Ammonia Nitridation Research Articles

Diffusion Nitride Surface Layers on Aluminum Substrates Produced by Hybrid Method Using Gas Nitriding

While gas nitriding of steel is currently used in industry, nitriding of aluminum alloys remains an open challenge. The main obstacle is aluminum’s high susceptibility to passivation. The oxide film provides an effective barrier to nitrogen diffusion. Attempts to overcome this problem have mainly focused on glow discharge nitriding using cathode sputtering of an oxide layer. The produced AlN layers exhibit no diffusion zone and show limited performance properties. In this work, the effect of hybrid treatment aimed at producing diffusion layers of nitrides other than AlN on aluminum alloys was investigated on the model system of iron nitride–aluminum substrate. Hybrid treatment combines an electrochemical process involving the removal of the aluminum oxide layer from the substrate, its subsequent iron plating, and a further gas nitriding in high-purity ammonia. The obtained results prove that the hybrid treatment allows the production, at 530 °C/10 h, of diffusion layers of Fe3N iron nitrides on aluminum substrates with a nitrogen diffusion zone range in aluminum of ca. 12 µm. In alloys containing magnesium, its unfavorable effect on the nitrogen diffusion and the functional properties of the layers was observed. An interesting direction for further research is hybrid treatment of precipitation-hardened alloys without magnesium.

Engineering Ir and Ni3N heterogeneous interfaces for promoted overall water splitting

The development of efficient and stable electrocatalysts is a necessary way to overcome the huge overpotential of hydrogen production from water splitting. In this work, a one-step ammonia nitridation process was proposed, in which noble metal Ir nanoparticles were directly embedded into the Ni3N nanosheet structure, and the Ir-Ni3N heterogeneous interface was established on the surface of the nickel foam substrate. X-ray photoelectron spectroscopy showed that the IrOx/NiOOH heterogeneous interfaces formed after the surface self-reconstruction played an important role in promoting the oxygen evolution reaction, and its over-potential was only 273.3 mV at the current density of 10 mA cm−2. Impressively, the nanosheet structure of Ir-Ni3N remained stable after 10 h of constant current oxidation. In addition, when Ir-Ni3N was used as the electrocatalyst for hydrogen evolution reaction at the current density of 10 mA cm−2, the overpotential was only 66.7 mV. As expected, when it was examined for overall water splitting, the Ir-Ni3N demonstrated a low voltage (1.49 V), outperforming most of the HER/OER bi-functional catalysts reported recently. This work will provide an important basis for the study of transition metal nitrides, especially in improving the stability of oxygen evolution reaction.

Temperature dependence of luminescence color of Mn2+-doped ZnCN2 phosphor

Photoluminescent materials which show multi-wavelength luminescence have potential applications in temperature sensing device. In this study, Mn2+-doped ZnCN2 phosphor was prepared by ammonia nitridation of Zn-Mn oxalate precursors in the presence of carbon nitride. Upon UV excitation, it showed photoluminescence band at 580 nm due to isolated Mn2+ ions as well as minor broad emission bands at 500 and 670 nm due to anion vacancies and Mn-Mn pairs in the ZnCN2, respectively. Since the intensities of the luminescent bands had different temperature dependencies, and the total luminescence color changed from orange at 80 K to yellow green at 500 K.

Reduction of supported GaN and its application in methane conversion

The reduction of supported GaN using amorphous SiO2, silicalite-1 (S1), and HZSM-5 (HZ) as the carriers was investigated and tested for methane conversion to acetonitrile (ACN). Ammonia nitridation was used to synthesize GaN samples. The highest ammonia and ACN productivities (433 and 383 μmol/g, respectively) were obtained by using 5 wt% GaN on HZ (5GaN/HZ). The N–Ga interaction of 5GaN/HZ was discovered to be weakened due to the presence of a neighboring Brønsted acid. The oxidation state of gallium was reduced during methane conversion, implying the loss of nitrogen from GaN. The decreased activity could be rejuvenated by a renitridation step. The GaN surface structures, including the adsorption surface, 11¯0, and the side surfaces, (001), (110), (111), and 111¯, were calculated via first-principles calculations. Among them, the (001) side surface of GaN was likely to be the origin of mobile nitrogen, which is important in the conversion of methane to ACN.

