope, whereas the red trace (sample 6) shows a more abrupt decrease until the reference frequency, where it flattens out. Comparing the results for the selected frequency of 7.125 GHz, it can be observed that the SE value of the single-layer CF sample is −60.03 dB, whereas the SE value of the double-layer CF sample shows an increase of 21.29 dB, reaching a SE value of −81.32 dB. Moreover, it is noted that the red trace shows an increase in slope starting at approximately 16 GHz. This phenomenon may be attributed to multiple reflections that occur between the two sheets of material.

**Figure 12.** Carbon-fiber-reinforced polymers. Comparison of one sample (black trace) and two superposed samples (red trace). **Figure 12.** Carbon-fiber-reinforced polymers. Comparison of one sample (black trace) and two superposed samples (red trace).

The following Table 5 provides a summary of the most representative results for each of the seven samples in three different frequency ranges. This table shows the maximum and minimum SE values in decibels.

The ability to detect these variations is essential for optimizing the performance of shielding materials in a given application. By identifying the frequency ranges in which a material provides the most significant attenuation in terms of attenuation, it is possible to design more effective shielding systems. These results demonstrate how this method is capable of detecting variations in the shielding performance of the material across different frequency ranges.

**Table 5.** Summary of the most representative results in three different frequency ranges.

#### **5. Conclusions**

The proposed measurement methodology shows significant advantages, including the simplicity of the sample machining, which means that very specific geometries or tiny dimensions are not required. This method allows the measurement of samples with low conductivity, contrary to the measurement method proposed in the ASTM D4935 standard. The sample insertion and removal process can be completed within a matter of seconds, which supposes that a wide range of measurements can be made with reasonable speed due to the easiness of the measurement procedure. The measured dynamic range is approximately −100 dB, allowing us to analyze the samples developed in this study in the frequency range where 5G technology operates.

On the other hand, EMI shielding effectiveness in the 700 MHz–18 GHz frequency range has been studied. Two types of materials have been compared, depending on the presence of fiber reinforcement. It has been observed the performance of the materials studied in a wider frequency spectrum than specified by ASTM4935-18 standard and controlling the surrounding effects by using the absorber box method.

Some of the samples analyzed have demonstrated to provide a significant attenuation. For those composites based on a polymer matrix with different concentrations of CNT, a value of −81.30 dB has been obtained for the frequency of 7.125 GHz for the 15w%CNT composite, which is a very significant SE value considering the nature of these materials. Whilst it is true that an increased volume fraction of filler may lead to a decrease in the mechanical performance of the host matrix by means of deterioration of its inherent morphology, it is necessary to incorporate a higher filler content in order to achieve higher SE. It is desirable to employ CNT/polymer composites at low filler loadings to produce cost-effective and versatile conductive composites.

The inclusion of a carbon fiber reinforcement has proven to be an effective strategy for achieving significant attenuation in composite materials, providing higher attenuation than the GF-reinforced composite with the same CNT concentration. This approach offers an advantage over using a high percentage of filler, which may lead to undesirable mechanical properties. This is due to the highly conductive nature of carbon fibers, which can effectively attenuate electromagnetic radiation. Furthermore, using CF reinforcement can also provide additional benefits such as increased stiffness and strength, as well as reduced weight. This, in turn, makes these materials an excellent alternative to replace traditional shielding materials.

It has to be highlighted that these types of characterizations are very relevant from a technological and industrial point of view. Specifically, for those sectors related to 5G technology, since the use of EMI shielding based on plastic materials has many advantages, such as manufacturing cost reduction.

