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Bala Vaidhyanathan

Loughborough University, Department of Materials, Faculty Member

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Bala Vaidhyanathan is a Professor of Advanced Materials and Processing, previous Associate Dean for Enterprise (ADE) of the School of Aeronautical, Automotive, Chemical and Materials Engineering at Loughborough University (LU), Leader of the Advanced Ceramics Research Group, member of the School Senior Management Team (SSMT), member of the advisory team of university’s research challenges and Chair of School Student Placement committee. Previously he has been a member of the research staff at the Pennsylvania State University, USA, and a Lead Scientist at General Electric-Global Research Corporation. He specializes in nanostructured functional materials and non-conventional field assisted processing. He has published over 150 peer-reviewed articles, 6 book chapters, holder of 14 patents and has delivered >40 key note and invited presentations across the globe. He has held/holds >40 Government/Industry sponsored project grants, worth >£9.2M. Vaidhyanathan has pioneered the development of energy efficient microwave, hybrid and flash methods for the advanced processing of functional ceramic materials and Loughborough is currently regarded as one of the world leaders in the utilization of these techniques and hosts the largest nanoceramics group in UK. With over 20 years of experience, he is one of the leading exponents in the field of microwave-assisted materials manufacturing, pioneered the development of hybrid two stage sintering methods and was the first to set up an atmosphere controlled, gradient flash sintering facility for the processing of oxide and non-oxide materials and devices. The range of products worked on has been very wide, from traditional to nanostructured materials, for energy, electronic, defense and healthcare applications. LU Materials department is also the home of the Loughborough Materials Characterisation Centre, a specialized facility for state-of-the-art materials characterisation in all length scales from surface to bulk, from microscopic to macroscopic structure determination and BV’s team commands significant analytical expertise on structure-property correlations.

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Papers by Bala Vaidhyanathan

Challenges to Develop and Design Ultra-high Temperature Piezoelectric Accelerometers

Conference proceedings of the Society for Experimental Mechanics, Sep 30, 2020

Field assisted processing of 3D printed ceramics

Microwave-Assisted Synthesis and Sintering of NZP Compounds

Journal of the American Ceramic Society, May 1, 2004

Page 1. Microwave-Assisted Synthesis and Sintering of NZP Compounds Balasubramaniam Vaidhyanathan... more

Microwave assisted processing of X8R nanocrystalline BaTiO3 based ceramic capacitors and multilayer devices

Open ceramics, Mar 1, 2022

Field Assisted Processing of 3D Printed Ceramics

Fabrication of complex ceramic components involves multiple steps, often is labour & energy inten... more Fabrication of complex ceramic components involves multiple steps, often is labour & energy intensive. 3D printing of ceramics and their sintering through the application of external fields could pave the way for solving these issues. In this work, the combination of Additive Manufacturing (AM) along with Field Assisted Sintering Techniques (FAST) were employed to manufacture functional ceramics suitable for electronic, communication and healthcare sectors. This poster highlights some of the recent advances.

Synthesis and dielectric characterisation of a low loss BaSrTiO3/ABS ceramic/polymer composite for fused filament fabrication additive manufacturing

Additive manufacturing, Jul 1, 2022

This is a repository copy of Synthesis and dielectric characterisation of a low loss BaSrTiO3/ABS... more

Microwave assisted synthesis of nanocrystalline YAG

Journal of Materials Science, Sep 1, 2006

Nanocrystalline particles of MgO were synthesized using microwave radiation in an ethylene glycol... more Nanocrystalline particles of MgO were synthesized using microwave radiation in an ethylene glycol solution. The antibacterial activities of the MgO nanoparticles were tested by treating Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) cultures with 1 mg mL ±1 of the nanoparticles. We have examined the importance of the size effect, pH, and the form of the active MgO species as a bactericidal agent. A clear size dependence of the nanoparticles is observed where the amount of eradicated bacteria was strongly dependent on the particle size.