Synthesis of a new class of high-entropy nitride ceramics and the effects of nitrogen vacancies on their magnetic properties

This research prepared a new type of (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)N0.956 high-entropy nitride (HEN) ceramic powder with a cubic single-phase zinc blende structure using the (Co,Cr,Fe,Mn,Ni)3O4 high-entropy spinel oxide as raw material through ammonia nitridation. Nitrogen vacancies are the main feature of HENs, considerably influencing the HEN their structure and magnetic properties. (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)N0.864 HENs with nitrogen vacancies were synthesized through hydrogenation. Theoretical calculations and experimental characterization results show that the samples obtained by heat treatment at 300 °C for 30 min in Ar/H2 atmosphere have the most nitrogen vacancies. Nitrogen vacancies affect the interactions among metal ions in the crystal, resulting in the spin tilt of some divalent metal ions requiring more energy; thus, coercivity (Hc) decreases from 411.98 to 251.40 Oe, and saturation magnetization (Ms) increases from 5.71 to 8.09 emu/g. We offer a fresh viewpoint to explain the structural origin of magnetic characteristics in high-entropy ceramics based on the theoretical calculations of the association between defects, structure, and magnetism as well as the characterization data.

Toward a Mechanistic Understanding of the Formation of 2D-GaNx in Epitaxial Graphene

Ultrathin 2D-GaNx can be formed by Ga intercalation into epitaxial graphene (EG) on SiC followed by nitridation in ammonia. Defects in the graphene provide routes for intercalation, but the nature and role of the defects have remained elusive. Here we examine the influence of graphene layer thickness and chemical functionalization on Ga intercalation and 2D-GaNx formation using a combination of experimental and theoretical studies. Thin buffer layer regions of graphene near steps on SiC readily undergo oxygen functionalization when exposed to air or a He/O2 plasma in contrast to thicker regions which are not chemically modified. Oxygen functionalization is found to inhibit Ga intercalation leading to accumulation of Ga droplets on the surface. In contrast, Ga readily intercalates between EG and SiC in the thicker graphene regions that do not contain oxygen. When NH3 annealing is carried out immediately after Ga exposure, 2D-GaNx formation is observed only in the oxygen-functionalized regions, and Ga intercalated under thicker nonfunctionalized graphene does not convert to GaNx. Density functional theory calculations demonstrate that oxygen functionalization of graphene alters the binding energy of Ga and NH3 species to the graphene surface. The presence of hydroxyl groups on graphene inhibits binding of Ga to the surface; however, it enhances the chemical reactivity of the graphene surface to NH3 which, in turn, enhances Ga binding and facilitates the formation of 2D-GaNx. By modifying the EG process to produce oxygen-functionalized buffer layer graphene, uniformly intercalated 2D-GaNx is obtained across the entire substrate surface.

STM/STS Study of the Density of States and Contrast Behavior at the Boundary between (7 × 7)N and (8 × 8) Structures in the SiN/Si(111) System

The origin of the contrast appearing in STM images at the boundary between diverse ordered structures is studied using the example of two structures, (7 × 7)N and (8 × 8), formed in the system of a two-dimensional silicon nitride layer on the Si(111) surface during ammonia nitridation. A significant dependence of the contrast between these structures on the voltage applied to the tunnel gap was found and studied both experimentally and theoretically. Variations in the contrast were quantitatively studied in the range from −3 V to +3 V, and they were studied in more detail for the positive biases on the sample from +1 V to +2.5 V, where the contrast was changed more than 2 times. Within the one-dimensional Wentzel–Kramers–Brillouin (WKB) model for the tunnel current, a comparatively simple procedure is proposed for the correction of the experimental STS-spectra of differential conductivity to identify the adequate (feasible) density of electron states (DOS). It is shown that the (8 × 8) structure DOS corresponds to a graphene-like layer of silicon nitride structure. The proposed correction procedure of the empirical differential conductivity spectra measured by STS will be useful for the quantitative determination of the DOS of new two-dimensional materials and surface structures.

