Low Hydrogen Content Research Articles

Hydrogen embrittlement of Zircaloy-4 fabricated by ultrasonic additive manufacturing

Ultrasonic additive manufacturing (UAM) was successfully applied to the zirconium material system to create a planar geometry. Following fabrication, SS-J3 type tensile specimens of the UAM and wrought Zircaloy-4 with a nominal gage section of 5 × 1.2 × 0.75 mm were machined for hydriding studies and mechanical testing. The SS-J3 type tensile specimens were gas-charged with hydrogen using a custom system that precisely controls hydrogen gas flow rate, hydrogen partial pressure, and temperature. To avoid altering the UAM material, the maximum process temperature was limited to 550 °C. Using different initial hydrogen gas pressures and flow rates, various hydrogen contents (70–1755 wppm) were achieved for Zircaloy-4 specimens. Tensile testing shows that, regardless of hydrogen content, all UAM specimens measured yield strengths in the range of 557 ± 16 MPa and ultimate tensile strengths of 660 ± 4 MPa. However, total elongation clearly decreased as a function of increasing hydrogen content. At the lowest hydrogen content, the total plastic elongation measured 21.5 %, and this value decreased to less than 2 % when the hydrogen content was increased to 1000 wppm. Cross-sectional optical microscopy images revealed that the hydride distributions are randomly oriented. For the low hydrogen content specimen, these randomly distributed hydrides are isolated from each other. A sandwiched structure, consisting of high-density and low-density layers, was also observed for the specimens with hydrogen content between 200 and 400 wppm. As the hydrogen content increases, the hydrides diffuse into the low-density hydride layers to form a network across the whole specimen. Although the tensile properties of UAM and wrought Zircaloy-4 exhibit the same behavior with increasing hydrogen content, the distinctions in grain orientations led to differences in the hydride orientations at low and intermediate hydrogen concentrations.

Effect of carbon segregation at prior austenite grain boundary on hydrogen-related crack propagation behavior in 3Mn-0.2C martensitic steels

The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness (JIC) and tearing modulus (TR), corresponding to crack growth resistance, were significantly improved by carbon segregation. In contrast, JIC and TR did not change by carbon segregation at the high hydrogen content (above 0.5 wt. ppm). Considering the non-linear relationship between the toughness properties and the PAGB cohesive energy, the experimentally evaluated toughness properties (JIC and TR) and the GB cohesive energy previously calculated by first-principles calculations were semi-quantitatively consistent even at the high hydrogen content. The microstructure observation confirmed that the plastic deformation associated with crack propagation, such as the local ductile fracture of uncracked ligaments and the formation of dislocation cell structures / nano-voids, played an important role in the non-linear relationship between the toughness properties and PAGB cohesive energy.

The impact of frequency and power on the ultrasonic purification of aluminum alloy

In this study, the variations in hydrogen content and oxide content in alloys under 0 W, 500 W, 1000 W, 1500 W, 2000 W, 2500 W, 3000 W, and at 20 kHz, 30 kHz, 40 kHz were investigated. Hydrogen content was assessed using porosity and computed tomography, while oxygen content in the alloy was measured using element analyzer and elemental scanning. Compared to other conditions, the melt had the lowest hydrogen and oxide contents at a frequency of 20 kHz and an ultrasonic power of 2500 W, with values of 0.099 cm3/100 g and 0.0015 %, respectively. Experimental observations also indicate that the variations in hydrogen content and oxide content in the alloy during ultrasonic treatment are almost similar. In most cases, lower hydrogen content corresponds to lower oxide content in the same alloy. This is because hydrogen bubbles and oxides become a single entity. At the same time, ultrasonic purification increases the tensile strength of the alloy to 200.1 MPa and the elongation rate to 0.72 %. This study primarily investigates the relationship between hydrogen bubbles and oxides in aluminum melt under different ultrasonic frequencies and power levels, providing significant reference for the purification of various fluids under ultrasonic fields.

