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"Reactive hard templating" : from carbon nitrides to metal nitridesFischer, Anna January 2008 (has links)
Nanostructured inorganic materials are routinely synthesized by the use of templates. Depending on the synthesis conditions of the product material, either “soft” or “hard” templates can be applied. For sol-gel processes, usually “soft” templating techniques are employed, while “hard” templates are used for high temperature synthesis pathways. In classical templating approaches, the template has the unique role of structure directing agent, in the sense that it is not participating to the chemical formation of the resulting material. This work investigates a new templating pathway to nanostructured materials, where the template is also a reagent in the formation of the final material. This concept is described as “reactive templating” and opens a synthetic path toward materials which cannot be synthesised on a nanometre scale by classical templating approaches. Metal nitrides are such kind of materials. They are usually produced by the conversion of metals or metal oxides in ammonia flow at high temperature (T > 1000°C), which make the application of classical templating techniques difficult.
Graphitic carbon nitride, g-C3N4, despite its fundamental and theoretical importance, is probably one of the most promising materials to complement carbon in material science and many efforts are put in the synthesis of this material. A simple polyaddition/elimination reaction path at high temperature (T = 550°C) allows the polymerisation of cyanamide toward graphitic carbon nitride solids. By hard templating, using nanostructured silica or aluminium oxide as nanotemplates, a variety of nanostructured graphitic carbon nitrides such as nanorods, nanotubes, meso- and macroporous powders could be obtained by nanocasting or nanocoating. Due to the special semi-conducting properties of the graphitic carbon nitride matrix, the nanostructured graphitic carbon nitrides show unexpected catalytic activity for the activation of benzene in Friedel-Crafts type reactions, making this material an interesting metal free catalyst. Furthermore, due to the chemical composition of g-C3N4 and the fact that it is totally decomposed at temperatures between 600°C and 800°C even under inert atmosphere, g-C3N4 was shown to be a good nitrogen donor for the synthesis of early transition metal nitrides at high temperatures. Thus using the nanostructured carbon nitrides as “reactive templates” or “nanoreactors”, various metal nitride nanostructures, such as nanoparticles and porous frameworks could be obtained at high temperature. In this approach the carbon nitride nanostructure played both the role of the nitrogen source and of the exotemplate, imprinting its size and shape to the resulting metal nitride nanostructure. / Die Nanostrukturierung anorganischer Materialien, d.h. die Kontrolle ihrer Form und Größe auf der Nanometerebene durch unterschiedliche Herstellungsverfahren, ist ein sich immer noch erweiterndes Forschungsgebiet. Eine solche Nanostrukturierung wird oft über sogenannte Templatierungsverfahren erreicht: Hier werden Formgeber (Template) mit definierter Morphologie und Größe verwendet und deren Struktur in ein neues Material abgebildet. Templatierungsverfahren können, je nach der Beschaffenheit des Templats, zwischen „weich“ und „hart“ unterschieden werden. Die Begriffe beziehen sich dabei vor allem auf die mechanische und thermische Stabilität der Template, d.h. weiche Template sind vornehmlich organischer, harte Template anorganischer Natur. Wo weiche Template in milden chemischen Verfahren eingesetzt werden, werden harte Template zur Herstellung von Materialien bei Hochtemperaturverfahren verwendet (z. B. poröse Kohlenstoffe). Allgemein dienen Template ausschließlich als Strukturgeber und gehen in keiner Weise in Form einer chemischen Reaktion in die Synthese des gewünschten Materials mit ein. Gegenstand dieser Arbeit ist ein neues Templatierungsverfahren: Die „reaktive Templatierung“. Hierbei wird das Templat - neben seiner Funktion als Strukturgeber – auch als Reagenz für die Synthese des Produktes verwendet. Dieser Synthese-Ansatz öffnet damit neue Wege für die Synthese von nanostrukturierten Materialien, die durch klassische Templatierungsansätze schwer zugänglich sind. Hierzu zählen zum Beispiel die Metallnitride. Üblicherweise werden Metallnitride über die Umsetzung von Metallen oder Metalloxiden in einem Ammoniakstrom bei Mindesttemperaturen von 1000°C gewonnen, was die Anwendung klassischer Templatierungsverfahren beinahe unmöglich macht. Darüber hinaus sind konzentrierte Lauge oder Flusssäure, welche zur Entfernung klassischer harter Template benötigt werden auch Aufschlussmittel für Metallnitride.