Highly durable core(Nb4N5)–shell(NbOx)-structured non-precious metal catalyst for oxygen reduction reaction in acidic media

In this study, we demonstrate an easy and eco-friendly synthesis method of Nb-based electrocatalysts using the combination of pulsed laser ablation in liquid and ammonia nitriding at various temperatures and times. Our electrocatalyst samples had a nanoparticle size with a core–shell structure, where the niobium nitride core was surrounded by a thin layer of niobium oxide shell. The nitriding conditions for the samples successfully modified the core structure to become more electrically conductive. The Nb4N5 @NbOx/C_5 h, the sample heat-treated at 900 °C for 5 h, exhibited a moderate onset potential and catalytic activity. More importantly, the oxygen reduction reaction performance of the Nb4N5 @NbOx/C_5h sample did not degrade over 10,000 potential cycles. Based on the above results, we suggest that the nitride core and the thin layer of the oxide shell might be responsible for oxygen reduction by facilitating electron transport and providing oxygen adsorption sites.

Controllable synthesis of Co3W3N supporting on N-doped GO as electrocatalysts for oxygen reduction reaction

The Co3W3N particles as oxygen reduction reaction (ORR) catalyst with small size and high dispersibility was supported on N-doped GO via hydrothermal and ammonia nitriding heat treatment processes. The bonds of WN, CoN, pyridinic-N and CN contribute positively to the oxygen reduction effect of the catalyst. Moreover, the synergistic effect of catalyst and support leads to superior ORR performance with more positive half-wave potential and rapid oxygen reduction kinetics, compared to the Pt/C catalyst in alkaline media. The Co3W3N particles decorated on GO might be a promising alternative for non-Pt catalysts for oxygen reduction.

Synthesis of Hollow Mesoporous TiN Nanostructures as An Efficient Catalyst Support for Methanol Electro-Oxidation

The development of efficient catalyst materials that can drive high catalytic performance is challenging. Here, we report a well-defined hollow mesoporous TiN nanostructure for use as Pt catalyst support material for methanol electro-oxidation. The hollow TiN nanostructure was synthesized by the ammonia nitridation of pre-synthesized mother hollow anatase TiO2, which was prepared by SiO2 template-assisted sol–gel synthesis followed by chemical etching, acid treatment, and sequential calcination. The variation in the ammonia nitridation temperature allowed the crystalline properties of the samples to be finely tuned. As the ammonia nitrification temperature increased, the crystallinity of the resulting hollow TiN continuously increased, and the corresponding Pt catalysts showed enhanced activity toward methanol electro-oxidation. The hollow TiN-800 sample (H-TiN-800), with a well-developed pure TiN phase, exhibited the highest electrical conductivity and the lowest resistance. The corresponding Pt/H-TiN-800 catalyst exhibited significantly enhanced catalytic activity. In this study, we systemically analyzed the physicochemical characteristics and electrochemical performance of hollow TiN samples and their corresponding Pt catalysts.

