Flash Sintering Research Perspective: A Bibliometric Analysis

  Citation: Gil-González, E.; Pérez-Maqueda, L.A.; Sánchez-Jiménez, P.E.; Perejón, A. Flash Sintering Research Perspective: A Bibliometric Analysis. Materials 2022,15, 416. https://doi.org/ 10.3390/ma15020416 Academic Editors: Mattia Biesuz and Peter Tatarko Received: 9 November 2021 Accepted: 2 January 2022 Published: 6 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 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/). materials Perspective Flash Sintering Research Perspective: A Bibliometric Analysis Eva Gil-González 1,2,* , Luis A. Pérez-Maqueda 1,* , Pedro E. Sánchez-Jiménez 1,3,* and Antonio Perejón1,3 1 Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas − Universidad de Sevilla, Calle Américo Vespucio 49, 41092 Sevilla, Spain; [email protected] 2Departamento de Ingeniería Química, Universidad de Sevilla, Escuela Politécnica Superior, Calle Virgen de África, 7, 41011 Sevilla, Spain 3Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, 41071 Sevilla, Spain *Correspondence: [email protected] (E.G.-G.); [email protected] (L.A.P.-M.); [email protected] (P.E.S.-J.) Abstract: Flash Sintering (FS), a relatively new Field-Assisted Sintering Technique (FAST) for ceramic processing, was proposed for the first time in 2010 by Prof. Rishi Raj’s group from the University of Colorado at Boulder. It quickly grabbed the attention of the scientific community and since then, the field has rapidly evolved, constituting a true milestone in materials processing with the number of publications growing year by year. Moreover, nowadays, there is already a scientific community devoted to FS. In this work, a general picture of the scientific landscape of FS is drawn by bibliometric analysis. The target sources, the most relevant documents, hot and trending topics as well as the social networking of FS are unveiled. A separate bibliometric analysis is also provided for Reaction or Reactive Flash Sintering (RFS), where not only the sintering, but also the synthesis is merged into a single step. To the best of our knowledge, this is the first study of this nature carried out in this field of research and it can constitute a useful tool for researchers to be quickly updated with FS as well as to strategize future research and publishing approaches. Keywords: flash sintering; bibliometric analysis; field assisted sintering; knowledge structure; ceramic materials 1. Introduction Flash Sintering (FS), an electric Field-Assisted Sintering Technique (FAST) [ 1 ] for the densification of ceramic materials at a greatly reduced temperature and time, has gained widespread attention since it was established in 2010 by Prof. Rishi Raj’s group from the University of Colorado at Boulder. It basically consists of simultaneously applying heat and a modest electric field to a green body [ 2 ] placed on a furnace, allowing the current to totally flow through the sample. The main material requirement is that it should possess a negative temperature coefficient of electrical resistance so that the electrical conductivity increases while heating. At a given applied electric field, there is a critical temperature at which there is a sudden non-lineal rise of the conductivity of the material, which is normally accompanied by instantaneous densification as well as photoluminescence [ 3 ]. This signals the flash event and it is now accepted that it is initiated by a thermal runaway induced by Joule Heating [ 4 , 5 ]. Much effort is also being devoted to understanding the underlying mechanisms of FS. A few driven mechanisms have been proposed, but none of them can solely explain the flash phenomenon, which still remains elusive [6–11]. Nevertheless, FS has many practical advantages. For instance, it has been proven to be an ecofriendly and versatile methodology, as a wide range of materials from insulators to conductors can be sintered within seconds at furnace temperature much lower than those employed in conventional processing [ 12 – 17 ], thereby minimizing the energy footprint. Additionally, in comparison to other FAST techniques such as Spark Plasma Sintering (SPS) [ 1 ], it does not require any sophisticated experimental setup; basically just a furnace and a power supply are the two essential components to carry out a FS experiment [ 18 ]. Materials 2022,15, 416. https://doi.org/10.3390/ma15020416 https://www.mdpi.com/journal/materials Materials 2022,15, 416 2 of 20 An inner working atmosphere is not a prerequisite either, but indeed it is possible to tune it to study its effect over the final properties of the material [ 14 , 19 , 20 ]. Moreover, some flashsintered materials have been granted special properties [ 