Graphitisches Kohlenstoffnitrid, g-C3N4, ist wohl eines der meistversprechendsten Materialien um Kohlenstoff in der Materialwissenschaft zu ergänzen. Es wurden bereits viele potentielle Syntheseansätze beschrieben. Eine durch Groenewolt M. erstellte Route ist die thermisch induzierte Polykondensation von Cyanamid (NCNH2) bei 550°C. Da g-C3N4 sich zwischen 600°C und 800°C vollständig in NH3 und CxNyH-Gase zersetzt, ist es eine geeignete Festkörper-Stickstoffquelle für die Herstellung von Metalnitriden. Daher boten sich nanostrukturierte graphitische Kohlenstoffnitride als geeignete reaktive Template oder Nanoreaktoren zur Herstellung von nano-strukturierten Metalnitriden an. Die Templatierung der g-C3N4-Matrix wurde über klassische Harttemplatierungsverfahren erreicht. So konnte eine Vielzahl nano-strukturierter g-C3N4 Materialien synthetisiert werden wie zum Beispiel Nanostäbchen, Nanoröhren, mesoporöse oder makroporöse graphitische Kohlenstoffnitride. Diese haben sich interessanterweise, als metalfreie Katalysatoren für die Aktivierung von Benzol in Friedel-Crafts-Acylierung und -Alkylierung erwiesen. Durch die Infiltrierung der nano-strukturierten g-C3N4-Materialien mit diversen Metal-Präkursoren und nachfolgendem Tempern bei 800°C unter Schutzgas, konnten entsprechende nano-strukturierte Metalnitride, als Nanoabdrücke der vorgegebenen Kohlenstoffnitrid Nanostrukturen hergestellt werden. So konnten TiN, VN, GaN, AlGaN und TiVN Nanopartikel synthetisiert werden, macroporöse TiN/Kohlenstoff Komposite sowie TiN Hohlkugeln. Die so hergestellten Materialien erwiesen sich als effektive basische Katalysatoren für Aldol-Kondensations Reaktionen.
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Investigation of self-heating and macroscopic built-in polarization effects on the performance of III-V nitride devicesVenkatachalam, Anusha. January 2009 (has links)
Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.
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Development of new cathodic interlayers with nano-architectures for lithium-sulfur batteriesZhao, Teng January 2018 (has links)
Issues with the dissolution and diffusion of polysulfides in liquid organic electrolytes hinder the advance of lithium–sulfur (Li-S) batteries for next generation energy storage. To trap and re-utilize the polysulfides, brush-like, zinc oxide (ZnO) nanowires based interlayers were prepared ex-situ using a wet chemistry method and were coupled with a sulfur/multi-walled carbon nanotube (S/MWCNT) composite cathode. The cell with this configuration showed a good cycle life at a high current rate ascribed to (a) a strong interaction between the polysulfides and ZnO nanowires grown on conductive substrates; (b) fast electron transfer and (c) an optimized ion diffusion path from a well-organized nanoarchitecture. A praline-like flexible interlayer consisting of titanium oxide (TiO2) nanoparticles and carbon (C) nanofiber was further prepared in-situ using an electrospinning method, which allows the chemical adsorption of polysulfides throughout a robust conductive film. A significant enhancement in cycle stability and rate capability was achieved by incorporating this interlayer with a composite cathode of S/MWCNT. These results herald a new approach to building functional interlayers by integrating metal oxides with conductive frameworks. The derivatives of the TiO2/C interlayer was synthesized by changing the precursor concentration and carbonization temperature. Finally, a dual-interlayer was fabricated by simply coating titanium nitride (TiN) nanoparticles onto an electro-spun carbon nanofiber mat, which was then sandwiched with a sulfur/assembled Ketjen Black (KB) composite cathode with an ultra-high sulfur loading. The conductive polar TiN nanoparticles not only have a strong chemical affinity to polysulfides through a specific sulfur-nitrogen bond but also improve the reaction kinetics of the cell by catalyzing the conversion of the long-chain polysulfides to lithium sulfide. Besides, carbon nanofiber mat ensures mechanical robustness to TiN layer and acts as a physical barrier to block polysulfides diffusion. The incorporation of dual interlayers with sulfur cathodes offers a commercially feasible approach to improving the performance of Li-S batteries.
