Mainak Saha1,21Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600036, Tamil Nadu, India2Department of Metallurgical and Materials Engineering, National Institute of Technology Durgapur-713209, West Bengal, IndiaEmail address: [email protected] number: +918017457062Type: Review

Recently, there have been a number of reports on the fabrication of Alumina-Zirconia (Al2O3-ZrO2 (AZ))-based eutectic ceramics using laser additive manufacturing (AM) techniques owing to the exceptional creep and oxidation resistance coupled with excellent microstructural stability in these materials. Moreover, a number of interesting microstructural features (in these materials) have been reported to be formed by the variation of process parameters (especially, laser scanning speed) associated with the aforementioned AM techniques. The present review provides an overview of the present state of research in the area of laser AM AZ-based eutectic ceramics and highlights the challenges and future outlooks in this avenue of research. In addition, the requirement of employing correlative microstructural characterisation in these materials has been highlighted in the outlooks section.

In the recent decade, metal-based conductive nano-inks comprising of nanometer-sized metallic particles of high electrical conductivity (mainly Ag and Cu) suspended in a polymer have revolutionized the field of printed electronics. The present review aims to provide an overview of the current state of research in synthesis and characterization techniques for metallic nano-inks and associated challenges. Moreover, the end of the review will discuss future perspectives in this field (from industrial and academic viewpoints).

Mainak Saha1,21Department of Metallurgical and Materials Engineering, National Institute of Technology, Durgapur-713209, India2Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai-600036, IndiaAbstract - While descending through different layers of atmosphere with tremendously high velocities, hypersonic re-entry nosecones fabricated using ultra-high temperature ceramic matrix composites (UHTCMCs) are subjected to repeated thermal shocks. This necessitates extensive investigations on the cyclic oxidation behaviour of UHTCMCs at temperatures ranging from 1100°C to 1300°C (service temperature of the nosecones). To this end, the present work is aimed at investigating the cyclic oxidation behaviour of ZrB2 -20 vol.%MoSi2 (ZM20) UHTCMC (a very widely investigated ZM CMC) by carrying out cycles for 6h, at 1cycle/h and estimating oxidation kinetic law. This has been followed by extensive characterisation using X-Ray Diffraction (XRD) to indicate the phases formed during oxidation and Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), in order to determine the chemical composition of the oxides formed between 1100°C and 1300°C.Keywords- Borides; ceramic composites; cyclic oxidation; kinetics; oxide layerAmong UHTCs, ZrB2-based ceramics have been reported to be potential candidates for the manufacture of reusable Thermal Protection Systems (TPS) in Hypersonic re-entry nosecones, due to very high thermal conductivity and relatively low density [1,2]. However, the low fracture toughness and poor thermal shock resistance of these ceramics pose major obstacles to their use in extreme environment [3]. Moreover, the poor oxidation resistance of ZrB2 at temperatures above 1200°C, due to formation of B2O3, and a non-protective porous scale of ZrO2 [4], poses restrictions to its use at elevated temperatures, especially above 1200°C. Thus, it becomes extremely important to find materials, which may highly enhance the oxidation resistance of ZrB2 [5-8]. A significant amount of work has already been done in that direction [8-12]. Besides, a significant amount of research has been done on reinforcing diborides like ZrB2, HfB2 and TiB2 with SiC, MoSi2, or ZrSi2 for enhanced oxidation resistance beyond 800°C [3- 23]. However, a limited amount of study has been made on cyclic oxidation of ZM20 at temperatures exceeding 1100°C, which is not at all unlikely, in the context of Hypersonic nosecones, during a high velocity descent through different layers of atmosphere. Thus, the scope of the present study is to investigate the cyclic oxidation behaviour of ZM20 between 1100 and 1300 °C.The important conclusions drawn from the results and discussions of this study have been elucidated. Cyclic oxidation behaviour of ZrB2-20 vol.% MoSi2 composite have been studied at 1100 °C, 1200 °C, 1250 °C and 1300 °C for 6hrs. Monitoring weight change and examining oxide scales draw following conclusions:(i) Weight gain for both the composites increased with increasing temperature and time. (ii) Weight gain occurred due to formation of ZrO2 and SiO2, at elevated temperatures. (iii) The main oxidation products were ZrO2, MoO3 and SiO2. (iv) At 1200 °C and above, the presence of SiC particles markedly improves the resistance to oxidation of the composite due to the formation of borosilicate glass.(v) Due to formation of oxide layer on the surface, the hardness of the samples i.e. its mechanical properties decreased from center to surface.(vi) The cyclic oxidation of the samples follow linear oxidation kinetics from 1100 to 1250 ºC while at 1300 ºC it follows parabolic oxidation kinetics due to the protective action of SiO2 above 1250 ºC.The results of the present study and their analyses lead to the following directions for future work: (i) The oxidation kinetics of the samples beyond 1300 ºC can be studied. (ii) Residual strain calculations can be carried out. (iii) Mathematical modelling study of the oxidation kinetics can be carried out. (iv) TEM study of the samples can be carried out for more precarious measurements. (v) Carrying out diffusion studies on oxide layer.AcknowledgementThe authors are grateful to the Department of Metallurgical and Materials Engineering, NIT Durgapur and Central Research Facility(CRF), IIT Kharagpur, for their support to carry out the work and hereby declare no conflict of interest.