Mixing dynamics and recovery factor during hydrogen storage in depleted gas reservoirs

Large-scale hydrogen storage is strategically important for society and depleted gas reservoirs have been proposed as a potential solution. However, the mixing with remaining hydrocarbon gas during injection/production cycles and the impact of different reservoir parameters on the hydrogen recovery factor (RFH2) have not been fully understood. We investigate mixing dynamics and analyze the effects of initial hydrocarbon recovery factor, reservoir permeability, well perforation length, intelligent completion, and fractures on RFH2. Fine-grid gas reservoir models with compositional simulation were used to simulate cyclic hydrogen storage. Mixing of injected hydrogen with remaining hydrocarbon gas occurred in three distinct phases in each cycle: 1) displacement of remaining hydrocarbon gas through pressure-driven flow during hydrogen injection, 2) density-driven flow of gases during the idle time after hydrogen injection, and 3) pressure-driven flow of gases towards the production intervals during the hydrogen production phase. Higher RFH2 s were obtained when the storage process was initiated at higher hydrocarbon gas recovery factors. Storing hydrogen in reservoirs with lower permeability also led to higher RFH2 s, provided the well pressure limits were not an issue. In conditions of a 5× to 15× increase in permeability, the injected hydrogen moved upward and spread laterally, positioning it farther from the wellbore. This made hydrogen production more difficult and placed the mixing zone and hydrocarbon gas closer to perforations. Shorter perforation lengths at the top of the formation resulted in the best RFH2 s. Additionally, intelligent completions (that closed perforations producing gas with low hydrogen content) improved RFH2 by providing the possibility of purer hydrogen production. The presence of natural fractures significantly reduced hydrogen recovery, especially in the first few cycles. However, the influence decreased over time as highly conductive flow paths provided by fractures can contribute to the production of the accumulated unrecovered hydrogen from previous cycles.

Effect of Europium and Gadolinium Alloying Elements on the Tribological Response of Low Hydrogen Content Amorphous Carbon.

Dopants and alloying elements are commonly introduced in amorphous carbon (a-C) materials to tailor their mechanical and tribological properties. While most published studies have focused on doping and alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements has only recently been explored. Notably, our understanding of the shear-induced structural changes occurring in rare-earth-element-containing a-C films is still elusive, even in the absence of any liquid lubricants. Here, the friction response of Eu- and Gd-containing a-C films with low hydrogen content deposited by HiPIMS on silicon was evaluated in open air and at room temperature. The load-dependent friction measurements indicated that the introduction of Gd ((2.3 ± 0.1) at.%) and Eu ((2.4 ± 0.1) at.%) into the a-C matrix results in a significant reduction of the shear strength of the sliding interfaces ((41 ± 2) MPa for a-C, (16 ± 1) MPa for a-C:Gd2.3at.%, and (11 ± 2) MPa for a-C:Eu2.4at.%). NEXAFS spectromicroscopy experiments provided evidence that no stress-assisted sp3-to-sp2 rehybridization of carbon atoms was induced by the sliding process in the near-surface region of undoped a-C, while the amount of sp2-bonded carbon progressively increased in a-C:Gd2.3at.% and a-C:Eu2.4at.% upon increasing the applied normal load in tribological tests. The formation of an sp2-bonded carbon-rich surface layer in a-C:Gd2.3at.% and a-C:Eu2.4at.% films was not only proposed to be the origin for the reduced duration of the running-in period in tribological test, but was also postulated to induce shear localization within the sp2-carbon-rich layer and transfer film formation on the countersurface, thus decreasing the interfacial shear strength. These findings open the path for the use of Gd- and Eu-containing a-C even under critical conditions for nearly hydrogen-free a-C films (i.e., humid air).