A Cost Effective Screening Technique for Testing UHTC Materials Using an Oxyacetylene Torch Flame

Sol-Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB<sub>2</sub>

Journal of the American Ceramic Society, Nov 28, 2013

HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protecti... more HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protection systems because of favourable temperature stability, good mechanical properties and oxidation resistance. In this work HfB 2 powder has been synthesised via a sol-gel based route using phenolic resin, hafnium chloride and boric acid as the source of carbon, hafnium and boron respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB 2 formation investigated using FTIR, TGA, DSC and XRD. The results showed that whilst temperatures as low as 1300 o C could be used to produce HfB 2 particles, the calcination needed to last for about 25 h and the long duration resulted in particle growth along the c-axis of the HfB 2 crystals yielding tube like structures of about 10 µm long. Equiaxed particles 1-2 µm in size were obtained when the precursor was calcined at 1600 o C for 2 h. The reaction mechanism involved carbo/borothermal reduction and the indications were that the formation of HfB 2 at 1300 o C is through

UHTC Composites for Hypersonic Applications

John Wiley & Sons, Inc. eBooks, Oct 10, 2014

Hydrothermal Aging Behavior of Nanocrystalline Y-TZP Ceramics

Journal of the American Ceramic Society, Mar 2, 2011

The influence of grain size and yttria content on the hydrothermal aging behavior of yttria‐stabi... more The influence of grain size and yttria content on the hydrothermal aging behavior of yttria‐stabilized zirconia ceramics has been investigated via the preparation of genuine, fully dense nanozirconia ceramics with &lt;100 nm grain size by two‐step sintering. The samples were aged at different temperatures for varying periods of time and XRD and micro‐Raman spectroscopy have been used to monitor the aging process while the hardness of the samples was used as a measure of the deterioration in the physical properties. The results were compared with those of a commercial submicrometer 3YSZ zirconia sample subjected to the same aging treatment and characterization. Nano3YSZ with &lt;100 nm grain size did not exhibit degradation, whereas the submicrometer 3YSZ samples underwent severe degradation.

Heat flux mapping of oxyacetylene flames and their use to characterise Cf-HfB<sub>2</sub>composites

Advances in Applied Ceramics, Feb 9, 2016

Cost effective and fast ultra-high-temperature testing methods such as oxyacetylene torch testing... more Cost effective and fast ultra-high-temperature testing methods such as oxyacetylene torch testing are extremely useful for the rapid screening of ultra-high-temperature ceramic (UHTC) materials for hypersonic applications. There is no report in the literature, however, of an organised study to measure the heat flux and how it varies with gas flow rate, gas flow ratio and distance from the nozzle tip for an oxyacetylene flame. In this paper, the authors report for the first time the heat flux mapping of an oxidising, neutral and reducing flame. The measured heat flux was as high as ∼17 MW m−2 at a distance of 10 mm from the nozzle, which is much higher than that previously reported in the literature. Torch testing was carried out for Cf-HfB2 UHTC composites at this heat flux and the results are presented along with detailed microstructural characterisation.

Synthesis and Static Oxidation Testing of Doped HfB2 Powders

The ballistic impact performance of nanocrystalline zirconia-toughened alumina (nZTA) and alumina ceramics

Journal of the European Ceramic Society

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of nanocrystalline barium titanate: Effect of microwave power on phase evolution

Journal of the European Ceramic Society, 2020

Abstract Nanocrystalline BaTiO3 (BT) powder was synthesised using a polymer precursor route and t... more Abstract Nanocrystalline BaTiO3 (BT) powder was synthesised using a polymer precursor route and the influence of conventional, microwave and combined hybrid heating methods on phase formation was investigated. A single-phase tetragonal BT (t-BT) nanocrystalline powder of about 20 nm primary particle size and decreased agglomeration were formed when high levels of microwave energy were used. This was accomplished at a lower processing condition of 700 °C for 30 min compared to conventional processing, which required 900 °C for 5 h, resulting in potential savings in time and energy. During the nano BT synthesis, the role of microwaves was determined by subjecting the samples to identical thermal histories, i.e. exactly the same time-temperature profiles, while using a range of different levels of microwave power. Significant reduction in the activation energy for the formation of the tetragonal phase was observed with increasing levels of microwave power and the results are explained in terms of a possible non-thermal mechanism. Furthermore, under otherwise identical thermodynamic conditions of temperature, time and (atmospheric) pressure, the co-occurrence of hexagonal crystal structure at

Development of Nano Zirconia Toughened Alumina for Ceramic Armor Applications

Advances in Ceramic Armor VII, 2011

Quantitative analysis of the residual stress and dislocation density distributions around indentations in alumina and zirconia toughened alumina (ZTA) ceramics