In the recent decade, metal-based conductive nano-inks comprising of nanometer-sized metallic particles of high electrical conductivity (mainly Ag and Cu) suspended in a polymer have revolutionized the field of printed electronics. The present review aims to provide an overview of the current state of research in synthesis and characterization techniques for metallic nano-inks and associated challenges. Moreover, the end of the review will discuss future perspectives in this field (from industrial and academic viewpoints).

In materials research involving additive manufacturing (AM)-based techniques for fabrication of a wide variety of materials, the latest trend at present is to focus largely on 3D printing (3DP) of nanoceramics, which at present is highly challenging, from both fundamental and industrial viewpoints inspite of the tremendous versatility offered by these techniques in terms of addressing design complexities [1]-[6]. The two main reasons for the same are: (i) low density and (ii) poor mechanical properties of nanoceramic parts fabricated using 3DP techniques [7]-[9]. The fundamental reason behind the two aforementioned features of 3DP-fabricated nanoceramic parts is the huge extent of microstructural inhomogeneity arising primarily due to variation in cooling rates during 'point by point', 'line by line' or 'layer by layer' deposition methodology followed in 3DP techniques [10]-[16], leading to a number of defects in the microstructure [17], [18]. Moreover, the industrial application of nanoceramic parts manufactured using 3DP techniques, is rather limited, primarily owing to the high manufacturing cost associated with these nanoceramic parts. Although, in the last ten years, there has been a considerable volume of work on 3DP-based techniques for manufacturing ceramic parts with enhanced densities and improved mechanical properties, however, there is limited understanding on the correlation of microstructure of 3DP-fabriated nanoceramic components with the mechanical properties. On the other hand, in the recent decade, the 'correlative' methodology of characterising microstructures from micro to nanoscale, involving a number of different structural and chemical characterisation techniques, for the study of a number of defects ranging from the equilibrium point (or 0-D) to non-equilibrium volume (or 3-D) defects, has been hugely employed in a number of metallic materials [19]-[21]. This has completely revolutionised the understanding of structure-property correlation and microstructural defects in these materials and paved a whole new dimension towards a systematic correlation of structure (ranging from bulk to nano-scale) to a wide range range of properties in these materials. However, in the context of 3DP-fabricated nanoceramic parts, at present, there is hardly report on understanding structure-property correlation using the aforementioned methodology. The present review is aimed to review some of the most commonly used 3DP techniques for the fabrication of nanoceramics and provide an overview of the future perspectives, associated with the necessity towards developing a systematic structure-property correlation through 'correlative' characterisation methodology in these materials.

