@article{jia_yang_xu_snyder_patrick_kumar_zhang_xu_2023, title={Polymer-derived SiOC reinforced with core-shell nanophase structure of ZrB2/ZrO2 for excellent and stable high-temperature microwave absorption (up to 900 degrees C)}, volume={13}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-023-27541-3}, abstractNote={AbstractMicrowave absorbing materials for high-temperature harsh environments are highly desirable for aerodynamically heated parts and engine combustion induced hot spots of aircrafts. This study reports ceramic composites with excellent and stable high-temperature microwave absorption in air, which are made of polymer-derived SiOC reinforced with core–shell nanophase structure of ZrB2/ZrO2. The fabricated ceramic composites have a crystallized t-ZrO2 interface between ZrB2 and SiOC domains. The ceramic composites exhibit stable dielectric properties, which are relatively insensitive to temperature change from room temperature to 900 °C. The return loss exceeds − 10 dB, especially between 28 and 40 GHz, at the elevated temperatures. The stable high-temperature electromagnetic (EM) absorption properties are attributed to the stable dielectric and electrical properties induced by the core–shell nanophase structure of ZrB2/ZrO2. Crystallized t-ZrO2 serve as nanoscale dielectric interfaces between ZrB2 and SiOC, which are favorable for EM wave introduction for enhancing polarization loss and absorption. Existence of t-ZrO2 interface also changes the temperature-dependent DC conductivity of ZrB2/SiOC ceramic composites when compared to that of ZrB2 and SiOC alone. Experimental results from thermomechanical, jet flow, thermal shock, and water vapor tests demonstrate that the developed ceramic composites have high stability in harsh environments, and can be used as high-temperature wide-band microwave absorbing structural materials.}, number={1}, journal={SCIENTIFIC REPORTS}, author={Jia, Yujun and Yang, Ni and Xu, Shaofan and Snyder, Alexander D. D. and Patrick, Jason F. F. and Kumar, Rajan and Zhang, Dajie and Xu, Chengying}, year={2023}, month={Jan} }
 @article{jia_mehta_li_chowdhury_horn_xu_2021, title={Additive manufacturing of ZrB2-ZrSi2 ultra-high temperature ceramic composites using an electron beam melting process}, volume={47}, ISSN={["1873-3956"]}, DOI={10.1016/j.ceramint.2020.09.082}, abstractNote={Owing to their high melting points and ability to resist extreme thermal stresses, ultra-high temperature ceramics (UHTCs) are important materials for critical applications such as hypersonic flights, space re-entry vehicles, and rocket engines. Traditional manufacturing processes restrict the freedom to manufacture UHTCs with complex geometries due to the limitations of die and mold designs. Electron beam melting (EBM) is an established powder-bed layer-by-layer additive manufacturing (AM) process for metal parts. In this research, an effort was made to evaluate the feasibility of EBM for the AM fabrication of UHTC-based materials, and to investigate the microstructures of the fabricated materials under different processing conditions. A mathematical model was developed to simulate and optimize the processing parameters for the fabrication of ZrB2-30 vol% ZrSi2 UHTC using EBM. The simulation results were compared with experimental observations. For EBM fabrication of ZrB2-30 vol% ZrSi2 composites, the optimal processing parameters are beam power of 500 W with scanning speeds of 500, 750, and 1000 mm/s, and beam power of 1000 W with scanning speed of 1000 mm/s. This study demonstrates the potential for additive manufacturing of UHTCs with complex geometries by the EBM technique.}, number={2}, journal={CERAMICS INTERNATIONAL}, author={Jia, Yujun and Mehta, Shashvat Tejaskumar and Li, Ryan and Chowdhury, Md Atiqur Rahman and Horn, Timothy and Xu, Chengying}, year={2021}, month={Jan}, pages={2397–2405} }
 @article{jia_chowdhury_xu_2020, title={Complex impedance spectra of polymer-derived SiC annealed at ultrahigh temperature}, volume={103}, ISSN={["1551-2916"]}, DOI={10.1111/jace.17395}, abstractNote={AbstractThis study reports the complex impedance and alternative current conductivity of polymer‐derived ceramic SiC (PDC‐SiC) annealed at ultrahigh temperatures. The PDC‐SiC shows an inductive response when annealed at temperatures of 1700°C‐1900°C due to the percolation of turbostratic carbon. The material returns to a capacitive response at an annealing temperature of 2000°C due to the dissolution of carbon into the SiC lattice. The electrical resistance of the carbon phase decreases with the increase in annealing temperature. These results provide new insights into the effects of processing temperature on microstructure evolution and electrical and dielectric property development of the PDC‐SiC ceramic system.}, number={12}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Jia, Yujun and Chowdhury, Md Atiqur Rahman and Xu, Chengying}, year={2020}, month={Dec}, pages={6860–6868} }