21 , 22 ] and it has been shown that it is possible to sinter unstable oxides with volatile components and complex composition while preserving their stoichiometry and properties [ 23 – 26 ]. Very recently, in 2018, it was reported that sintering and synthesis can be merged into a single step, giving rise to what has been named Reaction or Reactive Flash Sintering (RFS) and constitutes [ 27 ], by itself, another important branch of research. Given the advantages of FS, the widespread attention that it has received not only from the scientific community but also from the industry is not surprising [ 28 ]. Since it was proposed for the first time a decade ago, this research field has deeply evolved, constituting a true milestone in ceramic processing. The number of peer-reviewed articles has continued dramatically growing year by year with an annual growth rate of about 46% (see Figure 1). To the best of our knowledge, bibliometric analysis [ 29 ], which is a powerful tool to quantitatively identify essential variables in a particular research topic (such as top authors, institutions, trends of publication, collaboration, networking, etc.) has never been carried out on FS or RFS. Hence, the aim of this work is to gain insight into the knowledge structure, research perspective, and trends of this fascinating research field. It is noteworthy that the readings of these review articles are highly encouraged [ 10 , 18 , 28 , 30 , 31 ], as a comprehensive literature review is out of the scope of this work. The Biblioshiny web interface of the Bibliometrix R package [ 32 ] has been employed to conduct the bibliometric analysis. Peer-reviewed articles and reviews retrieved from the Web of Science (WoS) database have been analyzed based on several bibliometric indicators [ 33 ], i.e., quantity (productivity), quality (impact), and structural (networking) indicators. The results derived from this bibliometric analysis allow one to identify the target sources and trend topics in order to strategize future research and publishing approaches. Those publications that most influence this research field have been pointed out. Authors, institutions, and countries in terms of impact and the number of publications have also been identified. Additionally, the social structure of FS is shown by the clustering and collaboration networking from countries to institutions to authors. Moreover, a brief bibliometric analysis is also presented independently for RFS, given its impact and the scientific growth that it has experienced in a very short lifetime (just 3 years). Materials 2022, 15, x FOR PEER REVIEW 6 of 21 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 0 10 20 30 40 50 60 70 80 Annual Growth rate 46.56% Number of publications Year 0 50 100 150 200 250 300 350 Cumulative publications Figure 1. FS annual and cumulative publications from 2010 to December 2021. 3.1.3. Source-Level Analysis As mentioned, just peer-reviewed articles have been considered in this literature set. The analysis reveals that the documents have been published in 63 different journals (Table 1). Nevertheless, a close examination of Figure 2, which includes the sources where most of the articles have been published, shows that FS documents are concentrated in a few journals, i.e., Journal of European Ceramic Society and Journal of the American Ceramic Society. Indeed, those two are the core journals where more than one-third of the entire collection has been published. Additionally, these two journals together with Scripta Materialia (Letters journal of Acta Materialia), Ceramics International, and Acta Materialia have published more than 65% of the analyzed set of documents. These journals are mainly targeted towards ceramic materials as well as their relationship between processing, microstructure, and properties, which makes sense as the vast majority of flash-sintered materials are ceramics provided that a negative temperature coefficient of electrical resistance is possessed [28]. It is noteworthy that these five journals are top-ranked journals of the first quartile according to the Journal Citation Report (2020) in the category of Material Science–Ceramics (Journal of European Ceramic Society, Ceramics International, and Journal of the American Ceramic Society) or Metallurgy and Metallurgical Engineering-Sci- ence (Scripta and Acta Materialia). Reciprocally, these journals are also the most locally cited sources in the set of documents analyzed (see Table S1). A locally cited source is a journal included in at least one of the reference lists of the analyzed document collection. Additionally, there are also articles devoted to FS in general high-impact journals such as Nature Communications [74] or Science Advances [22]. This simple source-level analysis shows that FS research is mainly published in highranked, peer-reviewed journals and provides an idea about the high quality of the research carried out in this field and the interest that it generates and attracts within the scientific community. Figure 1. FS annual and cumulative publications from 2010 to December 2021. This article is structured in several sections. The section herein provides a basic and brief introduction about flash sintering, referring to the most relevant literature. The second section explains in detail the methodology used to carry out the bibliometric analysis. The Materials 2022,15, 416 3 of 20 third section provides the most relevant results of this work (target sources, authors, most influential publications, and hot and trending topics) along with data visualization and social networking. The last part of this section is dedicated to RFS. Finally, the conclusions and final remarks are presented. 2. Methods This section presents the design of this bibliometric study, which is schematically depicted in Scheme 1. As explained as follows, it implicitly contains the standard stages of bibliometric analysis, that is to say, the study design, data collection, data analysis, visualization, and interpretation [34,35]. Materials 2022, 15, x FOR PEER REVIEW 3 of 21 documents that contain these words in their titles, abstracts, or keywords were scanned. The search was restricted to Flash Sintering, excluding other derived hybrid methodologies such as Flash Spark Plasma Sintering (F-SPS) [36,37], Flash Microwave Sintering [38,39], or Ultrafast High-Temperature Sintering [40], as has been properly indicated in Scheme 1. The reaction or reactive flash sintering documents were also excluded, as a separate bibliometric analysis is provided about this topic (see Table S5). The document types were limited to Articles or Reviews and the time span from 2010 to 15 July 2021, whereas the language was set to English. Finally, manual abstract assessment and screening were performed for each document, resulting in a list of 318 documents, of which the meta-data fields were compiled to conduct the bibliometric analysis. 2.2. Bibliometric Analysis The data analysis was performed with the web interface Biblioshiny of the Bibliometrix R package (Version 1.4). The meta-data files extracted from the 318 documents were converted into R-readable files. Biblioshiny allows an analysis of three metric levels: Sources, authors, and documents. Hence, the most relevant sources, top authors, and documents in terms of productivity and impact can be identified. Additionally, the knowledge structure can be conceptual, intellectual, and socially analyzed, so that thematic mapping and evolution, co-citation, and collaboration networks can be built, among other things. The metrics analyzed and the derived results from this analysis are schematically depicted in the workflow diagram in Scheme 1. The source analysis points out the target sources, which are the journals where most of the documents are published. The author analysis as well as their countries and affiliations allow one to identify not only the most prolific authors but also the social structure of the FS community, showing co-authorship, institutional, and international networking. The document-level study identifies the most important publications in the field, while the keyword analysis provides a general overview of the most-studied topics and evolution over the years, which makes it possible to identify mainstream and trend topics. Scheme 1. Study design and workflow diagram. 2.1. Document Search Clarivate Analytics Web of Science (WoS), one of the largest databases of peer-reviewed articles in different disciplines, was used for the data collection. The documents included in WoS related to the topic FS were retrieved according to the search strategy shown in Scheme 1, which also includes the specific search query words and Boolean operators. As a search topic, the words Flash Sintering were used, which implies that WoS documents that contain these words in their titles, abstracts, or keywords were scanned. The search was restricted to Flash Sintering, excluding other derived hybrid methodologies such as Flash Spark Plasma Sintering (F-SPS) [ 36 , 37 ], Flash Microwave Sintering [ 38 , 39 ], or Ultrafast High-Temperature Sintering [ 40 ], as has been properly indicated in Scheme 1. The reaction or reactive flash sintering documents were also excluded, as a separate bibliometric analysis is provided about this topic (see Table S5). The document types were limited to Articles or Reviews and the time span from 2010 to 15 July 2021, whereas