Integration of Chemical Looping Combustion to a Gasified Stream with Low Hydrogen Content

Global population growth requires the use of various natural resources to satisfy the basic needs of humanity. Fossil fuels are mainly used to produce electricity, transportation and the artificial air conditioning of habitats. Nevertheless, countries around the world are looking for alternative energy sources due to the decrease in the availability of these fuels and their high environmental impact. The mixture of hydrogen and carbon monoxide (H2 + CO), commonly called syngas, is a high-value feedstock for various industrial applications. By varying the composition of syngas, especially the H2/CO molar ratio, it can be used to produce methanol, fuels or synthetic natural gas. However, when this ratio is very low, the separation of this gas usually represents a great problem when making the energy balance, which is why it is proposed to adapt a combustion process in chemical cycles, taking advantage of the energy of this gas, reducing the energy impact of the process. During the present project, mass and energy balances were developed for combustion in chemical cycles, using ilmenite as a carrier, integrating heat exchangers to take advantage of the residual energy at the output of the process, to preheat the inlet current in the regenerator. Here, a comparative was made at different temperatures of the air stream and evaluating the mechanism of the ilmenite when a syngas stream is used as fuel.

Non-Henrian behavior of hydrogen in plagioclase – basaltic melt partitioning

Plagioclase crystals in lunar anorthosite may have formed directly from the lunar magma ocean (LMO), with their elemental and isotopic compositions providing unique probes of LMO composition, age, and evolution. Low hydrogen concentrations have been reported in lunar plagioclase, but translating this to estimates of LMO hydrogen contents is complicated by incomplete knowledge of hydrogen partitioning between plagioclase and melt at low hydrogen contents. We conducted a series of high-temperature experiments to better quantify the plagioclase-melt partition coefficient of hydrogen in the Moon at low melt H abundances. Hydrogen contents of plagioclase and coexisting melt, reported in terms of water equivalent concentrations, were analyzed using Fourier Transform Infrared Spectroscopy. Resulting plagioclase-melt partition coefficients of water, Dplag-melt water, at melt water contents below 300 ppm, are 0.10 ± 0.05 to 0.36 ± 0.19. the highest Dplag-melt water reported to date. Our results, in conjunction with literature data, indicate a strong dependence of Dplag-melt water on water concentration in the melt (x in ppm): Dplag-melt water=20±5·x−0.88±0.03. This strong non-Henrian behavior is likely related to changes in the hydrogen activity in basaltic melt due to variations in H2O/OH speciation as a function of melt hydrogen content, implying that hydrogen partitioning in other mineral-melt systems may also be non-Henrian. At the low water contents in the lunar interior, Dplag-melt water is approximately 2 orders of magnitude higher than previously assumed. If hydrogen contents measured in lunar plagioclase crystals reflect pristine plagioclase-LMO equilibrium without subsequent modification, we conclude that <14 ppm H2O equivalent would have been present in the Moon when 95% of the initial LMO had crystallized.

A Review on The Effect of Post-Weld Heat Treatment (PWHT) on Its Thermal Analysis and Mechanical Properties of Welded Metallic Pipe

Welding is a commonly used process in manufacturing and engineering, and it may create residual stress in the welded region that can cause problems during service. Thermal expansion of the post-weld heat treatment (PWHT) of steel carbon induces residual stress. To relieve the internal stresses in the weld zone, PWHT is used to soften the hardening zone, improve microstructures, and lower hydrogen content in the welded region. The pipe size, heating width, insulation conditions, heating rates, soaking temperature, and holding time are factors that influence PWHT procedures. This paper will explain clearly about the PWHT process and the effect of PWHT on the metallic metal.

The effect of low hydrogen content on hydrogen embrittlement of additively manufactured 17–4 stainless steel

This work aims to evaluate the impact of small amounts of hydrogen on the hydrogen-assisted cracking (HAC) of 17-4 martensitic stainless steel (SS) prepared by additive manufacturing (AM). To elucidate the effect of processing on the hydrogen–material interactions, the obtained results were compared with a conventionally manufactured (CM) counterpart. It was found that the hydrogen uptake of AM 17-4 SS is higher compared to CM; however, its resistance to HAC is improved. These differences are attributed to the presence of stronger hydrogen trapping sites, retained austenite and the absence of Nb-rich precipitates in the AM 17-4 SS. The effect of processing on the microstructure and the susceptibility to hydrogen-induced damage and hydrogen embrittlement is discussed in detail.