 @article{jia_ajayi_xu_2020, title={Dielectric properties of polymer-derived ceramic reinforced with boron nitride nanotubes}, volume={103}, ISSN={["1551-2916"]}, DOI={10.1111/jace.17301}, abstractNote={AbstractThe electrical and dielectric properties of boron nitride nanotubes (BNNTs) reinforced ceramic composites using the polymer‐derived ceramic (PDC) processing route were investigated in this work. The electrical resistivity of the pristine PDC increases from 106 to 108 Ω m after the addition of BNNTs. When the BNNT loading was increased to 5 wt%, the average real relative permittivity of the PDC decreased from 2.94 to 2.80, while the quality factor (Q) of the PDC increased from 134.40 to 176.77. The BNNTs can increase the Q factor of the PDC due to the reduction in the porosity cause by the introduction of the BNNTs. Further increasing the BNNT content decreases the real relative permittivity of the nanocomposites and increases the Q factor at high frequency. The average real relative permittivity decreases to 2.29, while the average Q factor increases to 208.60 when the BNNT content is increased to 30 wt%. The dielectric loss after the addition of high fraction of BNNTs can be explained by the Lorentz resonance relaxation process. Results of this work showed that PDC‐BNNT nanocomposites are satisfactory electromagnetic transparent materials when the BNNT fraction is less than 10 wt%.}, number={10}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Jia, Yujun and Ajayi, Tosin D. and Xu, Chengying}, year={2020}, month={Sep}, pages={5731–5742} }
 @article{chowdhury_wang_jia_xu_2020, title={Electrical Conductivity and Structural Evolution of Polymer Derived SiC Ceramics Pyrolyzed From 1200°C to 1800°C}, volume={8}, ISSN={2166-0468 2166-0476}, url={http://dx.doi.org/10.1115/1.4046191}, DOI={10.1115/1.4046191}, abstractNote={Abstract
               Room temperature (RT) electrical conductivity and microstructure of polymer-derived SiC pyrolyzed at temperatures ranging from 1200 °C to 1800 °C were studied. We have shown that both free carbon content and pyrolysis temperature have significant effects on the DC conductivity of polymer derived ceramic (PDC) SiC. The RT DC conductivity of the PDC SiC increased gradually with increasing pyrolysis temperature, and it drastically increases 3 orders of magnitude after 1500 °C. This high electrical conductivity occurs due to the formation of a network of turbostratic carbon (percolative network). Below the percolation regime, hopping enables the electron movement from one carbon cluster site to another. Microstructural investigation with X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) analysis showed that the crystal size of SiC increases with increasing pyrolysis temperature, and carbon clusters act as an inhibitor for grain growth at lower pyrolysis temperature. Upon dissociation of clusters, accelerated grain growth occurs and graphitization of carbon occurs along the grains.}, number={2}, journal={Journal of Micro and Nano-Manufacturing}, publisher={ASME International}, author={Chowdhury, Md Atiqur and Wang, Kewei and Jia, Yujun and Xu, Chengying}, year={2020}, month={Feb} }
 @article{jia_ajayi_wahls_ramakrishnan_ekkad_xu_2020, title={Multifunctional Ceramic Composite System for Simultaneous Thermal Protection and Electromagnetic Interference Shielding for Carbon Fiber-Reinforced Polymer Composites}, volume={12}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/acsami.0c17361}, DOI={10.1021/acsami.0c17361}, abstractNote={Achieving a high electrical conductivity while maintaining a good thermal insulation is often contradictory in the material design for the goal of simultaneous thermal protection and electromagnetic interference shielding. The reason is that materials with a high electrical conductivity often pertain a high thermal conductivity. To address this challenge, this study reports a multifunctional ceramic composite system for carbon fiber-reinforced polymer composites. The fabricated multifunctional ceramic composite system has a multilayer structure. The polymer-derived SiCN ceramic reinforced with yttria-stabilized zirconia fibers serves as the thermal protection and impedance-matching layer, while the yttria-stabilized zirconia fiber-reinforced SiCN ceramic with carbon nanotubes provides the electromagnetic interference shielding. The thermal conductance of the multilayered ceramic