the language was set to English. Finally, manual abstract assessment and screening were performed for each document, resulting in a list of 318 documents, of which the meta-data fields were compiled to conduct the bibliometric analysis. Materials 2022,15, 416 4 of 20 2.2. Bibliometric Analysis The data analysis was performed with the web interface Biblioshiny of the Bibliometrix R package (Version 1.4). The meta-data files extracted from the 318 documents were converted into R-readable files. Biblioshiny allows an analysis of three metric levels: Sources, authors, and documents. Hence, the most relevant sources, top authors, and documents in terms of productivity and impact can be identified. Additionally, the knowledge structure can be conceptual, intellectual, and socially analyzed, so that thematic mapping and evolution, co-citation, and collaboration networks can be built, among other things. The metrics analyzed and the derived results from this analysis are schematically depicted in the workflow diagram in Scheme 1. The source analysis points out the target sources, which are the journals where most of the documents are published. The author analysis as well as their countries and affiliations allow one to identify not only the most prolific authors but also the social structure of the FS community, showing co-authorship, institutional, and international networking. The document-level study identifies the most important publications in the field, while the keyword analysis provides a general overview of the most-studied topics and evolution over the years, which makes it possible to identify mainstream and trend topics. 3. Results and Discussion 3.1. Flash Sintering (FS) 3.1.1. General Descriptive Information Table 1summarizes the essential information extracted from the analysis of the set of 318 documents retrieved from WoS from 2010 to 2021, using the word search query indicated in Scheme 1. Detailed information about each metric level is commented as follows. Table 1. Main information. Description Results Period 2010–2021 Documents 318 Sources (Journals, Books, etc.) 63 Average citations per documents 25.07 References 4852 Authors Authors 670 Author appearances 1498 Authors of single-authored documents 7 Authors of multi-authored documents 663 Authors Collaboration Single-authored documents 17 Documents per Author 0.475 Authors per Document 2.11 Co-Authors per Documents 4.71 Collaboration Index 2.2 3.1.2. Scientific Production FS has constituted a breakthrough in material processing, and since its inception in 2010, it has become a hot topic and a new paradigm in ceramic processing. As it is shown in Figure 1, the number of publications has kept on growing, with an average of approximately 32 documents per year and an annual growth rate of 46.56%. Note that hybrid methodologies such as F-SPS and Reaction or Reactive FS (RFS) have been excluded and only peer-reviewed articles and reviews have been considered. Indeed, the cumulative publications since 2010 follow an exponential trend, as Figure 1shows. If the topic development continues along the same trend, by a simple fitting to an exponential Materials 2022,15, 416 5 of 20 growth equation, the predicted number of publications in 10 years will be 4400, one order of magnitude higher than nowadays. There are probably several reasons behind such dramatic growth in such a short period of its lifetime. The main reason is, of course, the scientific relevance of FS for the scientific community, mainly for those interested in ceramic research. Moreover, the simplicity of the FS experimental setup is significant, and as mentioned, just a furnace and a power supply are strictly needed, unlike other FAST methodologies that require complex and expensive equipment [ 1 ]. Despite its simplicity, FS is an extremely powerful sintering method that can be successfully used for most ceramic materials, from dielectrics (BaTiO 3 [ 41 – 47 ] or (Bi 0.2 Na 0.2 K 0.2 Ba 0.2 Ca 0.2 )TiO 3 [ 48 ]) to ionic (Zirconia, YSZ [ 2 , 49 – 52 ], CeO 2 or doped- CeO 2 [ 53 – 58 ]) or electronic (TiO 2 [ 19 , 22 , 59 – 62 ], BiFeO 3 or substituted-BiFeO 3 [ 24 , 27 ]) conductors. Interestingly, it can be also applied for processing ceramic composites of complex stoichiometry, metastable phases, or materials constituted by volatile species at the temperatures required for their sintering such as YSZ-Al 2 O 3 composites [ 63 – 65 ], different types of solid state electrolytes [ 25 , 66 , 67 ], BiFeO 3 [ 68 , 69 ], or K 0.5 Na 0.5 NbO 3 [ 26 , 70 – 73 ]. Moreover, ceramics prepared by FS present very interesting properties rarely reported for materials obtained by convectional procedures. For example, it has been observed that