**Author Contributions:** Conceptualization, A.S., J.T., A.B. and R.R.; Methodology, A.A. (Andrea Amaro), P.A.M. and A.A. (Antonio Alcarria); Validation, A.S., J.T. and P.A.M.; Formal analysis, A.A. (Andrea Amaro), R.H. and A.A. (Antonio Alcarria); Investigation, A.A. (Andrea Amaro), A.S. and R.H.; Resources, A.B., R.R., P.G. and A.P.; Writing—original draft, A.A. (Andrea Amaro), A.S., R.H., P.G. and A.P.; Writing—review & editing, J.T., P.A.M, A.B. and R.R.; Project administration, J.T.; Funding acquisition, J.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The project with reference PLEC2021-007994 has been funded by MCIN/AEI/10.13039/<PHONE_NUMBER>33 and by the European Union NextGenerationEU/PRTR.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.


*Editorial* **Special Issue on New Advances and Illustrations in Applied Geochemistry in China**

**Qingjie Gong 1,\* and Zeming Shi 2,\***

## **1. Introduction**

The 9th national conference on applied geochemistry in China will be held in Chengdu, Sichuan province, in October 2023, hosted by the committee of applied geochemistry, the Chinese Society for Mineralogy, Petrology and Geochemistry (CSMPG). In order to facilitate the academic exchange on applied geochemistry, this Special Issue is intended for the presentation of new advances and case illustrations in the field of applied geochemistry.

This Special Issue gathers 14 scientific papers that capture various hot and challenging issues in applied geochemistry. These articles belong to three broad groups: (i) new advances and case illustrations in resource exploration; (ii) new advances and case illustrations in environment assessment; and (iii) new advances in the basic theory of geochemistry.

## **2. Resource Exploration**

Geochemical exploration has played an important role in resource prospecting. This issue includes six papers on geochemical exploration. As discoveries of world-class mineral deposits continue to decline, shallow deposits are no longer enough to meet the daily needs of human beings, and exploration in covered areas has received increasing attention. The first paper, authored by J. Li, B. Zhang, Q. Gong, H. Liu, and N. Liu, presents the finegrained soil-prospecting method in the Jiaojia gold deposit in Shandong province [1], where three lines of evidence on gold concentrations, microscopic morphology, and geochemical lithogenes were illustrated to explain the gold migration from ore to surface soils in a covered area. The second paper introduces the theory of the geoelectrochemical exploration method and presents three case studies on gold prospecting in covered areas [2], authored by M. Kang, H. Guo, W. Zhu, X. Luo, and J. Yang. Unlike the above two papers, the third paper, authored by B. Zhou, Z. Zhang, Z. Shi, H. Song, and L. Yu, focuses on the geochemistry and geochronology of a pluton in western Kunlun orogen [3], where the formation of the pluton and its mineralization potential on polymetallic deposits were illustrated. The paper by T. Dong, L. Kang, Y. Zhang, and Y. Gao presents prospecting on shale oil [4], where shales with higher TOC and lower pore fractal parameters in the Songliao basin, NE China, are favorable reservoirs.

The remaining two papers focus on determining geochemical anomalies in geochemical survey data. The first paper, authored by B. Zhao, D. Zhang, R. Zhang, Z. Li, P. Tang, and H. Wan, describes an advanced workflow of an object-based geochemical graph learning approach [5], where a case study on multiple mineral anomalies was illustrated in Southeastern Inner Mongolia, North China. The other paper illustrates a creative method to determine the tungsten composite anomalies using the geochemical gene technique [6], authored by J. Li, Q. Gong, B. Zhang, N. Liu, X. Wu, T. Yan, X. Li, and Y. Wu. The mineralization similarities of the geochemical metallogene can be viewed as useful integrated indices on geochemical tungsten exploration with the elimination of the lithological influence.

**Citation:** Gong, Q.; Shi, Z. Special Issue on New Advances and Illustrations in Applied Geochemistry in China. *Appl. Sci.* **2023**, *13*, 8220. https://doi.org/ 10.3390/app13148220

Received: 11 July 2023 Accepted: 12 July 2023 Published: 15 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

## **3. Environment Assessment**

The soil quality survey and environment assessment are enduring topics in applied geochemistry. This issue includes three papers that focus on the evaluation of soil nutrient status, pollution assessment of heavy metals, and geochemical survey of geoherbs, respectively.