composite is about 22.5% lower compared to that of the carbon fiber-reinforced polymer composites. The thermal insulation test during the steady-state condition shows that the hybrid composite can be used up to 300 °C while keeping the temperature reaching the surface of carbon fiber-reinforced polymer composites at around 167.8 °C. The flame test was used to characterize the thermal protection capability under transient conditions. The hybrid composite showed temperature differences of 72.9 and 280.7 °C during the low- and high-temperature settings, respectively. The average total shielding efficiency per thickness of the fabricated four-layered ceramic composite system was 21.45 dB/mm, which showed a high reflection-dominant electromagnetic interference shielding. The average total shielding efficiency per thickness of the eight-layered composite system was 16.57 dB/mm, revealing a high absorption-dominant electromagnetic interference shielding. Typical carbon fiber-reinforced polymer composites reveal a reflection-dominant electromagnetic interference shielding. The electrons can freely move in the percolated carbon nanotubes within the inner layers of the composite material, which provide the improved electromagnetic interference shielding ability. The movement of electrons was impeded by the top and bottom layers whose thermal conduction relies on the lattice vibrations, resulting in a satisfactory thermal insulation of the composite materials and impedance matching with the free space. Results of this study showed that materials with a good thermal insulation and electromagnetic interference shielding can be obtained simultaneously by confining the electron movement inside the materials and refraining their movement at the skin surface.}, number={52}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Jia, Yujun and Ajayi, Tosin D. and Wahls, Benjamin H. and Ramakrishnan, Kishore Ranganath and Ekkad, Srinath and Xu, Chengying}, year={2020}, month={Dec}, pages={58005–58017} }
 @article{jia_ajayi_roberts_chung_xu_2020, title={Ultrahigh-Temperature Ceramic-Polymer-Derived SiOC Ceramic Composites for High-Performance Electromagnetic Interference Shielding}, volume={12}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.0c08479}, abstractNote={High-performance electromagnetic interference (EMI) shielding materials for high-temperature harsh environment are highly required for electronics and aerospace applications. Here, a composite made of ultra-high temperature ceramic and polymer derived SiOC ceramic (PDC-SiOC) with high EMI shielding was reported for such applications. A total EMI shielding efficiency (SET) of 26.67 dB with a thickness of 0.6 mm at Ka-band (26.5-40 GHz) was reported for ZrB2 fabricated by spark plasma sintering, which showed reflection dominant shielding. A unique interface of t-ZrO2 was formed after the introduction of PDC-SiOC into ZrB2. This interface has better electrical conductivity than SiOC. The composites also displayed reflection dominant shielding. Accordingly, the composite with a normalized ZrB2 fraction of 50 % pyrolyzed at 1000 °C exhibited significantly SET of 72 dB (over 99.99999 % shielded) with a thickness of 3 mm at the entire Ka-band. A maximum SET of 90.8 dB (over 99.9999999 % shielded) was achieved with a thickness of 3 mm at around 39.7 GHz.}, number={41}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Jia, Yujun and Ajayi, Tosin D. and Roberts, Mark A., Jr. and Chung, Ching-Chang and Xu, Chengying}, year={2020}, month={Oct}, pages={46254–46266} }
 @article{jia_yang_wang_chowdhury_chen_su_nickerson_xu_2019, title={Aligned carbon nanotube/carbon (CNT/C) composites with exceptionally high electrical conductivity at elevated temperature to 400 degrees C}, volume={6}, ISSN={["2053-1591"]}, DOI={10.1088/2053-1591/ab4385}, abstractNote={To obtain a high electrical conductivity for aerospace and defense applications, in this paper, we investigated the electrical properties of carbon nanotube reinforced carbon (CNT/C) composites at elevated temperature up to 400 °C in air. The CNT/C composites were made of aligned CNT sheet and pyrolytic carbon (PyC) by chemical vapor infiltration (CVI) process. The electrical conductivity of the aligned CNT/C composites is 3.2 × 106 S · m−1 in room temperature and shows a positive temperature dependence as a function of the measuring temperature, and the conductivity reaches to the order of ∼107 S · m−1 at 400 °C. We concluded that the conduction in the aligned CNT/C composite is mainly dominated by the tunneling conductivity mechanism. These unique features are related to the structural change of composites by infiltrating amorphous PyC into the space in between of the CNT bundles. The exceptionally high conductivity might be contributed to the electron transport mechanism between the CNT bundles and the PyC matrix.}, number={11}, journal={MATERIALS RESEARCH EXPRESS}, author={Jia, Yujun and Yang, Jinshan and Wang, Kewei and Chowdhury, Md Atiqur Rahman and Chen, Banghao and Su, Yifeng and Nickerson, Bill C. and Xu, Chengying}, year={2019}, month={Sep} }