FS specimens deform plastically before fracture when compressed at high strain, due to their extraordinarily high density of defects, such as stacking faults, dislocations, and twins [ 22 , 74 ] or that chemically inert ceramics are converted into active catalytic compounds by enhancing the concentration and reactivity of the ionic species [ 21 , 75 ]. Furthermore, the understanding of the flash phenomena still remains elusive and requires contributions from different scientific fields. This challenge has raised the interest of many researchers with different expertise, including theoreticians and experimentalists [ 76 – 79 ]. Another significant feature of FS is its ecofriendly nature and potential to possibly scale-up, as it requires less energy than conventional processing processes [ 80 ] and, therefore, it contributes to reducing CO 2 emissions. The possibility of working under continuous FS with rolling electrodes has been proposed in the literature [ 81 ]. This idea is already being explored at a larger scale by the company Lucideon [ 82 ] in a pilot plant. Very recently, the possibility of homogenously sintering 3D-complex shaped ceramics by the application of a three-phase power supply has been reported, giving rise to what has been named Multiphase Flash Sintering (MPFS) [ 83 ]. MPFS is presented as a feasible option to overcome the shape restrictions of conventional FS specimens. Moreover, the capabilities of FS combined with other FAST techniques are evolving in other interesting “flash-based methodologies” that have experienced great development as well. That is the case of Flash Spark Plasma Sintering (F-SPS), where pressure and pulsed currents are simultaneously applied, enabling the homogeneous and energetically efficient sintering (by different electrodes architectures) of both electric conductive and insulating materials [ 36 , 37 , 84 – 88 ]. Contactless Flash Sintering (Contactless-FS) is another flash-based methodology. Plasma electrodes are used instead of the traditional metallic wires. The plasma not only heats the material but also carries the current to trigger the flash, minimizing some of the thermal management issues encountered in conventional flash sintering due to the sample–electrodes contact [ 76 ]. Similarly, Flame-assisted Flash Sintering (FAFS) uses a flame as an electrode and heating source [ 89 ]. It has proved to be an effective technique for the sintering of ceramic coatings on metallic substrates. The combination of FS with Cold Sintering has resulted in Cold Flash Sintering (CFS), where the presence of relatively small amounts of liquids, such as water of acetic acid, on the pellets are used as electrolytes and enables the flash event even at room temperature [ 90 , 91 ]. Last but not least, the efforts made by the pioneers in the field, Prof. Raj and others, have been relevant by spreading the topic through the scientific community by inviting visitors to their labs, collaborating with other groups, giving lectures, and organizing successful International Conferences on the topic “Electromagnetic/Electric Fields in materials processing” such as those held on 2016 and 2019 in Tomar (Portugal) or the symposiums arranged by the Materials Research Society (MRS). Materials 2022,15, 416 6 of 20 3.1.3. Source-Level Analysis As mentioned, just peer-reviewed articles have been considered in this literature set. The analysis reveals that the documents have been published in 63 different journals (Table 1). Nevertheless, a close examination of Figure 2, which includes the sources where most of the articles have been published, shows that FS documents are concentrated in a few journals, i.e., Journal of European Ceramic Society and Journal of the American Ceramic Society. Indeed, those two are the core journals where more than one-third of the entire collection has been published. Additionally, these two journals together with Scripta Materialia (Letters journal of Acta Materialia),Ceramics International, and Acta Materialia have published more than 65% of the analyzed set of documents. These journals are mainly targeted towards ceramic materials as well as their relationship between processing, microstructure, and properties, which makes sense as the vast majority of flash-sintered materials are ceramics provided that a negative temperature coefficient of electrical resistance is possessed [ 28 ]. It is noteworthy that these five journals are top-ranked journals of the first quartile according to the Journal Citation Report (2020) in the category of Material Science–Ceramics (Journal of European Ceramic Society,Ceramics International, and Journal of the American Ceramic Society) or Metallurgy and Metallurgical