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Synthesis and design of alternative plasmonic materials for core-multishell nanowire photonic devicesHansen, Katherine E. 05 November 2020 (has links)
One of the keys to successful commercialization of photonic devices is compatibility with complementary metal-oxide-semiconductor technology (CMOS), the major platform of the microelectronics industry. Silicon photonics, with plasmonic materials are promising candidates for next generation chip-scale technology. The majority of plasmonics research has focused on noble metals, which are not CMOS compatible. Transition metal nitrides are an emerging class of alternative plasmonic materials that are complementary metal-oxide-semiconductor compatible and have shown promising results when compared to devices utilizing noble metals.
This dissertation highlights, a CMOS compatible method to produce such alternative plasmonic materials using atomic layer deposition (ALD), specifically ultrathin plasmonic titanium nitride, aluminum metal and zirconium nitride. A post-deposition hydrogen plasma treatment is also introduced to improve the metallic properties of the ultrathin films. Additionally, this dissertation proposes a core-multishell (CMS) nanowire (NW) device structure that utilizes these materials to enable the creation of photonic devices, specifically detailing designs for cloaking and photoelectrochemical (PEC) water splitting applications. It is shown theoretically that zirconium nitride cloaks a silicon nanowire without substantially compromising the absorption of light, resulting in a less-intrusive, better performing silicon nanowire photosensor, and outperforms a gold cloak in the wavelength region of 400-500 nm. It is demonstrated theoretically that emerging plasmonic materials TiN and ZrN are promising candidates to improve the ideal photocurrent density hematite photoanodes in core-multishell nanowire devices, allowing hematite to remain electrically thin enough to effectively transport charge carriers while absorbing light similar to thick hematite features.
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Thermodynamic investigations of transition metal systems containing coabon and nitrogenTeng, Lidong January 2004 (has links)
<p>In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF<sub>2</sub><sup> </sup>single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).</p><p>In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr<sub>3</sub>C<sub>2</sub> were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach.</p><p>In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)<sub>7</sub>C<sub>3</sub> carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software. </p><p>In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M<sub>23</sub>C<sub>6</sub>, M<sub>7</sub>C<sub>3</sub> and M<sub>5</sub>C<sub>2</sub> (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M<sub>7</sub>C<sub>3</sub>/graphite was partly constructed at 1073 K. </p><p>In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)<sub>4</sub>(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased.</p><p><b>Keywords:</b> Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;</p>
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Toughness enhancement in transition metal nitridesSangiovanni, Davide Giuseppe January 2011 (has links)
Toughness enhancements can be induced in cubic-B1 transition metal nitride alloys by an increased occupation of the d-t2g metallic states. In this Licentiate Thesis I use density functional theory to investigate the mechanical properties of TiN and VN and of the ternaries obtained by replacing 50% of Ti and V atoms with M (M = V, Nb, Ta, Mo, and W) to form ordered structures with minimum number of inter-metallic bonds. The calculated values of elastic constants and moduli show that ternary alloys with high valence electron concentrations (M = Mo and W), have large reductions in shear moduli and C44 elastic constants, while retaining the typically high stiffness and incompressibility of ceramic materials. These results point to significantly improved ductility in the ternary compounds. This important combination of strength and ductility, which equates to material toughness, stems from alloying with valence electron richer dmetals. The increased valence electron concentration strengthens metal–metal bonds by filling metallic d-t2g states, and leads to the formation of a layered electronic configuration upon shearing. Comprehensive electronic structure calculations demonstrate that in these crystals, stronger Ti/V – N and weaker M – N bonds are formed as the valence electron concentration is increased. This phenomenon ultimately enhances ductility by promoting dislocation glide through the activation of an easy slip system.
