Browsing by Author "Ni, Chaoying"

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    Assessment of Directionally Solidified Eutectic Sm–Fe(Co)–Ti Alloys as Permanent Magnet Materials
    (IEEE Transactions on Magnetics, 2023-05-29) Gabay, Alexander M.; Han, Chaoya; Ni, Chaoying; Hadjipanayis, George C.
    Sm–Fe–Ti and Sm–Fe 0.8 Co 0.2 –Ti alloys were prepared via arc-melting and directionally solidified on a water-cooled copper hearth. The as-solidified alloys featured cells of the Sm(Fe,Co,Ti) 12 –Ti(Fe,Co) 2+δ –(α-Fe) lamellar eutectic. The lamellae of Sm(Fe,Co,Ti) 12 phase with a crystal structure of the ThMn12 type were less than 0.2 μm thick, and had their [001] easy-magnetization directions oriented along the temperature gradient of the solidification. The eutectic microstructure led to an increased coercivity, especially in the Co-added alloys. Below 250 °C, this coercivity was found not to vary much with temperature with a temperature coefficient of -0.18 %/°C. However, the modest absolute values, reaching only 0.7 kOe, are insufficient for utilization of the directionally solidified alloys as anisotropic permanent magnets.
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    Band alignment and p-type doping of ZnSnN2
    (American Physical Society, 2017-05-31) Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Tianshi Wang, Chaoying Ni, and Anderson Janotti; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson
    Composed of earth-abundant elements, ZnSnN2 is a promising semiconductor for photovoltaic and photoelectrochemical applications. However, basic properties such as the precise value of the band gap and the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap vary from 1.4 to 2.0 eV. In addition, doping in ZnSnN2 remains largely unexplored. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ZnSnN2, its band alignment to GaN and ZnO, and the possibility of p-type doping. We find that the position of the valence-band maximum of ZnSnN2 is 0.39 eV higher than that in GaN, yet the conduction-band minimum is close to that in ZnO, which suggests that achieving p-type conductivity is likely as in GaN, yet it may be difficult to control unintentional n-type conductivity as in ZnO. Among possible p-type dopants, we explore Li, Na, and K substituting on the Zn site. We show that while LiZn is a shallow acceptor, NaZn and KZn are deep acceptors, which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.
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    Carbon Additive Manufacturing with a Near-Replica “Green-to-Brown” Transformation
    (Advanced Materials, 2023-05-30) Zhang, Chunyan; Shi, Baohui; He, Jinlong; Zhou, Lyu; Park, Soyeon; Doshi, Sagar; Shang, Yuanyuan; Deng, Kaiyue; Giordano, Marc; Qi, Xiangjun; Cui, Shuang; Liu, Ling; Ni, Chaoying; Fu, Kun Kelvin
    Nanocomposites containing nanoscale materials offer exciting opportunities to encode nanoscale features into macroscale dimensions, which produces unprecedented impact in material design and application. However, conventional methods cannot process nanocomposites with a high particle loading, as well as nanocomposites with the ability to be tailored at multiple scales. A composite architected mesoscale process strategy that brings particle loading nanoscale materials combined with multiscale features including nanoscale manipulation, mesoscale architecture, and macroscale formation to create spatially programmed nanocomposites with high particle loading and multiscale tailorability is reported. The process features a low-shrinking (<10%) “green-to-brown” transformation, making a near-geometric replica of the 3D design to produce a “brown” part with full nanomaterials to allow further matrix infill. This demonstration includes additively manufactured carbon nanocomposites containing carbon nanotubes (CNTs) and thermoset epoxy, leading to multiscale CNTs tailorability, performance improvement, and 3D complex geometry feasibility. The process can produce nanomaterial-assembled architectures with 3D geometry and multiscale features and can incorporate a wide range of matrix materials, such as polymers, metals, and ceramics, to fabricate nanocomposites for new device structures and applications.
