Nitrogen-rich Layer Research Articles

Flame retardant behavior of bio-based epoxy resin bearing pyridazinone and dynamic ester bond via self-blow-out and vesicular carbon formation

Aromatic nitrogen heterocyclic structure is a key building block of high-performance polymer materials and nitrogen-doped porous carbons, yet systematic studies on its flame-retardant effects and carbonization mechanisms in thermoset materials is still lacking. Here, a pyridazinone embedded bio-based epoxy resin (CSPZ-EP) cured by 4,4ʹ-methylenedianiline (DDM) was prepared. Benefiting from the rigid biphenyl-like pyridazinone structure and the high cross-linking density originated from the self-catalyzed transesterification, the cured CSPZ-EP/DDM can easily pass the UL-94 test with a V-0 rating suggesting excellent intrinsic flame retardancy. The cone calorimeter results further demonstrated that the peak heat release rate, the total heat release and flame growth rate of CSPZ-EP/DDM were significantly reduced by 84.4 %, 28.4 % and 90.5 %, respectively, compared with those of bisphenol A epoxy resin. The higher graphitization degree of the char residues and a large amount of non-flammable pyrolysis gas ejection synergistically prevented the further spread of fire. Strong yet tough nitrogen-rich char layer formed after thermal ablation together with the renewable features and the facile preparation route of CSPZ-EP/DDM make it a potential precursor for heteroatom porous carbons.

Improvement in Fatigue Strength of Chromium–Nickel Austenitic Stainless Steels via Diamond Burnishing and Subsequent Low-Temperature Gas Nitriding

Chromium–nickel austenitic stainless steels are widely used due to their high corrosion resistance, good weldability and deformability. To some extent, their application is limited by their mechanical characteristics. As a result of their austenitic structure, increasing the static and dynamic strength of the components can be achieved by surface cold work. Due to the tendency of these steels to undergo intercrystalline corrosion, another approach to improving their mechanical characteristics is the use of low-temperature thermo-chemical diffusion processes. This article proposes a new combined process based on sequentially applied diamond burnishing (DB) and low-temperature gas nitriding (LTGN) to optimally improve the fatigue strength of 304 steel. The essence of the proposed approach is to combine the advantages of the two processes (DB and LTGN) to create a zone of residual compressive stresses in the surface and subsurface layers—the enormous surface residual stresses (axial and hoop) introduced by LTGN, with the significant depth of the compressive zone characteristic of static surface cold working processes. DB (both smoothing and single-pass hardening), in combination with LTGN, achieves a fatigue limit of 600 MPa, an improvement of 36.4% compared to untreated specimens. Individually, smoothing DB, single-pass DB and LTGN achieve 540 MPa, 580 MPa and 580 MPa, respectively. It was found that as the degree of plastic deformation of the surface layer introduced by DB increases, the content of the S-phase in the nitrogen-rich layer formed by LTGN decreases, with a resultant increased content of the ε-phase and a new (also hard) phase: stabilized nitrogen-bearing martensite.

Enhancing the tribological properties of TA1 pure titanium by modulating the energy of pulsed laser nitriding

Surface nitriding of titanium and titanium alloys is a commonly used method to enhance their wear properties. In this work, the pulsed laser was used to nitride TA1 commercially pure titanium and the microstructure and wear properties of the nitride layers prepared with different pulsed laser energies was investigated. The nitride layers prepared by laser nitriding are bi-layered with the upper layer being an N-element enriched layer (mainly composed of TiN) and the lower layer being an N-element diffusion layer [mainly composed of α(N)-Ti]. With the rise in laser energy, the nitrogen-rich layer gradually thickens and the mechanical properties improved to provide good wear resistance. Therefore, the nitride layer prepared with laser energy of 400 mJ has the best wear resistance, whereas the nitride layer prepared with laser energies of 100, 200, and 250 mJ fails completely during the wear test.

Effect of the bottom electrode on the digital and analog resistive switching behavior of SiNx-based RRAM

This Letter is about the role of the bottom electrode in the resistive switching of SiNx-based resistive random-access memory. Titanium nitride (TiN) and platinum (Pt) are used as bottom electrodes to fabricate devices whose I–V characteristics are compared. The devices with Pt as the bottom electrode have digital resistive switching behavior with a main memory window. However, the devices with TiN as the bottom electrode provide an analog resistive switching with the gradual operation. We propose that this switching difference is due to the different work functions of the top and bottom electrodes and the nitrogen-rich layer formed at the SiNx/TiN interface. The work function of the bottom electrode is larger than the top electrode in the device with Pt electrodes, which are considered non-reactive. However, the difference in the work functions between the bottom and top electrodes is not much for the device with a TiN electrode. As the nitrogen-rich layer formed at the SiNx/TiN interface has nitrogen accommodation ability, the nitrogen ions are more likely to drift to the bottom electrode, and resistive switching changes from digital to analog for the device with TiN electrodes.