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Plasmonic Metasurfaces Utilizing Emerging Material PlatformsKrishnakali Chaudhuri (6787016) 02 August 2019 (has links)
<p>Metasurfaces are broadly defined as
artificially engineered material interfaces that have the ability to
determinately control the amplitude and phase signatures of an incident
electromagnetic wave. Subwavelength sized optical scatterers employed at the
planar interface of two media, introduce abrupt modifications to impinged light
characteristics. Arbitrary engineering of the optical interactions and the
arrangement of the scatterers on plane, enable ultra-compact, miniaturized
optical systems with a wide array of applications (e.g. nanoscale and nonlinear
optics, sensing, detection, energy harvesting, information processing and so
on) realizable by the metasurfaces. However, maturation from the laboratory to
industry scale realistic systems remain largely elusive despite the expanding
reach and vast domains of functionalities demonstrated by researchers. A large
part of this multi-faceted problem stems from the practical constraints posed
by the commonly used plasmonic materials that limit their applicability in
devices requiring high temperature stability, robustness in varying ambient,
mechanical durability, stable growth into nanoscale films, CMOS process
compatibility, stable bio-compatibility, and so on. </p>
<p>Aiming to create a
whole-some solution, my research has focused on developing novel,
high-performance, functional plasmonic metasurface devices that utilize the
inherent benefits of various emerging and alternative material platforms. Among
these, the two-dimensional MXenes and the refractory transition metal nitrides
are of particular importance. By exploiting the plasmonic response of thin films of the titanium carbide
MXene (Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub>) in the near infrared spectral
window, a highly broadband metamaterial absorber has been designed, fabricated
and experimentally demonstrated. In another work, high efficiency photonic spin
Hall Effect has been experimentally realized in robust phase gradient
metasurface devices based on two different refractory transition metal nitrides
–titanium nitride (TiN) and zirconium nitride (ZrN). Further, taking advantage
of the refractory nature of these plasmonic nitrides, a metasurface based temperature
sensor has been developed that is capable of remote, optical sensing of very
high temperatures ranging up to 1200<sup>o</sup>C.</p>
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Thermodynamic investigations of transition metal systems containing coabon and nitrogenTeng, Lidong January 2004 (has links)
In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF2single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr3C2 were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach. In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)7C3 carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software. In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M23C6, M7C3 and M5C2 (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M7C3/graphite was partly constructed at 1073 K. In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)4(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased. Keywords: Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;
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Atomistic simulation and experimental studies of transition metal systems involving carbon and nitrogenXie, Jiaying January 2006 (has links)
The present work was initiated to investigate the stability, structural and thermodynamic properties of transition metal carbides, nitrides and carbo-nitrides by atomistic simulations and experimentations. The interatomic pair potentials of Cr-Cr, Mn-Mn, Fe-Fe, C-C, Cr-C, Mn-C, Fe-C, Cr-Fe, Cr-N and Mn-N were inverted by the lattice inversion method and ab initio cohesive energies, and then employed to investigate the properties of Cr-, Mn- and Fe-carbides by atomistic simulations in this work. For the binary M7C3 carbide, the structural properties of M7C3 (M = Cr, Mn, Fe) were investigated by atomistic simulations. The results show that the stable structure for these compounds is hexagonal structure with P63mc space group. The cohesive energy of M7C3 calculated in this work indicates that the stability of carbides decreases with the increasing in metal atomic number. Further, the vibrational entropy of Cr7C3 was calculated at different temperatures and compared with the entropy obtained by experimentations. The comparison demonstrates that the main contribution to the entropy is made by the vibrational entropy. For the binary τ-carbides, the structural properties of Cr23C6 and Mn23C6, as well as the vibrational entropy of Cr23C6 were computed. Further, the site preference of ternary element Fe among 4a, 8c, 32f and 48h symmetry sites in Cr23-xFexC6 was studied. It has been seen that Fe atoms would firstly occupy 4a sites and then 8c sites. The lattice constant and stability of Cr23-xFexC6 were also computed with different Fe content. In order to understand the relative stability of the transition metal carbides and nitrides, the standard formation Gibbs energies of carbides and nitrides for Cr, Mn and Fe were compared. The order of carbon and nitrogen affinities for Cr, Mn and Fe was further clarified by the comparison of the interatomic pair potentials among Cr-C, Mn-C, Fe-C, Cr-N and Mn-N. It was found that Cr-N interaction was very strong in comparison with other binary interactions above and consequently, nitrogen addition would lead to a strong decrease in the thermodynamic activity of chromium in Cr-containing alloys. This was confirmed by the investigations of thermodynamic activities of Cr in the Fe-Cr-N and Fe-Cr-C-N alloys. The activities were measured in the temperature range 973-1173 K by solid-state galvanic cell method involving CaF2 solid electrolyte under the purified N2 gas. In addition, the analysis of nitrogen content and phase relationships in the Fe-Cr-N and Fe-Cr-C-N alloys equilibrated at 1173 K were carried out by inert-gas fusion thermal conductivity method, X-ray diffraction and scanning electron microscopy technique. The experimental results show that the solubility of nitrogen in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the alloys was found to have a strong negative impact on the Cr activity in Fe-Cr-N and Fe-Cr-C-N systems. / QC 20100929
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Growth, Optimization, and Characterization of Transition Metal Nitrides and Transition Metal Oxides for Electronic and Optical ApplicationsBiegler, Zachary J. January 2019 (has links)
No description available.
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