Combined thermochemical treatment of steel construction parts to increase their corrosion resistance

Electrochemical corrosion of construction parts made of structural steels appears due to different electrochemical potentials (ECP) of contacting materials. The values of ECP of carbon steels may be more or less compensated by forming of alloyed layers or coatings by methods of thermochemical treatment (TCT). The following combinations of TCT were examined for surface modification of steels: (a) application of zinc coatings by a method of cold zinc-plating with subsequent classical gas nitriding in ammonia; (b) joint diffusion saturation by zinc and nitrogen; (c) joint diffusion saturation with chromium and nickel. Metallographic and microhardness methods were used for the study of modified layers and coatings. Strengthened layers with a thickness up to 140…150 microns were formed in low-carbon steels after nitriding of zinc coatings at 540°C for 24 hours, and after combined TCT with multicomponent saturation by Cr+Ni at 1000°C for 7 hours. Both methods of TCT increase the corrosion resistance of low-carbon steels and low-alloyed steels. Corrosion products examination in 5% NaCl salt fog shows that the dissolution of nitrided Zn coating and formation of corrosion products slow down with the increase of zinc concentration in the coating.

NITRO-OXIDATION PRODUCTS FROM STRUCTURAL STEELS

The article examines the technology of nitro-oxidation, which consists of nitriding in separated ammonia at the first organize and oxidation in water vapor at the second stage of the process, studied the structure and phase composition of the diffusion nitride-oxide layer, providing high physico-mechanical and physicochemical surface characteristics, competitively capable of galvanic strategies of getting a coating, the conceivable outcomes of regulating the structure and phase composition of the nitride-oxide layer are being investigated to obtain a diffusion coating composition with predetermined properties.

Synthesis of gallium nitride nano-particles by ammonia nitridation of mixed β-gallium oxide and gallium nitride powders

Synthesis of gallium nitride nano-particles by ammonia nitridation of mixed β-gallium oxide and gallium nitride powders

Nitridated Ca2NaMg2V3O12: Eu3+ Vanadate Garnet Phosphor-in-Glass.

In this study, Ca2NaMg2V3O12 coordination compound undoped and doped with Eu3+ obtained in the air and ammonia atmosphere by solid-state reaction was investigated. Ca2NaMg2V3O12: Eu3+ obtained in the ammonia atmosphere was then investigated as a phosphor-in-glass (PiG). Annealed Ca2NaMg2V3O12: Eu3+ phosphor forms a single phase with the cubic garnet structure and Ia3d space group. Nitridation in ammonia causes a widening of a VO43− group emission band on the low energy side and it red-shifts by 20 nm. The emission of Eu3+ is extended as compared to the non-nitridated phosphor. Ca2NaMg2V3O12: Eu3+ after nitridation shows higher emission quantum yield (QY): 49 vs. 45 and lower correlated color temperature (CCT) and 4179 vs. 4998 as compared to phosphor without nitridation. The QY for PiG is 55. The thermal stability of nitridated phosphors is superior to phosphor obtained in the air atmosphere and is further enhanced for PiG.

TiN nanotube supported Ni catalyst Ni@TiN-NTs: experimental evidence of structure-activity relations in catalytically hydrolyzing ammonia borane for hydrogen evolution.

With commercial TiO2 as the precursor, titanium nitride nanotubes (TiN-NTs) were fabricated through a hydrothermal – ammonia nitriding route, and next non-noble metal nanosized Ni particles were evenly and firmly anchored on the surface of the TiN-NTs via a PVP-mediated non-aqueous phase reduction–deposition strategy, to obtain the supported catalyst Ni@TiN-NTs. The X-ray powder diffraction (PXRD), field emission scanning and transmission electron microscopy (FE-SEM/TEM) and specific surface area measurements were used to characterize and analyze the phase composition, surface microstructure and morphological features of the product. The catalytic activity of the Ni@TiN-NTs for hydrolyzing ammonia borane to generate hydrogen (H2) under different conditions was evaluated systematically. The results reveal that the as-fabricated TiN-NTs are composed of TiN and a small amount of TiNxOy with the approximate molar atomic ratio of Ti to N at 1 : 1, existing as hollow microtubules with mean tube diameter of 130 nm and length of about 1 μm. Via in situ reduction and deposition, Ni nanoparticles can be uniformly anchored on the surface of TiN-NTs. The catalytic activities of Ni(x)@TiN-NTs with different Ni loading amounts are all higher than that of single metal Ni nanoparticles. The temperature has a positive effect on the catalytic activity of Ni(20)@TiN-NTs, and its total turnover frequency for hydrolyzing ammonia borane is 11.73 mol(H2) (mol Ni)−1 min−1, with an apparent activation energy of 52.05 kJ mol−1 at 303 K. After 5 cycles, the Ni(20)@TiN-NTs catalyst still maintains 87% of the initial catalytic activity. It could be suggested that these tactics can also be extended to the fabrication of other metal or alloy catalysts supported by TiN-NTs, with great application potential and development prospects.