The first paper, authored by J. Zhang, Y. Liu, S. Hong, M. Wen, C. Zheng, and P. Liu, presents the geochemical soil survey of heavy metals in a typical mining area [7], where single and integrated pollution indices were used to assess the environmental quality of soils, and the speciation of heavy metals were analyzed to discover their mobility. The second paper presents a LightGBM model to assess the soil nutrient status, authored by Z. Liang, T. Zou, J. Gong, M. Zhou, W. Shen, and J. Zhang [8], where the tobacco planting soil in Debao County, Guangxi province, was illustrated as an example. Finally, the quality of the top-geoherb in Chengde City, Hebei province, was geochemically surveyed in the paper authored by Z. Sun, W. Shen, W. Fang, H. Zhang, Z. Chen, L. Xiong, and T [9], where the geochemical signatures of trace elements (especially rare earth elements) and medicinal components of Scutellaria baicalensis were described.

## **4. Basic Theory**

In this Special Issue, five papers focus on the basic theory of geochemistry, such as the elemental abundance, the equation of state of multicomponent fluids, geochemical indices and mechanism, and a new geochemical concept.

The successful return of Chang'E-5 (CE5) samples implores the study of the Moon in geochemistry. The paper authored by Z. Hou, Q. Gong, N. Liu, B. Jiang, J. Li, Y. Wu, and J. Huang compiled the reported geochemical data on Moon samples and derived the elemental abundance of Moon samples [10], where the statistical distributions of normal, log-normal, and additive log-ratio transformation methods were used to derive the elemental abundance of major components, trace elements, and rare earth elements, respectively. Furthermore, Moon samples were classified into two types, low-Ca and high-Ca samples.

An equation of state (EOS) of CH4-N2 fluid mixtures in terms of Helmholtz free energy is presented in the paper authored by J. Zhang, S. Mao, and Z. Shi [11], which can reproduce the pressure–volume–temperature–composition (PVTx) and vapor–liquid equilibrium (VLE) properties of CH4-N2 fluid mixtures. Dendrite is a typical self-similar morphology. The paper authored by N. Liu, Y. He, S. Xu, L. Xiong, Y. Wei, J. Li, P. Li, and Q. Gong presents a dendrite developed along the bedding plane in limestone and dolomite strata in Zhoukoudian, Beijing city [12], where the formation's dynamic mechanism was simulated on the diffusion-limited aggregation method on a model of fractal growth.

Sediment provenance is a main topic in research on soils, sediments, and sedimentary rocks. The paper authored by Z. Dong, L. Zhang, and B. Jin illustrates a case study on sediment provenance along the Subei coast to the Yangtze estuary [13], where the detrital mica composition was used as an efficient approach in sediment provenance analysis and transport tracing. With respect to the provenance and tracing of geological materials, a promising concept, the geochemical gene, has been presented recently [14], which can be used not only to classify geological materials [1,15] but also to determine geochemical anomalies [6]. The last paper, authored by Y. An, X. Yin, Q. Gong, X. Li, and N. Liu, presents an excellent illustration of the classification and provenance of geological materials [16], where the rock–soil–sediment system in the Wangquan area of Zhangjiakou, North China, was investigated and traced on geochemical lithogenes.

Although this Special Issue has been closed, more in-depth research into new advances and illustrations in applied geochemistry are expected to be presented at the national conference on applied geochemistry.

**Author Contributions:** Q.G.: Conceptualization, Writing—original draft preparation. Z.S.: Methodology, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We are grateful to all contributors who made this Special Issue a success. Our thanks and congratulations are extended to all the authors for submitting their work. Our sincere gratefulness is also put forward to all reviewers for their effort and time spent to help the authors improve their papers. We want to express our gratitude to the editorial team of *Applied Sciences* for their effective and untiring editorial support for the success of this issue. We hope this issue serves as an inspiration for future research in applied geochemistry.

**Conflicts of Interest:** The author declares no conflict of interest.

## **References**

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.