The effect of direct energy deposition arc mode on the porosity of AA5087 aluminum alloy

The paper evaluates the effect of the welding mode of the CMT-based direct energy deposition arc (DED-Arc) process on the porosity of AA5087 aluminum alloy walls. Three welding modes were used to produce the walls: cold metal transfer (CMT), CMT-Pulse (CMT-P), and CMT Cycle-Step (CMT-CS) with heat inputs of 128.5 J/mm, 150.4 J/mm, and 152 J/mm, respectively. Porosity was assessed with a light microscopy (LM) and computer tomography (CT). The wall produced with the lowest heat input had the lowest average porosity of 0.641 % compared to CMT-P (0.799 %) and CMT-CS (0.766 %). CT results followed the same trend with porosity of 0.21 %, 0.32 %, and 0.27 % for CMT, CMT-P, and CMT-CS, respectively. The porosity level strongly correlates with the hydrogen content. The lowest porosity (CMT) at the lowest hydrogen content (5.29 ppm) is associated with lower hydrogen solubility in a molten pool at lower heat inputs. Most of the detected pores in all modes ranged from 51 to 100 μm in size.

Low hydrogen permeability and high durability proton exchange membrane with three-dimensional acid-base crosslink structure for water electrolysis

Introducing a acid–base cross-linked network structure and reinforcement layer into a proton exchange membrane (PEM) can effectively improve its performance, durability, and hydrogen permeability for high-pressure PEM water electrolysis (PEMWE). Here, a hyperbranched polymer containing imidazole terminal groups (HBP-MZ) is designed and prepared, then is used to form a novel PEM possessing a cross-linked network structure consisting of HBP-MZ, perfluorosulfonic acid (PFSA) and reinforced with a poly(ether-ether-ketone) (PEEK) mesh. The microstructure of the PEM is characterized using thermal performance analysis. Benefiting from the acid–base crosslinked structure, the obtained PEM has a more compact structure, induces a drastic increase in dimensional stability, mechanical properties, proton conductivity and stability in water. The HBP-MZ/PFSA@PEEK membrane exhibits excellent water electrolysis performance, low high-frequency impedance, and superior stability: the HBP-MZ/PFSA@PEEK membrane provides an electrolytic performance of up to 1.76 V at 2.0 A cm−2 and 1.84 V at 3.0A cm−2 (60 °C, with an IrO2 loading of 1.0 mg cm−2), and exhibits a 6.2 μV h−1 average voltage degradation rate during durability testing. Benefiting from the characteristics of HBP and the existing cross-linked structures, the obtained HBP-MZ/PFSA@PEEK membrane exhibits a lower hydrogen crossover current density of 1.06 mA cm−2 and maintains a lower hydrogen content in oxygen under conditions of high current density and high pressure. This work provides a promising way to prepare a PEM with high durability and low gas permeability that could be applied in high-pressure water electrolysis.

Stability of industrial gallium-doped Czochralski silicon PERC cells and wafers

The carrier lifetime stability of gallium-doped silicon wafers and performance stability of industrial PERC solar cells produced from sister wafers were investigated under four different illumination conditions and temperatures. The seven investigated materials feature a resistivity variation of 0.4–1.0 Ωcm and lifetime samples were processed to create high hydrogen content (with PECVD SiNx) or low hydrogen content (with ALD Al2O3 or HfO2). Our results confirm that the material itself is prone to light and elevated temperature induced degradation (LeTID), however experiments on PERC cells produced utilising the same silicon material indicate that the production process can successfully suppress LeTID. In contrast to earlier studies, we observe only small levels of degradation at the cell level, with some showing an improvement in cell parameters under LeTID testing conditions. Our results indicate that LeTID is not necessarily a major issue for the performance of modern passivated emitter and rear cells made from gallium-doped silicon substrates.