Preparation and thermal stability of oxynitride perovskite solid solution Sr1-xLaxTa1-xTixO2N

The thermal stability of oxynitride perovskites is very important for the fabrication of their dense ceramics. In this study, a solid solution of the oxynitride perovskite Sr1-xLaxTa1-xTixO2N was prepared via the ammonia nitridation of oxide precursors. The oxynitrides products obtained at x ≤ 0.2 and x ≥ 0.7 had tetragonal and triclinic perovskite-type structures, respectively. A neutron diffraction study on the oxynitride product obtained at x = 0.2 indicated that this tetragonal perovskite had a cis-type anion distribution similarly to SrTaO2N. The tetragonal perovskite released a part of nitrogen at approximately 1000 °C in a nitrogen atmosphere while maintaining the perovskite structure at least to 1450 °C. The oxynitride perovskites at x ≥ 0.5 gradually decomposed to a mixture of LaTiO2N, LaTiO3, La(OH)3, La2O3, TiN and TaO above 1100 °C.

Application of Laser Radiation for Intensification of Chemical Heat Treatment

The processes of chemical heat treatment were analyzed. The areas of their application, productivity, energy intensity, advantages and disadvantages are highlighted. The surface nitrided layers with hardness up to 800–900 HV were obtained. In this case, nitriding thickness does not exceed 0.15–0.2 mm, which is insufficient, especially if the value of residual deformations exceeds the thickness of the diffusion layer. It is shown that to increase the efficiency of nitriding, it is advisable to pre-modify the microstructure of the layer of the sample by laser irradiation. Modeling of thermal processes occurring in steels under irradiation and experimental studies have determined the conditions under which highly dispersed, with a high dislocation density, disoriented in space structures are formed in the surface layers. Dislocations, located on the grain boundaries and having a complex configuration, form paths along which nitrogen atoms can move, forming nitrides with alloy elements. It was shown that in U8 tool carbon steel (AISI M2) after laser treatment and subsequent nitriding in ammonia at a temperature of 780–800 °К for 12 h, a nitrided layer is formed. The depth of the nitrided layer without prior laser treatment is 0.115 mm, on laser-treated it varies within 0.12–0.46 mm depending on the processing speed. The results of the combined nitriding of 40Cr13 stainless steels (AISI 420 Stainless Steel) and 12Cr18Ni10Ti (AISI 321 Stainless Steel), in which the thickness of the hardened layer with a hardness of 815.7–917.7 HV is 0.4–0.5 mm, with a significantly shorter (by 3–5 times) process time and its cost, are presented. The expediency of manufacturing discretely located nitrided zones on the working surfaces of products is discussed.

Hierarchical analysis of alloying element effects on gas nitriding rate of Fe alloys: A DFT, microkinetic and kMC study