Engineering-Science (Scripta and Acta Materialia). Reciprocally, these journals are also the most locally cited sources in the set of documents analyzed (see Table S1). A locally cited source is a journal included in at least one of the reference lists of the analyzed document collection. Additionally, there are also articles devoted to FS in general high-impact journals such as Nature Communications [ 74 ] or Science Advances [22]. Materials 2022, 15, x FOR PEER REVIEW 7 of 21 J ASIAN CERAM SOC ADV APPL CERAM J ALLOYS COMPD J APPL PHYS J NUCL MATER MATERIALIA J MATER SCI MRS BULL MATERIALS J CERAM SOC JPN ACTA MATER CERAM INT SCRIPTA MATER J AM CERAM SOC J EUR CERAM SOC 0 5 10 15 20 25 30 35 40 45 50 55 60 65 3 4 6 6 7 9 11 29 48 60 61 Number of Publications Sources 4 4 4 4 Figure 2. Top 15 sources with 3 or more published papers. 3.1.4. Author-Level Analysis and Networking As shown in Table 1, the author-level analysis reveals that this set of documents involved 670 authors from 29 different countries. Seven of those authors published singleauthored documents, whereas the rest participated in co-authored documents. Generally speaking, each document is written by 2.11 authors on average and the Collaboration Index, defined as the total authors of multi-authored documents divided by the total number of multi-authored articles, is 2.2. Additionally, Figure S1 (Supplementary Materials) depicts the country’s scientific production map, where it is qualitatively shown that the USA, China, and Italy are the most involved countries. Table 2 includes a list of authors that have published, so far, more than 10 articles directly related to FS along with their local h-index. The local h-index has been calculated using the conventional procedure (number h of publications that have been cited h times or more [92]) but only considering the set of 318 documents analyzed here. More details about authors with the highest numbers of local citations (citations included in this set of 318 documents) and global citations can be found in Table S2. It must be noted that those lists have been exclusively elaborated with the information retrieved from articles directly related to FS, as explained in the methods section. Those dealing with other similar techniques, such as F-SPS and others, have not been contemplated in the lists. In any case, Table 2 and S2 should not be considered as any type of author’s ranking, as most authors in those lists have a significantly larger number of publications and citations in different related scientific topics that have not been included here. Moreover, some authors have been involved in FS almost since the early days, while others have become involved in recent times. As a way of example, the production of the authors included in Table 2 over the 11-year period of lifetime of FS is depicted in Figure S2, where the bubble size is proportional to the number of documents and the color darkness to the citations. Regardless of this, Prof. Raj not only introduced the FS topic for the first time in 2010, but he is the most prolific author in terms of the quantity of papers, citations, and the local h-index. The high level of interaction among authors working in FS is very relevant, as shown in the collaboration maps of Figure 3. This type of figure displays the knowledge structure of the research field by providing a general overview about its social structure or, in other words, how the FS scientific community interacts at different levels (authors, institutions, and countries) [35]. Qualitatively, the bubble size in these collaboration maps represents the number of documents, while the strength of the relationship is represented by the Figure 2. Top 15 sources with 3 or more published papers. This simple source-level analysis shows that FS research is mainly published in highranked, peer-reviewed journals and provides an idea about the high quality of the research carried out in this field and the interest that it generates and attracts within the scientific community. 