Journal of the European Ceramic Society, 2014

Alumina, 10% and 20% nano ZTA with 1.5 mol% yttria stabiliser were subjected to Vickers indentati... more Alumina, 10% and 20% nano ZTA with 1.5 mol% yttria stabiliser were subjected to Vickers indentation testing with loads from 1 to 20 kg. Cr 3+ fluorescence and Raman spectroscopy were applied to the indent centre and around the indentation in order to investigate the origin of the signal, the effect of indentation loads and zirconia phase transformation on the residual stress and plastic deformation in the plastic zone. The results suggested that with very strong laser scattering, the depth resolution of ZTA materials was very poor, which lead to a very significant amount of the signal being collected from the subsurface regions below the plastic zone. It was also found that zirconia phase transformation reduced the compressive residual stress in the alumina matrix within the plastic zone, except at the indentation centre, due to the tensile residual microstress generated by the zirconia phase transformation. In addition, the dislocation density on the indent surface of the ZTA samples was significantly reduced due to the restriction of crack propagation and energy absorption during the phase transformation process. At the indent centre, the zirconia phase transformation was suppressed by the high compressive stress, therefore, no significant difference between alumina and ZTA in terms of their residual stress and dislocation density were observed. Using TEM observation, it was found that the plastic zone microstructure of pure alumina is different from that of ZTA, which is consistent with the Cr 3+ fluorescence results. the microcrack density in the plastic zone of the ZTA was confirmed to be lower than that of the alumina sample.

Direct ink writing of bismuth molybdate microwave dielectric ceramics

Ceramics International, 2021

Additive manufacturing via direct ink writing and microwave dielectric characterisation of commer... more Additive manufacturing via direct ink writing and microwave dielectric characterisation of commercially produced low sintering temperature bismuth molybdenum oxide ceramics, have been both performed for the first time, following a powder-to-product holistic approach. We demonstrated that direct ink writing is an excellent candidate for producing dielectric substrates to be used for wireless telecommunication applications operating at microwave (MW) frequencies, with great repeatability and properties comparable to ceramics fabricated via conventional processing routes. The optimum density (relative density of ρr ≈ 93%) of the 3D printed test samples was obtained by sintering at 660 °C for 2 hours, resulting in a relative permittivity εr = 35.7, dielectric loss tanδ = 0.0004 and microwave quality factor Q×f = 14,928 GHz. Sintering at higher temperatures promoted a porosity increase due to mismatching grain growth mechanisms and phase decomposition, that collectively hindered the test samples' microwave dielectric performance in terms of achievable relative permittivity (εr) and dielectric loss (tanδ).

Multi-material additive manufacture and microwave-assisted sintering of a metal/ceramic metamaterial antenna structure

Applied Materials Today

3D Antennas, Metamaterials, and Additive Manufacturing

2019 IEEE MTT-S International Wireless Symposium (IWS), 2019

This paper presents several demonstrators that utilize advanced additive manufacturing (AM) to ra... more This paper presents several demonstrators that utilize advanced additive manufacturing (AM) to rapidly prototype antennas, RF components for microwave and mm-wave applications. Antenna substrates with tailored permittivity; lenses, reflectarray with locally varied dielectric properties; and metamaterials can be fabricated by using extrusion based AM processes such as fused filament fabrication method to realize high gain antenna systems with low material cost, time-saving and low impact to the environment.

Challenges to Develop and Design Ultra-high Temperature Piezoelectric Accelerometers

Conference proceedings of the Society for Experimental Mechanics, Sep 30, 2020

Field assisted processing of 3D printed ceramics

Microwave-Assisted Synthesis and Sintering of NZP Compounds

Journal of the American Ceramic Society, May 1, 2004

Page 1. Microwave-Assisted Synthesis and Sintering of NZP Compounds Balasubramaniam Vaidhyanathan... more

Microwave assisted processing of X8R nanocrystalline BaTiO3 based ceramic capacitors and multilayer devices

Open ceramics, Mar 1, 2022

Field Assisted Processing of 3D Printed Ceramics

Fabrication of complex ceramic components involves multiple steps, often is labour & energy inten... more Fabrication of complex ceramic components involves multiple steps, often is labour & energy intensive. 3D printing of ceramics and their sintering through the application of external fields could pave the way for solving these issues. In this work, the combination of Additive Manufacturing (AM) along with Field Assisted Sintering Techniques (FAST) were employed to manufacture functional ceramics suitable for electronic, communication and healthcare sectors. This poster highlights some of the recent advances.