Unveiling the structure, chemistry, and formation mechanism of an in-situ phosphazene flame retardant-derived interphase layer in LiFePO4 cathode

The safety risks posed by the liquid electrolytes of lithium-ion batteries have drawn large-scale concern recently due to the increased reports of fires in electric vehicles and portable devices fuelled by these electrolytes. Efforts to address the fire safety of electrolytes have predominantly focused on the incorporation of flame retardant additives which generally lead to poor electrochemical properties when used in large quantities. Conversely, low amounts (∼5 vol%) of fluorinated phosphazene-based flame retardants have been reported to yield non-flammable electrolytes while improving the electrochemical properties. Herein, an advanced fire testing method, cone calorimetry, is employed to analyze a model electrolyte (ethoxy (pentafluoro) cyclotriphosphazene (EPCP) based electrolyte), which reveals that the flourinated phosphazene-based flame retardant electrolyte with up to 6 vol% only exhibits ignition delay rather than the widely acknowledged non-flammable behavior. Besides, for the first time, by employing a time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and transmission electron microscopy, we clarify the chemistry and structure of a flame retardant-derived cathod electrolyte interphase (CEI) layer formed on the LiFePO4 cathode surface. The CEI layer is characterized by a phosphorus and nitrogen (PN) rich layer, which inhibits the formation of a thick parasitic LiF layer, which improves the electrochemical integrity of cells.

Tracking the role of nitrogen in the improvement of the high temperature oxidation resistance of titanium by mechanical treatments

We report the role of ultrasonic shot-peening (SP) and laser-shock peening (LSP) on the insertion of atmospheric nitrogen during high temperature oxidation of pure titanium and the improvement of its oxidation resistance. Ion Beam analysis showed that for short oxidation durations, a layer of Ti2N is formed at the oxide-metal interface. However, only the LSP treatment leads to the formation of a long-term stable nitrogen-rich layer which acts as a barrier for the diffusion of oxygen. This explains the efficiency of the LSP treatment to reduce the oxidation of Ti and prevent embrittlement.

Molecular dynamics simulation of amine groups formation during plasma processing of polystyrene surfaces

Plasma treatment and plasma polymerization processes aiming to form amine groups on polystyrene surfaces were studied in-silico with molecular dynamics simulations. The simulations were compared with two experiments, (i) plasma treatment in N2/H2 bipolar pulsed discharge and (ii) plasma polymerization in cyclopropylamine/Ar radio frequency (RF) capacitively coupled discharge. To model favorable conditions for the incorporation of primary amine groups, we assumed the plasma treatment as the flux of NH2 radicals and energetic NH3 ions, and the plasma polymerization as the flux of cyclopropylamine molecules and energetic argon ions. It is shown in both the simulation and the experiment that the polystyrene treatment by the bipolar pulsed N2/H2 plasmas with an applied voltage of about ±1 kV formed a nitrogen-rich layer of a thickness of only a few nm. The simulations also showed that, as the NH3 incident energy increases, the ratio of primary amines to the total number of N atoms on the surface decreases. It is because the energetic ion bombardment brakes up N–H bonds of primary amines, which are mostly brought to the surface by NH2 radical adsorption. Our previous experimental work on the CPA plasma polymerization showed that increased RF power invested in the plasma leads to the deposition of films with lower nitrogen content. The MD simulations showed an increase of the nitrogen content with the Ar energy and a limited impact of the energetic bombardment on the retention of primary amines. Thus, the results highlighted the importance of the gas-phase processes on the nitrogen incorporation and primary amines retention in the plasma polymers. However, the higher energy flux towards the growing film clearly decreases amount of hydrogen and increases the polymer cross-linking.