3.1.4. Author-Level Analysis and Networking As shown in Table 1, the author-level analysis reveals that this set of documents involved 670 authors from 29 different countries. Seven of those authors published singleauthored documents, whereas the rest participated in co-authored documents. Generally speaking, each document is written by 2.11 authors on average and the Collaboration Index, defined as the total authors of multi-authored documents divided by the total number of multi-authored articles, is 2.2. Additionally, Figure S1 (Supplementary Materials) depicts Materials 2022,15, 416 7 of 20 the country’s scientific production map, where it is qualitatively shown that the USA, China, and Italy are the most involved countries. Table 2includes a list of authors that have published, so far, more than 10 articles directly related to FS along with their local h-index. The local h-index has been calculated using the conventional procedure (number hof publications that have been cited htimes or more [ 92 ]) but only considering the set of 318 documents analyzed here. More details about authors with the highest numbers of local citations (citations included in this set of 318 documents) and global citations can be found in Table S2. It must be noted that those lists have been exclusively elaborated with the information retrieved from articles directly related to FS, as explained in the methods section. Those dealing with other similar techniques, such as F-SPS and others, have not been contemplated in the lists. In any case, Table 2and Table S2 should not be considered as any type of author’s ranking, as most authors in those lists have a significantly larger number of publications and citations in different related scientific topics that have not been included here. Moreover, some authors have been involved in FS almost since the early days, while others have become involved in recent times. As a way of example, the production of the authors included in Table 2over the 11-year period of lifetime of FS is depicted in Figure S2, where the bubble size is proportional to the number of documents and the color darkness to the citations. Regardless of this, Prof. Raj not only introduced the FS topic for the first time in 2010, but he is the most prolific author in terms of the quantity of papers, citations, and the local h-index. Table 2. Authors with the highest number of publications in the analyzed documents set along with their local h-index. Authors Articles h-Index Raj R 53 27 Sglavo VM 36 17 Biesuz M 30 14 Jha SK 22 15 Yamamoto T 18 8 Tsakalakos T 16 9 Wang YG 16 9 Wang HY 16 8 Wang H 15 9 Yoshida H 15 9 Charalambous H 14 9 Phuah XL 14 7 Tokunaga T 14 6 Liu JL 13 8 Lebrun JM 12 11 Liu DG 12 8 Chaim R 12 7 Muccillo R 12 7 Grasso S 12 6 Luo J 11 9 The high level of interaction among authors working in FS is very relevant, as shown in the collaboration maps of Figure 3. This type of figure displays the knowledge structure of the research field by providing a general overview about its social structure or, in other words, how the FS scientific community interacts at different levels (authors, institutions, and countries) [ 35 ]. Qualitatively, the bubble size in these collaboration maps represents the number of documents, while the strength of the relationship is represented by the links; the thicker, the stronger. Moreover, the position represents the influence, placing the most influential items, i.e., author, institution, or country, at the center of the maps. The different colors or clusters denote common collaboration networks or sub-networks. Figure 3a includes the collaboration map just among authors included in Table 2. Interestingly, all authors in Table 2, apart from Prof. Chaim, have very strong collaborations with others Materials 2022,15, 416 8 of 20 from the same list. Actually, in some cases, they even belonged to the same research group. That is the case of Biesuz who was a former PhD student in Sglavo’s lab (Trento University, Trento, Italy), Charalambous in Tsakalakos’ lab (Rutgers University, New Brunswick, NJ, USA), H Wang and Phuah in Haiyan Wang’s lab (Purdue University, West Lafayette, IN, USA), while Lebrun was a postdoc in Raj’s lab (University of Colorado at Boulder, Boulder, CO, USA). Moreover, there is a significant movement of authors from one institution into another that is helping to spread the topic. For instance, Jha who is currently an Assistant Professor at the Indian Institute of Technology Kanpur was a former PhD student of Raj and, later on, a postdoc in Tsakalakos’s lab. Dianguang Liuwas a former PhD student at Northwestern Polytechnical University (working with Wang, Yiguang) and then moved to Southwest Jiaotong University where he works with Jinling Liu. Grasso is currently a professor at the Southwest Jiaotong University after having worked at the Queen Mary University of London. Moreover, as seen in Figure 3a by the solid connectors, there are many fruitful collaborations among different groups, resulting in co-authored publications. As mentioned, the behavior observed in Figure 3a is limited to authors from Table 2. Nevertheless, it is quite general and can be extrapolated to the whole FS community, as explained as follows by Figure 3b,c, which represents the countries and institutional collaboration maps, respectively, and aims to provide a broader overview of the scientific landscape of FS. As