Impact and Detection of Hydrogen in Metals

AbstractThe widespread use of hydrogen as an energy carrier is considered one of the most important keys to achieving the decarbonization necessary for the energy transition in numerous areas of technology and society. Not least due to the associated contact of metallic components with (pressurized) hydrogen, there is a latent risk of hydrogen-induced cracking (“hydrogen embrittlement”). The cause of damage is the hydrogen absorbed by the material, which is mobile via interstitial lattice diffusion. In high-strength steels with a tensile strength of more than 800 MPa, even very low diffusive hydrogen contents of less than 1 ppm (parts per million) can have a crack-inducing effect. Hence, dedicated, highly accurate analytical and testing methods are required for the detection of hydrogen and its effect on the mechanical properties of metals. This paper summarizes the current state of knowledge regarding hydrogen embrittlement and reviews the analytical, mechanical, and fractographic investigation methods for detecting hydrogen in metals.

Study on flame propagation and inherent instability of hydrogen/ammonia/air mixture

To improve the safety and reliability of hydrogen energy, this paper studied the flame propagation and instability characteristics of hydrogen/ammonia/air mixture under different hydrogen contents, equivalence ratios and initial pressures. The results show that three typical phenomena are observed in the premixed hydrogen/ammonia/air flame propagation process under different experimental conditions. In a stable flame, the roughness of the flame surface does not increase, while during unstable flame expansion, the cracking of the flame surface splits and increases. In addition, under the condition of low equivalence ratio and hydrogen content, significant flame uplift is observed. With the increase in hydrogen content, the flame thickness and critical instability radius of the hydrogen/ammonia/air mixture decrease monotonously. The increase of hydrogen content mainly strengthens the hydrodynamic instability, leading to the weakening of flame stability. With the increase of the equivalence ratio, the Markstein length and the critical instability radius increase monotonically. The minimum value of flame thickness appears at the equivalence ratio of 1.0. The Lewis number is always less than 1 for poor combustion and greater than 1 for rich combustion. An increase in the hydrogen content only moves the Lewis number further away from 1. In rich burn, the thermal-diffusion is beneficial to flame stability, and the flame thickness increases with the increasing equivalence ratio. Under the combined effect of thermal-diffusion and hydrodynamic instability, the overall stability of the hydrogen/ammonia/air flame gradually increases with the increasing equivalence ratio. With the increase of initial pressure, the flame thickness decreases gradually. The critical instability radius and Markstein length both decrease rapidly with the increase of initial pressure.

Mesoscale quantification of grain boundary character and local plasticity in hydrogen-assisted intergranular and intergranular-like cracking paths in tempered martensitic steel

The effects of the misorientation of prior austenite grain boundary (PAGB) segments on the local plasticity evolution in intergranular (IG) and IG-like fractures were investigated using a tempered martensitic steel. When a diffusible hydrogen content of 6.3 mass ppm was introduced, IG fractures with smooth fracture surfaces occurred in the elastic regime of the tensile test. According to the grain orientation spread (GOS) distribution, local plasticity did not contribute to the mesoscopic (block size) scale, irrespective of the PAGB segment misorientation. With a relatively low hydrogen content (4.0 mass ppm), an IG-like feature appeared on the fracture surface, which included plasticity-related traces such as tear ridges or nanovoids. The GOS values near the IG-like cracks were higher than those near the IG cracks and exhibited significant misorientation dependence. Specifically, at the low-angle PAGB segments, the GOS values of the grains neighboring the IG-like cracks increased with a decreasing misorientation. On the other hand, most of the cracked high-angle PAGB segments exhibited nearly constant GOS values. Remarkably, a Σ3 PAGB segment exhibited an abnormally high GOS value. These facts and corresponding microstructural observation results indicate that the low angle and Σ3 PAGB segments allow crack-tip blunting before crack growth.