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    Load and release of gambogic acid via dual-target ellipsoidal-Fe3O4@SiO2@mSiO2-C18@dopamine hydrochloride -graphene quantum dots-folic acid and its inhibition to VX2 tumor cells
    (Nanotechnology, 2022-12-19) Dong, Mengyang; Liu, Wenwen; Yang, Yuxiang; Xie, Meng; Yuan, Hongming; Ni, Chaoying
    Ellipsoidal-Fe3O4@SiO2@mSiO2-C18@dopamine hydrochloride-graphene quantum dots-folic acid (ellipsoidal-HMNPs@PDA-GQDs-FA), a dual-functional drug carrier, was stepwise constructed. The α-Fe2O3 ellipsoidal nanoparticles were prepared by a hydrothermal method, and then coated with SiO2 by Stöber method. The resulting core–shell structure, Fe3O4@SiO2@mSiO2-C18 magnetic nano hollow spheres, abbreviated as HMNPs, was finally grafted with graphene quantum dots (GQDs), dopamine hydrochloride (PDA) and folic acid (FA) by amide reaction to obtain HMNPs@PDA-GQDs-FA. Transmission electron microscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy and element analysis proved the successful construction of the HMNPs@PDA-GQDs-FA nanoscale carrier-cargo composite. The carrier HMNPs@PDA-GQDs-FA has higher load (51.63 ± 1.53%) and release (38.56 ± 1.95%) capacity for gambogic acid (GA). Cytotoxicity test showed that the cell survival rate was above 95%, suggesting the cytotoxicity of the carrier-cargo was very low. The cell lethality (74.91 ± 1.2%) is greatly improved after loading GA because of the magnetic targeting of HMNPs, the targeting performance of FA to tumor cells, and the pH response to the surrounding environment of tumor cells of PDA. All results showed that HMNPs@PDA-GQDs-FA had good biocompatibility and could be used in the treatment of VX2 tumor cells after loading GA.
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    Screen-Printable Contacts for Industrial N-TOPCon Crystalline Silicon Solar Cells
    (IEEE Journal of Photovoltaics, 2022-01-13) Lu, Meijun; Mikeska, Kurt R.; Ni, Chaoying; Zhao, Yong; Chen, Feibiao; Xie, Xianqing; Xu, Yawen; Zhang, Changgen
    Optimally prepared industrial n -type bifacial tunnel oxide passivated contacts c-Si solar cells (156 × 156 mm) fabricated with cost effective screen-printable front-side (FS) and rear-side (RS) silver pastes had a median solar cell efficiency of 22.21% ± 0.10% and bifaciality efficiency factor of 82.9%. A FS paste comprising silver, metallic aluminum, and inorganic frit was designed to contact p+ boron-diffused Si emitter surfaces with SiN x :H–Al 2 O 3 antireflection-passivation layers. A RS paste comprising silver and inorganic frit was designed to contact n+ phosphorous-doped surfaces with tunnel-SiO x / n+ poly-Si/SiN x :H layers. The bifacial electrical data indicates efficiency is being limited by the FS contact. The final FS bulk silver metal region microstructure shows isolated metallic aluminum particles surrounded by solidified liquid phase within the bulk sintered silver conductor line. The FS silver metal- p+ boron-diffused emitter contact region shows continuous interfacial (IF) films decorated with silver colloids located between the bulk silver metal and emitter surface. The final RS silver metal- n+ phosphorus diffused contact region again shows continuous IF films between the bulk silver metal and semiconductor surface. A microstructural model suggests electrical contact for both the FS and RS contact regions occurs by a tunneling mechanism though the residual IF films.
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    Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces
    (American Physical Society, 2017-02-22) Ye, Ning; Feser, Joseph P.; Sadasivam, Sridhar; Fisher, Timothy S.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Ning Ye, Joseph P. Feser, Sridhar Sadasivam, Timothy S. Fisher, Tianshi Wang, Chaoying Ni, and Anderson Janotti; Ye, Ning; Feser, Joseph P.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson
    Silicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi2, CoSi2, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations.While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of CoSi2, a comparison ismade with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green’ function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for CoSi2, NiSi, and TiSi2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration onmetal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (n or p type) ≈1 × 1019 cm−3, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.