depicted in Figure 3b, there are authors from 26 different countries with at least one mutual publication. Note that the whole FS community involved authors from 29 countries, as is shown in Figure S1. This highlights, once again, the strong interconnection of this scientific community. Similar to Figure 3a, the USA is the country with the highest number of publications and the strongest collaboration network. Authors from China and Italy also have important scientific production and collaboration networks. Interestingly, even authors from countries such as Germany, Spain, India, and France, among others, have become involved in FS more recently and, therefore, while their number of publications is not that high, they have very strong collaboration networks. For example, in the whole document set, only five papers have been issued by corresponding authors from Spain, all of them being internationally collaborative publications. Figure 3c shows the collaboration network at an institutional level. The FS community involves authors from 217 institutions. Thus, for the sake of clarity and visualization, the institutional collaboration map has been limited to those 18 institutions with the highest number of published papers. Analogously to Figure 3a, the bubble size of the University of Colorado at Boulder (USA) and Trento University (Italy) is not surprising; as mentioned, the mostproductive authors in terms of publications belong to these institutions, i.e., Raj, Sglavo, and Biesuz. Moreover, both institutions have strong collaboration networks worldwide. For instance, Trento University, Southwest Jiaotong University, one of the most prolific Chinese institutions, and Queen Mary University of London (UK) have established a strong collaboration sub-network. As commented for Figure 3a, Southwest Jiaotong University and the Queen Mary University of London are the current and previous affiliations of Prof. Grasso, respectively. At the same time, other researchers from Southwest Jiaotong University (Liu Dg and Liu Jl among others) work with researchers from other academic centers, such as the Northwestern Polytechnical University (China) and the University of Central Florida (USA). Additionally, Figure 3c also unveils a strong collaboration subnetwork within American institutions composed of Rutgers University, Purdue University, Argonne National Laboratory, the University of California San Diego, and the University of California Davis. In albeit extreme simplification, this subnetwork is partially shown in Figure 3a, proving again its similarities with the whole FS scientific community. Most of the works of this American co-authorship sub-network involve some kind of in situ measurements during FS or to flash-sintered samples, such as energy-dispersive X-ray diffraction [15,53,69,93]. Materials 2022,15, 416 9 of 20 Materials 2022, 15, x FOR PEER REVIEW 10 of 21 Figure 3. (a) Selected authors from Table 2, (b) countries, and (c) institution’s collaboration networks. * UC San Diego: University of California San Diego, IPEN: Instituto de Pesquisas Energéticas e Nucleares, QMUL: Queen Mary University of London, SWJU: Southwest Jiaotong University, NPU: Northwestern Polytechnical University, NIMS: National Institute for Materials Science, Technion: Technion–Israel Institute of Technology. Figure 3. ( a ) Selected authors from Table 2, ( b ) countries, and ( c ) institution’s collaboration networks. * UC San Diego: University of California San Diego, IPEN: Instituto de Pesquisas Energéticas e Nucleares, QMUL: Queen Mary University of London, SWJU: Southwest Jiaotong University, NPU: Northwestern Polytechnical University, NIMS: National Institute for Materials Science, Technion: Technion–Israel Institute of Technology. Materials 2022,15, 416 16 of 20 Author Contributions: Conceptualization, E.G.-G. and L.A.P.-M.; methodology, E.G.-G.; software, E.G.-G.; validation, L.A.P.-M., P.E.S.-J. and A.P.; formal analysis, E.G.-G. and L.A.P.-M.; investigation, E.G.-G.; resources, L.A.P.-M.; data curation, E.G.-G.; writing—original draft preparation, E.G.-G. and L.A.P.-M.; writing—review and editing, P.E.S.-J.; visualization, E.G.-G.; supervision, L.A.P.-M., P.E.S.-J.; project administration, L.A.P.-M.; funding acquisition, L.A.P.-M.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Spanish Government Agency Ministerio de Ciencia, Innovación y Universidades and FEDER grant numbers CTQ2017–83602-C2–1-R and CTQ2017– 83602-C2–2-R), Junta de Andalucía-Consejería de Economía, Conocimiento, Empresas y Universidad grant number P18-FR-1087, FEDER grant number US-1262507 and INTRAMURAL-CSIC grant numbers 201960E092 and 202060I004. 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