Quenching and tempering effect on the corrosion resistance of nitrogen martensitic layer produced by SHTPN on AISI 409 steel

This work presents the application of Solution Heat Treatment after Plasma Nitriding (SHTPN) followed by requenching and tempering to the AISI 409 stainless steel aiming to increase hardness with no compromise of corrosion resistance. The SHTPN process consists of applying solution treatment to a previously plasma nitrided surface, promoting the dissolution of nitrides and the formation of a nitrogen-rich layer, in which nitrogen is in solid solution. When applied to ferritic stainless steels, SHTPN allows the obtention of a high‑nitrogen martensitic layer. It is presented an evaluation of the effects of requenching (from 950 and 1050 °C) and tempering (at 250, 450 and 650 °C) processes on the microstructure, hardness, and corrosion resistance of the nitrided layer of samples previously treated by SHTPN. Corrosion resistance was assessed through cyclic potentiodynamic polarization tests carried out in NaCl 0.5 mol/L, according to ASTM G150. Optical and scanning electron microscopy, wavelength dispersive spectroscopy, and X-ray diffraction were applied to microstructural and phase characterization. Surface hardening was evaluated by Vickers microhardness tests. Results have shown that the requenching of SHTPN samples promoted a deepening of the nitrided layer and microstructure refinement. The process with requenching from 1050 °C and tempering at 250 °C led to increasing both hardness and corrosion resistance. These results confirmed that nitrogen in solid solution increases the hardness and the potential for pit nucleation in the AISI 409 stainless steel.

Low-Temperature Oxy-Nitriding of AISI 304 Austenitic Stainless Steel for Combat Corrosion and Wear in HCl Medium

Time-dependent experiments were carried out to study the corrosion behavior of AISI 304 austenitic stainless steel before and after low-temperature oxy-nitriding in HCl solution. The weight loss of the untreated sample was higher than that of the nitrided sample after HCl corrosion. When immersed for a short time, the nitrided sample remained relatively intact, while when immersed for a long time, the nitrogen-rich layer was damaged. XPS analysis showed that the passive film of the low-temperature oxy-nitrided sample was complete. The structure of passive film on the LTON sample was characterized by Mott–Schottky test, and the effect of nitrogen on the corrosion resistance of the material was discussed. The wear test results show that the oxy-nitrided samples after HCl immersion corrosion still maintained high wear resistance.

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

A detailed study of the near-surface structure and composition of Nb, the material of choice for Superconducting Radio Frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse X-ray scattering, X-ray reflectivity and X-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex-situ during annealing in UHV as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to 800{\deg}C in vacuum promotes partial reduction of the natural surface oxides (Nb2O5, NbO2, NbO) into NbO. Upon cooling to 120{\deg}C, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 hours. Oxygen enrichment below the Nb/oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides was observed upon a stepwise annealing up to 250{\deg}C. Nitrogen introduction to the system at 250{\deg}C neither promotes N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to 500{\deg}C in a nitrogen atmosphere, the growth of a new subsurface Nb$_x$N$_y$ layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

Sodium Metal Batteries: Anode‐Free Sodium Metal Batteries Based on Nanohybrid Core–Shell Templates (Small 37/2019)

In article number 1901274, Seungyong Han, Young Soo Yun, and co-workers design a nanohybrid template based on high aspect-ratio silver nanofibers and nitrogen-rich carbon thin layers as a core–shell structure to improve the coulombic efficiency (CE) and cycling performance of anode-free sodium metal batteries, showing exceptionally long cycle lives for more than 3000 cycles with a highly stable CE of >99.9%.

Corrosion Resistance of Low Temperature Plasma Nitrided X12CrMoWVNbN10-1-1 Martensitic Stainless Steel

This paper deals with affecting of corrosion resistance of X12CrMoWVNbN10-1-1 martensitic stainless steel after plasma nitriding. This steel was subjected to plasma nitriding at lower temperature of 400 °C for 15 h in the reverse nitriding atmosphere 1H2:3N2 (l/h), tested and then compared to untreated one. The microstructure and microhardness of the untreated and nitrided stainless steel were evaluated. The anodic potentiodynamic polarization tests in neutral 2.5% NaCl deaerated solution were executed and the corrosion properties of the untreated and plasma nitrided steel samples were evaluated. The results showed a nitride layer, consisting of nitrogen rich diffusion layer but without compound layer on the surface of the plasma nitrided X12CrMoWVNbN10-1-1 stainless steel. The surface hardness of the martensitic stainless steel after plasma nitriding was increased significantly. The corrosion resistance of the X12CrMoWVNbN10-1-1 stainless steel was increased only partially. The pitting was evaluated, and the pitting coefficient was calculated. The plasma nitrided steel showed higher (more positive) corrosion potentials, lower current densities and decreased corrosion rates and pitting during electrochemical corrosion tests compared to not nitrided steel.

Anode-Free Sodium Metal Batteries Based on Nanohybrid Core-Shell Templates.