Synthesis and dielectric characterisation of a low loss BaSrTiO3/ABS ceramic/polymer composite for fused filament fabrication additive manufacturing

Additive manufacturing, Jul 1, 2022

This is a repository copy of Synthesis and dielectric characterisation of a low loss BaSrTiO3/ABS... more

Microwave assisted synthesis of nanocrystalline YAG

Journal of Materials Science, Sep 1, 2006

Nanocrystalline particles of MgO were synthesized using microwave radiation in an ethylene glycol... more Nanocrystalline particles of MgO were synthesized using microwave radiation in an ethylene glycol solution. The antibacterial activities of the MgO nanoparticles were tested by treating Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) cultures with 1 mg mL ±1 of the nanoparticles. We have examined the importance of the size effect, pH, and the form of the active MgO species as a bactericidal agent. A clear size dependence of the nanoparticles is observed where the amount of eradicated bacteria was strongly dependent on the particle size.

A Cost Effective Screening Technique for Testing UHTC Materials Using an Oxyacetylene Torch Flame

Sol-Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB<sub>2</sub>

Journal of the American Ceramic Society, Nov 28, 2013

HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protecti... more HfB 2 is an ultra-high temperature ceramic that holds great potential for use in thermal protection systems because of favourable temperature stability, good mechanical properties and oxidation resistance. In this work HfB 2 powder has been synthesised via a sol-gel based route using phenolic resin, hafnium chloride and boric acid as the source of carbon, hafnium and boron respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB 2 formation investigated using FTIR, TGA, DSC and XRD. The results showed that whilst temperatures as low as 1300 o C could be used to produce HfB 2 particles, the calcination needed to last for about 25 h and the long duration resulted in particle growth along the c-axis of the HfB 2 crystals yielding tube like structures of about 10 µm long. Equiaxed particles 1-2 µm in size were obtained when the precursor was calcined at 1600 o C for 2 h. The reaction mechanism involved carbo/borothermal reduction and the indications were that the formation of HfB 2 at 1300 o C is through

UHTC Composites for Hypersonic Applications

John Wiley & Sons, Inc. eBooks, Oct 10, 2014

Hydrothermal Aging Behavior of Nanocrystalline Y-TZP Ceramics

Journal of the American Ceramic Society, Mar 2, 2011

The influence of grain size and yttria content on the hydrothermal aging behavior of yttria‐stabi... more The influence of grain size and yttria content on the hydrothermal aging behavior of yttria‐stabilized zirconia ceramics has been investigated via the preparation of genuine, fully dense nanozirconia ceramics with &lt;100 nm grain size by two‐step sintering. The samples were aged at different temperatures for varying periods of time and XRD and micro‐Raman spectroscopy have been used to monitor the aging process while the hardness of the samples was used as a measure of the deterioration in the physical properties. The results were compared with those of a commercial submicrometer 3YSZ zirconia sample subjected to the same aging treatment and characterization. Nano3YSZ with &lt;100 nm grain size did not exhibit degradation, whereas the submicrometer 3YSZ samples underwent severe degradation.

Heat flux mapping of oxyacetylene flames and their use to characterise Cf-HfB<sub>2</sub>composites

Advances in Applied Ceramics, Feb 9, 2016

Cost effective and fast ultra-high-temperature testing methods such as oxyacetylene torch testing... more Cost effective and fast ultra-high-temperature testing methods such as oxyacetylene torch testing are extremely useful for the rapid screening of ultra-high-temperature ceramic (UHTC) materials for hypersonic applications. There is no report in the literature, however, of an organised study to measure the heat flux and how it varies with gas flow rate, gas flow ratio and distance from the nozzle tip for an oxyacetylene flame. In this paper, the authors report for the first time the heat flux mapping of an oxidising, neutral and reducing flame. The measured heat flux was as high as ∼17 MW m−2 at a distance of 10 mm from the nozzle, which is much higher than that previously reported in the literature. Torch testing was carried out for Cf-HfB2 UHTC composites at this heat flux and the results are presented along with detailed microstructural characterisation.