 @article{chowdhury_wang_jia_xu_2020, title={Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200 degrees C to 1800 degrees C}, volume={103}, ISSN={["1551-2916"]}, DOI={10.1111/jace.16961}, abstractNote={AbstractThis paper studies the effect of pyrolysis temperature on the semiconductor‐conductor transition of pristine polymer‐derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C‐1800°C is presented. At relatively lower pyrolysis temperatures (1200°C‐1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C‐1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal‐like conductive response and their resistivity increases monotonically with the increasing measuring temperature.}, number={4}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Chowdhury, Md Atiqur Rahman and Wang, Kewei and Jia, Yujun and Xu, Chengying}, year={2020}, month={Apr}, pages={2630–2642} }
 @article{jia_ajayi_morales_chowdhury_sauti_chu_park_xu_2019, title={Thermal properties of polymer-derived ceramic reinforced with boron nitride nanotubes}, volume={102}, ISSN={["1551-2916"]}, DOI={10.1111/jace.16670}, abstractNote={AbstractWe report the thermal properties of boron nitride nanotube (BNNT) reinforced ceramic composites using the polymer derived ceramic (PDC) processing route. The nano‐composites had a BNNT loading of up to 35.4 vol.%. TGA results showed that nano‐composites have good thermal stability up to 900°C in air. BNNTs in nano‐composites survived in an oxidizing environment up to 900°C, revealing that nano‐composites can be used for high temperature applications. Thermal conductivity of PDC reinforced with 35.4 vol.% BNNT was measured as 4.123 W/(m·K) at room temperature, which is a 2100 % increase compared to that of pristine PDC. The thermal conductivity value increases with the increase of BNNT content. A thermal conductivity percolation phenomenon appeared when the BNNT content increased to 36 ± 5 vol.%. The results of this study showed that BNNTs could effectively improve the thermal conductivity of PDC materials. BNNT reinforced PDC could be used as thermal structural materials in a harsh environment at temperatures up to 900°C.}, number={12}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, author={Jia, Yujun and Ajayi, Tosin D. and Morales, Justin and Chowdhury, Md Atiqur Rahman and Sauti, Godfrey and Chu, Sang-Hyon and Park, Cheol and Xu, Chengying}, year={2019}, month={Dec}, pages={7584–7593} }
 @article{jia_chowdhury_zhang_xu_2019, title={Wide-Band Tunable Microwave-Absorbing Ceramic Composites Made of Polymer-Derived SiOC Ceramic and in Situ Partially Surface-Oxidized Ultra-High-Temperature Ceramics}, volume={11}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.9b16475}, abstractNote={Microwave absorbing materials in high-temperature harsh environment are highly desired for electronics and aerospace applications. This study reports a novel high-temperature microwave absorbing ceramic composites made of polymer derived SiOC ceramic and in-situ partially surface-oxidized ultra-high temperature ceramic (UHTC) ZrB2 nanoparticles. The fabricated composites with a normalized weight fraction of ZrB2 nanoparticles at 40% has a significantly wide microwave absorption bandwidth of 13.5 GHz (26.5-40GHz) covering the entire Ka-band. This is attributed to the extensive nanointerfaces introduced in the composites, attenuation induced by the interference of electromagnetic wave, attenuation from the formed current loops and the electronic conduction loss provided by the partially surface-oxidized ZrB2 nanoparticles. The minimum reflection coefficient (RC) was -29.30 dB at 29.47 GHz for a thickness of 1.26 mm for the composites with a normalized weight fraction of ZrB2 nanoparticles at 32.5%. The DC conductivity of the nanocomposites showed a clear percolation phenomenon as the normalized weight fraction of ZrB2 nanoparticles increases to 30.49 %. The results provide new insights in designing microwave absorbing materials with a wide absorption frequency range and strong absorption loss for high-temperature harsh environment applications.}, number={49}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Jia, Yujun and Chowdhury, Md Atiqur Rahman and Zhang, Dajie and Xu, Chengying}, year={2019}, month={Dec}, pages={45862–45874} }