Hydride reorientation in fuel cladding under interim storage conditions with low hoop stress

Abstract. The restart of the search for a final repository for high-level nuclear waste in Germany and the resulting so-called white map lead to a reassessment of the safety of interim storage as well. Due to the resulting increase in interim storage periods with a prospected timescale until 2068, among other things, renewed consideration of cladding tube integrity is required for the handling of the fuel assemblies during repackaging into the final storage containers. Besides oxide formation during operation, hydrides can have a negative influence on stability during interim storage and handling later on. The hydrides, which are initially precipitated tangentially in the tube material during operation, can reorient during interim storage as a result of the temperature and load history and precipitate again in a radial direction during cooling under load. In recent years, a number of investigations have already been carried out with respect to this topic, with cladding conditions that are present in high burn-up fuel assemblies with corresponding high hydrogen contents. In these cases, reorientation effects did occur with stresses that can be encountered in interim storage, but the proportions of radial hydrides were very low and were classified as not being harmful. In addition to the tests with cladding tubes containing high hydrogen contents, sporadic tests with low hydrogen contents were also carried out, for example, by Hardie and Shanahan (1975) and later by Chu et al. (2008). These indicated that strong reorientation effects could occur because of the complete dissolution of hydrogen under high temperature and tensile stress. In recent years, Kaufholz et al. (2018) also formulated a hypothesis that describes this effect as being the consequence of free nucleation without the presence of precipitates. This results in the possible formation of extremely pronounced radial hydrides, which can even extend through the tube wall. This is possible because residual hydrides that can hinder free precipitation or represent preferential points for nucleation are absent. The results of the present investigations confirm this behaviour. They show that massive reorientations can already occur at stresses at the lower end of the literature values and that, until now, these reorientations were interpreted as being uncritical. Considering the hysteresis in solubility between terminal solid solubility for dissolution (TSSD) and precipitation (TSSP), it becomes apparent that precipitation at hydrogen contents of 100 ppmw (parts per million by weight) already starts at approx. 538 K and that the hydrides precipitate radially under a stress of 52 MPa and form extremely pronounced hydrides. This results in a need for further research, since the thermal and mechanical loads under which the tests were carried out are well below the permissible limit loads in interim storage and below the values expected so far. This applies in particular to materials and cladding tubes that either absorb less hydrogen during operation or have only low burn-ups. The study provides an overview of the status quo, presents part of the investigations carried out during the project MUDZ (Modellierung und Untersuchung der Degradation von Hüllrohrmaterialien aus Zr-Legierungen durch Hydridbildungs- und Hydridverteilungsprozesse im Hinblick auf die Langzeitzwischenlagerung) in the KEK (Kompetenzerhalt in der Kerntechnik) initiative and the results obtained, and discusses the ramifications of the results.

The Role of Surface in Hydride Formation Processes

Several LaNi5-based hydrogen storage alloys were studied using secondary ion mass spectrometry (SIMS) technique. Ar+ ions with the energy of 10 - 18 keV were used as primary ions. The study of the initial stages of the processes of LaNi5-based alloys interaction with hydrogen under the experimental conditions showed that on the areas of clean surface, hydrogen formed chemical compounds with the both of the main components of the alloy: nickel and lanthanum. As hydrogen accumulates on the surface and in the near-surface region, a hydrogen-containing structure is formed, which is characterized by a certain stoichiometric ratio of components. Nickel in this structure has strong chemical bonds with two hydrogen atoms, and lanthanum – with two or more hydrogen atoms. Along with such compounds, some structures with lower hydrogen content are also formed. The formed hydrogen-containing structure includes both main alloy components, La and Ni for all the studied samples, even though only lanthanum is generally accepted to be the hydride-forming element in such alloys. The SIMS studies of the chemical composition of the surface monolayers of the intermetallic alloy LaNi5, in the process of its interaction with oxygen, showed the following. As a result of the oxygen interaction with the alloy, a complex chemical structure including oxygen, lanthanum and nickel is formed on the surface and in the near-surface region of LaNi5. Oxygen in such a structure, similarly to hydrogen, forms strong chemical bonds with both components of the alloy. This is indicated by the presence in the mass spectra of a large set of oxygen-containing emissions of positive and negative secondary ions with lanthanum and nickel, as well as oxygen-containing lanthanum-nickel cluster secondary ions. The formed oxide compounds have a three-dimensional structure and occupy tens of monolayers. Oxygen poisoning of the surface of the hydride-forming alloy LaNi5 can occur regardless of whether the surface of the alloy was clean from the very beginning or it was covered with a layer of hydrogen-containing chemical compounds.