Anode-free sodium metal batteries (AF-SMBs) can deliver high energy and enormous power, but their cycle lives are still insufficient for them to be practical as a power source in modern electronic devices and/or grid systems. In this study, a nanohybrid template based on high aspect-ratio silver nanofibers and nitrogen-rich carbon thin layers as a core-shell structure is designed to improve the Coulombic efficiency (CE) and cycling performance of AF-SMBs. The catalytic nanohybrid templates dramatically reduce the voltage overshooting caused by metal nucleation to one-fifth that of a bare Al foil electrode (≈6 mV vs ≈30 mV), and high average CE values of >99% are achieved over a wide range of current rates from 0.2 to 8 mA cm-2 . Moreover, exceptionally long cycle lives for more than 1600 cycles and an additional 1500 cycles are achieved with a highly stable CE of >99.9%. These results show that AF-SMBs are feasible with the nanohybrid electrode system.

Nitrogen-doped graphite encapsulated Fe/Fe3C nanoparticles and carbon black for enhanced performance towards oxygen reduction

Non-noble metal (NNM) catalysts have recently attracted intensive interest for their high catalytic performance towards oxygen reduction reaction (ORR) at low cost. Herein, a novel NNM catalyst was synthesized by the simple pyrolysis of carbon black, urea and a Fe-containing precursor, which exhibits excellent ORR catalytic activity, superior durability and methanol tolerance versus the Pt/C catalyst in both alkaline and acidic solutions. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterizations demonstrate that the product is a nitrogen-doped hybrid of graphite encapsulated Fe/Fe3C nanoparticles and carbon black. X-ray photoelectron spectrum (XPS) and electrochemical analyses indicate that the catalytic performance and chemical stability correlate closely with a nitrogen-rich layer on the Fe/Fe3C nanoparticle after pyrolysis with presence of urea, leading to the same four-electron pathway towards ORR as the Pt/C catalyst. The hybrid is prospective to be an efficient ORR electrocatalyst for direct methanol fuel cells with high catalytic performance at low cost.

Cavitation Erosion Resistance of Laser Beam Nitride Layers of X5CrNi18-10 Stainless Steel

The use of nitration in technical applications, as thermochemical treatment, regardless of the used method (ionic, gas or laser beam nitration) is realized to obtain a nitrogen rich layer, which increase the resistance to shock or abrasion, as a result of the high hardness obtained. All the previous cavitation researches showed that hardness is an important mechanical properties, for cavitation erosion resistance increase. The present paper show some results regarding the cavitation erosion behavior of laser beam nitrated layers. As basic material was used the austenitic stainless steel X5CrNi18-10. This material is frequently used for manufacturing details subjected to cavitation, such as valves, drawers of hydraulic distribution and regulation devices, or the retaining ring for butterfly valves. There were used three power regimes of the laser beams: 120 W, 180 W and 240 W. To obtain cavitation erosion was used the standard device with piezo-ceramic crystals of Timisoara Polytechnic University Cavitation Laboratory. The cavitation erosion comparisons, both with the basic material subjected only to the common thermochemical treatments and with the laboratory cavitation standard steel (OH12NDL), show that the nitrated surfaces presents increased cavitation erosion behavior, the principal factor being the important hardness increase of the nitrated layer. We mention also, that for higher laser beam powers the thickness of the nitrated layer increases. All the images obtained at the end of tests show that the cavitation exposure was stopped before overtaking the nitrated layer. So, all our results concern only the hardened layers.

A study on Ga[sbnd]Si interdiffusion during (Al)GaN/AlN growth on Si by plasma assisted molecular beam epitaxy

The GaSi interdiffusion during (Al)GaN/AlN growth on Si substrate by plasma assisted molecular beam epitaxy (PA-MBE) is studied. The epilayers were grown using a combination of different III/V ratios for GaN and AlN layers. The columnar morphology of the nitrogen-rich (III/V < 1) AlN nucleation layer allows the migration of Ga metal into Si, leading to the interdiffusion. The presence of less Ga at the GaN/AlN interface and the two-step growth process of AlN with different column sizes on the top and bottom AlN completely eliminates the possibility of GaSi interdiffusion. Alternatively, a thin silicon nitride as a nucleation layer for the growth of (Al) GaN layers was also found to prevent the GaSi interdiffusion thereby circumventing the process of melt-back etching.