Nitriding is the most widely employed thermochemical surface treatment to enhance the mechanical properties of steel. Specifically, gas nitriding, which is a low-temperature process for efficiently producing high-performance steels, has a disadvantage in that it consumes a large amount of time. To enhance the nitriding rate, we studied the surface alloying of iron (Fe) and its effect on ammonia (NH3) nitriding of Fe using a hierarchical protocol with density functional theory (DFT)-based microkinetics and real-time simulations. First, we considered the NH3 decomposition and nitrogen (N) diffusion mechanism on clean and alloyed (Fe-X) Fe (100) surfaces using DFT. In this study, the alloying elements including transition metals and period III to VI elements in the periodic table were considered for DFT-based computational screening. For the candidate Fe-X systems selected to improve the nitriding rate in the previous step, we calculated all the energy barriers for every elementary reaction step by varying the alloying elements and performed microkinetic analysis using those kinetic energy barriers to determine their influence on the nitriding rate. After adding consideration of thermodynamic factors, selected candidate alloys were subjected to detailed DFT calculations of the nitriding mechanism with N coverage, and based on these results, a kinetic Monte Carlo (kMC) simulation was performed to reconfirm the results under the actual nitriding process conditions. Through a hierarchical protocol, we performed a theoretical analysis and simulation of the effects of alloying elements on the nitriding rate that were not explained experimentally and suggested the best alloying element with the improved nitriding rate.

Hierarchical Analysis of Alloying Element Effects on Gas Nitriding Rate of Fe Alloys: A DFT, Microkinetic and kMC Study

Nitriding is the most widely employed thermochemical surface treatment to enhance the mechanical properties of steel. Specifically, gas nitriding, which is a low-temperature process for efficiently producing high-performance steels, has a disadvantage in that it consumes a large amount of time. To enhance the nitriding rate, we studied the surface alloying of iron (Fe) and its effect on ammonia nitriding of Fe using a hierarchical protocol with density functional theory (DFT)-based microkinetics and real-time simulations. First, we considered the NH3 decomposition and nitrogen (N) diffusion mechanism on clean and alloyed (Fe-X) Fe (100) surfaces using DFT. In this study, the alloying elements including transition metals and period III to VI elements in the periodic table were considered for DFT-based computational screening. For the candidate Fe-X systems selected to improve the nitriding rate in the previous step, we calculated all the energy barriers for every elementary reaction step by varying the alloying elements and performed microkinetic analysis using those kinetic energy barriers to determine their influence on the nitriding rate. After adding consideration of thermodynamic factors, selected candidate alloys were subjected to detailed DFT calculations of the nitriding mechanism with N coverage, and based on these results, a kinetic Monte Carlo (kMC) simulation was performed to reconfirm the results under the actual nitriding process conditions. Through a hierarchical protocol, we performed a theoretical analysis and simulation of the effects of alloying elements on the nitriding rate that were not explained experimentally and suggested the best alloying element with the improved nitriding rate.

Constructing hierarchical porous carbon via tin punching for efficient electrochemical energy storage

To achieve large-scale preparation of high-performance carbon materials for electrochemical energy storage with a simple and cost-effective method remains a challenge. Here, we report a novel approach to synthesize hierarchical porous carbon with a low melting point metal tin (Sn) as pore forming agent. An aqueous processed tin chloride-polyethylene glycol (SnCl2-PEG) gel is used as precursor to form Sn/Carbon (Sn/C) composites with homogeneously distributed ultrafine Sn particles (<2 nm) by pyrolysis. After etching Sn nanoparticles, hierarchical porous carbon with high specific surface area (SSA: 846 m2 g−1) and abundant pore structure (coexistence of mesopore and micropore) was obtained. The SSA, pore size distribution and pore volume were successfully tuned by controlling the size of the Sn nanoparticle. The capacitive performance of the as-prepared hierarchical porous carbon was evaluated in 1 M H2SO4, which exhibits excellent specific capacitance of 240 F g−1. In order to further increase the electrochemical performance, N-doped porous carbon was fabricated by thermal nitridation in ammonia, which shows favorable features for electrochemical energy storage such as high specific surface area (1175 m2 g−1), uniform micropore volume (0.46 cm3 g−1) and rich nitrogen-doping (4.45 wt%). This N-doped sample exhibits outstanding specific capacitances of 360 F g−1 and excellent cycling stability in aqueous electrolytes. The above achievements indicate that combining low melting point metal as pore forming agent with a sol-gel protocol can be a unique and reliable method for preparation of high performance supercapacitor electrode materials.