Hydrogen extraction from methane-hydrogen mixtures from the natural gas grid by means of electrochemical hydrogen separation and compression

One interesting option to store and transport hydrogen in the future is to blend it into the natural gas grid. In this paper, we present technological insights regarding electrochemical hydrogen separation and compression (EHC/S), a technology that enables extraction of compressed and purified hydrogen from such blends. Our contribution discusses two use cases, namely i) hydrogen separation to recover purified hydrogen under pressure, and ii) hydrogen removal to protect downstream methane consumers from excessive hydrogen levels. Our experimental work with a laboratory scale EHC/S centers on efficiency aspects. We show that at low hydrogen contents in dry feed gas, gas humidification causes relevant thermal energy consumption, that cannot be provided by EHC/S waste heat. We also show that hydrogen recovery is restricted by mass transport if the residence time in the EHC device is too short. Simultaneous hydrogen separation and compression from 5% hydrogen in methane is demonstrated yielding hydrogen at 6 bara at the cathode.

Optimizing the gradient stress sandwich structure thin-film encapsulation for super flexible organic light-emitting devices

To enable organic light-emitting diodes (OLEDs) to be rolled and folded, we need a flexible encapsulation layer that can protect organic materials and metal electrodes that are susceptible to moisture and oxygen. Thin films that encapsulate organic electronic devices need to have excellent mechanical properties to prevent cracks during bending. Using plasma-enhanced chemical vapor deposition (PECVD) and other techniques, we fabricated high-density, stress gradient sandwich-structured films and studied the residual stress of deposited films on encapsulating films and their effect on delamination and cracking. We found that by adjusting the H2/N2 gas ratio and optimizing the Si:N:H ratio of PECVD SiNx:H films, denser, more etch-resistant, higher compressive stress and lower hydrogen content films can be deposited, thereby enable better flexible thin film encapsulation (TFE). We also deposited an inorganic/organic/inorganic sandwich structure film and utilized stress gradient changes to relieve the tensile stress on the outer film during bending. After standardized testing, the OLED with the stress gradient encapsulation structure has no dark spots after bending 10 000 times (bending radius 2 mm). This technique can be used in flexible TFE for various organic devices, showing promising applications in bendable and wearable products.

Low-temperature fabrication of silicon nitride thin films from a SiH4+N2 gas mixture by controlling SiNx nanoparticle growth in multi-hollow remote plasma chemical vapor deposition

High-quality amorphous silicon nitride (SiNx) thin films were fabricated by the controlled growth of nanoparticles during SiH4+N2 multi-hollow remote plasma chemical vapor deposition (CVD) at low substrate temperature 100 °C. Measurements from quartz crystal microbalances showed that a higher amount of nanoparticle incorporation in the SiNx film corresponded to a higher ratio of N/Si in the film, implying that the nanoparticles were nitrided in the plasma phase. We controlled the size of the nanoparticles by tuning the gas flow ratio of N2/SiH4 and the total gas flow rate. Transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that smaller nanoparticles in the plasma led to a higher ratio of N/Si in the film and a lower hydrogen content. We attribute these results to the low heat capacity and large specific surface area of the nanoparticles, which enabled active chemical reactions on their surface in the plasma.