Electrocatalytic and energy storage performance of bio-derived sulphur-nitrogen-doped carbon

Sulphur and nitrogen-rich carbon layers (S/N-CLs) have been derived successfully via a simple calcination for high energy applications including supercapacitor and electrocatalytic hydrogen evolution reaction (HER). The flexible working electrode was fabricated using thoroughly analyzed S/N-CLs composite as an active electrode energy material for supercapacitor and HER. The S/N-CLs composite shows a higher specific capacitance of 266Fg−1 at a current density of 0.5Ag−1 and a satisfied cycling stability with 84% capacitance retention even after 5000cycles of galvanostatic charge-discharge. On the other hand, the synthesized S/N-CLs composite was used as an electrocatalyst for HER and it exhibits an excellent HER performance with an onset overpotential of −75mV, and a Tafel slope of 73mVdec−1 in 0.5M H2SO4 aqueous electrolyte. Also, the S/N-CLs composite displayed good stability and strong durability. The high electrocatalytic activity and stability of S/N-CLs composite toward HER due to the active site of a compound which may originate from the heteroatom-functional groups including N and S in the carbon structure. Overall, the high-performance supercapacitors and enhancing HER based on heteroatom-doped carbon structures could be promising in replacing traditional supercapacitors for many electronic devices and electrocatalyst for HER.

Corrosion Resistance of High Temperature Plasma Nitrided X12CrMoWVNbN10-1-1 Martensitic Stainless Steel

The X12CrMoWVNbN10-1-1 martensitic stainless steel was subjected to plasma nitriding at an increased temperature of 550 °C for 15 h and compared to untreated one. The microstructure and microhardness of the untreated and nitrided stainless steel were evaluated. The corrosion properties of the untreated and plasma nitrided steel samples were evaluated using the anodic potentiodynamic polarization tests in neutral 2.5% NaCl deaerated solution. The results showed that plasma nitriding process on the X12CrMoWVNbN10-1-1 stainless steel produced a nitride layer consisting of compound layer and a nitrogen rich diffusion layer. Plasma nitriding process significantly increased the surface hardness of the martensitic stainless steel but also decreased the corrosion resistance of the X12CrMoWVNbN10-1-1 stainless steel. The pitting was evaluated, and the pitting coefficient was calculated. During electrochemical corrosion tests, the nitrided steel showed lower (more negative) corrosion potentials, higher current densities and greatly increased corrosion rates and pitting.

Turning Carbon Atoms into Highly Active Oxygen Reduction Reaction Electrocatalytic Sites in Nitrogen-Doped Graphene-Coated Co@Ag

In recent years, 3d transition metals or alloys encapsulated by graphene layers (M@NG) have been emerging as prospective electrocatalysts especially for hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs). However, this strategy is limited in preparation of high-performance oxygen reduction reaction (ORR) catalysts. Herein we prepared Co@Ag bimetallic nanoparticles embedded in nitrogen-rich graphene layers via pyrolysis metal organic frameworks (MOFs). The catalyst displays extraordinarily high ORR performance with a high onset potential of 0.989 V and a half-wave potential of 0.872 V in 0.1 M KOH. Moreover, it shows a superb long-term stability performance after 5000 cycles on account of the carbon layers that protect the material from corrosion as well as high methyl alcohol tolerance under methanol environments. Density functional theory calculations suggest that carbon atoms, which are adjacent to nitrogen dopants in Co@Ag@NC, are active sites for ORR. Especially, Ag “mantle” in ...

The Effect of Liquid Nitriding on the Corrosion Resistance of AISI 304 Austenitic Stainless Steel in H2S Environments

AISI 304 austenitic stainless steel was low-temperature liquid nitrided in a molten salt bath at 703 K for 8 hours, which produced a 3-layered structure consisting of a top oxide layer, an intermediate nitrogen-rich layer and a bottom carbon-rich layer. The effect of nitriding on its corrosion resistance was investigated in a H2S environment. The corrosion rate of the untreated sample is about 3.3 times that of the nitrided sample after H2S corrosion. Corrosion pits can be clearly observed on the surface of the untreated sample, while the nitrided sample surface remained relatively intact. Both the oxide layer and the nitrogen-rich layer can help reduce the hydrogen permeation, which is beneficial for combating hydrogen embrittlement. The corrosion products mainly consisted of oxides, hydroxides, and sulfates. The nitrided layers can serve as a barrier to corrosion, thus preventing the corrosion of the substrate material. Active nitrogen in the nitrided layer reacts with H ions to form NH4+, which effectively prevents further acidification of the local area and inhibits the occurrence of pitting corrosion and the dissolution rate of the metal in the etching hole, thus improving the local corrosion resistance of the stainless steel.