Synthesis and Static Oxidation Testing of Doped HfB2 Powders

The ballistic impact performance of nanocrystalline zirconia-toughened alumina (nZTA) and alumina ceramics

Journal of the European Ceramic Society

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of nanocrystalline barium titanate: Effect of microwave power on phase evolution

Journal of the European Ceramic Society, 2020

Abstract Nanocrystalline BaTiO3 (BT) powder was synthesised using a polymer precursor route and t... more Abstract Nanocrystalline BaTiO3 (BT) powder was synthesised using a polymer precursor route and the influence of conventional, microwave and combined hybrid heating methods on phase formation was investigated. A single-phase tetragonal BT (t-BT) nanocrystalline powder of about 20 nm primary particle size and decreased agglomeration were formed when high levels of microwave energy were used. This was accomplished at a lower processing condition of 700 °C for 30 min compared to conventional processing, which required 900 °C for 5 h, resulting in potential savings in time and energy. During the nano BT synthesis, the role of microwaves was determined by subjecting the samples to identical thermal histories, i.e. exactly the same time-temperature profiles, while using a range of different levels of microwave power. Significant reduction in the activation energy for the formation of the tetragonal phase was observed with increasing levels of microwave power and the results are explained in terms of a possible non-thermal mechanism. Furthermore, under otherwise identical thermodynamic conditions of temperature, time and (atmospheric) pressure, the co-occurrence of hexagonal crystal structure at

Development of Nano Zirconia Toughened Alumina for Ceramic Armor Applications

Advances in Ceramic Armor VII, 2011

Quantitative analysis of the residual stress and dislocation density distributions around indentations in alumina and zirconia toughened alumina (ZTA) ceramics

Journal of the European Ceramic Society, 2014

Alumina, 10% and 20% nano ZTA with 1.5 mol% yttria stabiliser were subjected to Vickers indentati... more Alumina, 10% and 20% nano ZTA with 1.5 mol% yttria stabiliser were subjected to Vickers indentation testing with loads from 1 to 20 kg. Cr 3+ fluorescence and Raman spectroscopy were applied to the indent centre and around the indentation in order to investigate the origin of the signal, the effect of indentation loads and zirconia phase transformation on the residual stress and plastic deformation in the plastic zone. The results suggested that with very strong laser scattering, the depth resolution of ZTA materials was very poor, which lead to a very significant amount of the signal being collected from the subsurface regions below the plastic zone. It was also found that zirconia phase transformation reduced the compressive residual stress in the alumina matrix within the plastic zone, except at the indentation centre, due to the tensile residual microstress generated by the zirconia phase transformation. In addition, the dislocation density on the indent surface of the ZTA samples was significantly reduced due to the restriction of crack propagation and energy absorption during the phase transformation process. At the indent centre, the zirconia phase transformation was suppressed by the high compressive stress, therefore, no significant difference between alumina and ZTA in terms of their residual stress and dislocation density were observed. Using TEM observation, it was found that the plastic zone microstructure of pure alumina is different from that of ZTA, which is consistent with the Cr 3+ fluorescence results. the microcrack density in the plastic zone of the ZTA was confirmed to be lower than that of the alumina sample.

Direct ink writing of bismuth molybdate microwave dielectric ceramics

Ceramics International, 2021

Additive manufacturing via direct ink writing and microwave dielectric characterisation of commer... more Additive manufacturing via direct ink writing and microwave dielectric characterisation of commercially produced low sintering temperature bismuth molybdenum oxide ceramics, have been both performed for the first time, following a powder-to-product holistic approach. We demonstrated that direct ink writing is an excellent candidate for producing dielectric substrates to be used for wireless telecommunication applications operating at microwave (MW) frequencies, with great repeatability and properties comparable to ceramics fabricated via conventional processing routes. The optimum density (relative density of ρr ≈ 93%) of the 3D printed test samples was obtained by sintering at 660 °C for 2 hours, resulting in a relative permittivity εr = 35.7, dielectric loss tanδ = 0.0004 and microwave quality factor Q×f = 14,928 GHz. Sintering at higher temperatures promoted a porosity increase due to mismatching grain growth mechanisms and phase decomposition, that collectively hindered the test samples' microwave dielectric performance in terms of achievable relative permittivity (εr) and dielectric loss (tanδ).

Multi-material additive manufacture and microwave-assisted sintering of a metal/ceramic metamaterial antenna structure

Applied Materials Today

3D Antennas, Metamaterials, and Additive Manufacturing

2019 IEEE MTT-S International Wireless Symposium (IWS), 2019

This paper presents several demonstrators that utilize advanced additive manufacturing (AM) to ra... more This paper presents several demonstrators that utilize advanced additive manufacturing (AM) to rapidly prototype antennas, RF components for microwave and mm-wave applications. Antenna substrates with tailored permittivity; lenses, reflectarray with locally varied dielectric properties; and metamaterials can be fabricated by using extrusion based AM processes such as fused filament fabrication method to realize high gain antenna systems with low material cost, time-saving and low impact to the environment.