At present, fabrication of ceramics using AM-based techniques mainly suffers from two primary limitations, viz: (i) low density and (ii) poor mechanical properties of the finished components. It is worth mentioning that the present state of research in the avenue of AM-based ceramics is focussed mainly on fabricating ceramic and cermet components with enhanced densities and improved mechanical properties. However, to the best of the authors' knowledge, not much is known about the microstructure evolution and its correlation with the mechanical properties of the finished parts. Addressing the aforementioned avenue is highly essential for understanding the utilisation of these components for structural applications. To this end, the present review article is aimed to address the future perspectives in this avenue has been provided with a special emphasis on the need to establish a systematic structure-property correlation in these materials.

In materials research involving additive manufacturing (AM)-based techniques for fabrication of a wide variety of materials, the latest trend at present is to focus largely on 3D printing (3DP) of nanoceramics, which at present is highly challenging, from both fundamental and industrial viewpoints inspite of the tremendous versatility offered by these techniques in terms of addressing design complexities [1]-[6]. The two main reasons for the same are: (i) low density and (ii) poor mechanical properties of nanoceramic parts fabricated using 3DP techniques [7]-[9]. The fundamental reason behind the two aforementioned features of 3DP-fabricated nanoceramic parts is the huge extent of microstructural inhomogeneity arising primarily due to variation in cooling rates during 'point by point', 'line by line' or 'layer by layer' deposition methodology followed in 3DP techniques [10]-[16], leading to a number of defects in the microstructure [17], [18]. Moreover, the industrial application of nanoceramic parts manufactured using 3DP techniques, is rather limited, primarily owing to the high manufacturing cost associated with these nanoceramic parts. Although, in the last ten years, there has been a considerable volume of work on 3DP-based techniques for manufacturing ceramic parts with enhanced densities and improved mechanical properties, however, there is limited understanding on the correlation of microstructure of 3DP-fabriated nanoceramic components with the mechanical properties. On the other hand, in the recent decade, the 'correlative' methodology of characterising microstructures from micro to nanoscale, involving a number of different structural and chemical characterisation techniques, for the study of a number of defects ranging from the equilibrium point (or 0-D) to non-equilibrium volume (or 3-D) defects, has been hugely employed in a number of metallic materials [19]-[21]. This has completely revolutionised the understanding of structure-property correlation and microstructural defects in these materials and paved a whole new dimension towards a systematic correlation of structure (ranging from bulk to nano-scale) to a wide range range of properties in these materials. However, in the context of 3DP-fabricated nanoceramic parts, at present, there is hardly report on understanding structure-property correlation using the aforementioned methodology. The present review is aimed to review some of the most commonly used 3DP techniques for the fabrication of nanoceramics and provide an overview of the future perspectives, associated with the necessity towards developing a systematic structure-property correlation through 'correlative' characterisation methodology in these materials.

The last two decades have witnessed a large volume of research revolving around structure-property correlation in carbon-based nanocomposites, synthesized by several methods. In the simplest of terms, the electronic properties of these nanomaterials, which form the present context of discussion, vary mainly as a function of three parameters, out of which two are process parameters (viz. (i) the kind of reinforcement and (ii) method of synthesis), and one is a structure-dependent parameter. The structure-dependent parameter is highly influenced by the two process parameters and plays a vital role in determining the ionic and electronic transport phenomenon in these materials. In other words, the interaction between electrons and the equilibrium 0-D (point) defects, along with different types of 2-D interfaces, plays a crucial role in the understanding of electronic properties, apart from physical and chemical properties of these materials. The present chapter provides a brief overview of the state-of-the-art on research along with detailed discussions on some recent developments in understanding electronic properties of some conventional carbon-based nanocomposites (synthesized by different techniques) based on the structure-property correlation in these materials. Finally, some of the significant challenges in this field have been addressed from both industrial and fundamental viewpoints.