Formation Of Δ-phase Research Articles

Unveiling the role of Cs ion in perovskite phase formation during solid–vapor reactions

The preparation of perovskite solar cells (PSCs) using a solid–vapor reaction has shown great potential due to its scalability and lack of organic solvent. The phase formation process in the solid–vapor reaction is crucial for the crystallization of perovskite films. Previous studies mainly introduced Cs into the solid–vapor reaction, but insufficient attention was paid to the role of Cs in purifying the phase formation path during the crystallization process. In this study, we investigated the effect of Cs on the phase formation process in the solid–vapor reaction and found that Cs can decrease the crystallinity of lead halide films, induce a purified phase formation process to prevent the formation of δ-phase and other impurity phases, regulate the crystallization rate of perovskite, and produce high-quality, low-defect perovskite films. The result was achieving a champion power conversion efficiency (PCE) of 21.54 % for a small area (0.148 cm2) and 18.65 % for larger mini-modules (5 × 5 cm2). Furthermore, the devices maintained over 80 % of their initial efficiencies after 450 h of continuous operation under AM 1.5 G irradiation. This work underscores the importance of purified phase formation paths in solid–vapor reactions for outstanding crystal quality in high-performance perovskite solar cells.

Advancements in perovskite solar cell efficiency through blade coating techniques and post-treatment optimization

Addition of lead chloride (PbCl2) improves the crystallization kinetics of perovskites by suppressing the formation of δ-phase during large-area perovskite film fabrication. However, this addition results in formation and accumulation of lead iodide (PbI2) platelet atop the film, which adversely affects carrier transport. To eliminate the negative effect of PbI2 accumulation and simultaneously obtain a dense, mirror-like perovskite film, a post-treatment method involving blade coating formamidinium iodide (FAI) under air atmosphere is used. Instead of being washed away, as reported in literatures using spin coating process, PbI2 platelets are converted into cesium lead halide perovskites (CsPbX3) through reaction with Cs and halide present in the original perovskite Cs0.22FA0.78Pb(I0.85Br0.15)3. An efficiency enhancement of (>10%) is achieved by FAI treatment, which does not require fast solvent removal or nitrogen (N2) atmospheres, indicating that FAI blade coating is an effective and straightforward method for the post-treatment of excess PbI2.

Effects of nitride precipitation on delta phase formation in additively manufactured nickel superalloys

Additively manufactured Inconel 625 is particularly susceptible to the formation of niobium-rich δ-phase precipitates in the interdendritic regions during high temperature exposure. With specific minor alloying element combinations and nitrogen mass fractions on the order of 0.1 %, the formation of δ-phase can be suppressed through the precipitation of nitrides. For example, mass fractions of 0.39 % silicon and 0.03 % titanium led to the precipitation of Z-phase and η-nitrides, which consumed the excess niobium in the interdendritic regions and limited δ-phase formation. Even when holding the material at a temperature of 870 °C for 1000 h, the δ-phase volume fraction was approximately 2 %, which is far below the 6 % level observed in the wrought condition. When the titanium mass fraction was increased to 0.21 % and the silicon mass fraction decreased to 0.05 %, titanium-rich MN nitrides formed within the interdendritic regions instead. Since much of the Nb in these regions was not consumed by the nitrides, δ-phase formation was promoted and reached volume fractions above 10 %. The addition of a hot isostatic pressing step prior to high temperature exposure in both alloys produced a more uniform niobium distribution and minimized δ-phase formation at shorter times. At extended times, trends in the δ-phase volume fractions were similar to those observed in the as-deposited condition, with the initial alloy compositions driving differences in the distribution of excess niobium available for δ-phase formation.

A detailed investigation of the structural stability, ionic conductivity and in situ δ-phase formation of YxBi2xO3

The in situ δ-phase formation for a series of yttrium-doped Bi2O3 materials was studied by Raman spectroscopy and high resolution X-ray diffraction under variable temperature conditions. Contrary to previous reports, for dopant concentrations less than 25%, the cubic ô-phase was not fully stabilized at room temperature - tetragonal phase content was evident in both Raman spectra and powder diffractograms. Arrhenius plots also showed evidence of a phase transformation by the distinct large step change in conductivity in the temperature region where the tetragonal-cubic phase transformation was noted. Both the thermal expansion and ionic conductivity of the cubic phase decreased with increasing dopant content.

Revisiting δ-PVDF based piezoelectric nanogenerator for self-powered pressure mapping sensor

δ-phase comprising polyvinylidene fluoride (PVDF) nanoparticles are fabricated through an electrospray technique by applying a 0.1 MV/m electric field, which is 103 times lower than the typical value, required for δ-phase transformation. X-ray diffraction and selected area electron diffraction patterns clearly indicate the δ-phase formation that limits the infrared vibrational spectroscopic technique due to identical molecular chain conformations to that of non-polar α-phase. The piezo- and ferro-electric response of δ-PVDF nanoparticles have been demonstrated through a scanning probe microscopic technique based on piezoresponse force microscopy. The localized piezoelectric response, indicated by d33 coefficient, is found to be ∼−11 pm/V. To utilize the distinct electromechanical response of δ-PVDF nanoparticles, the piezoelectric nanogenerator (PNG) has been fabricated. Due to the stress confinement effect in the spherical shape of δ-PVDF nanoparticles, the PNG exhibits synergistic effect than that of the film-based counterpart. The maximum power, i.e., 930 μW/m2 determined by the PNG under ∼4.5 N of periodic force impact, indicates the potential to use it as a self-powered sensor. As a proof of concept, a self-powered pressure sensor mapping has been demonstrated for representing its realistic technological applicability.

Characterization of Ni-Cr-Mo alloy phase structures under DLD process

Direct laser deposition of Ni-Cr-Mo alloy has been carried out to study the phase structure transformation under various operating parameters. To assess the phase composition SEM, TEM and XRD analysis were performed. It clearly seen that the precipitation of straightening phases (γ' and γ") depends on the temperature gradients caused by different laser power. The nucleation of γ' phase starts at 750 W while γ" phase at 1000 W. The changes in temperature gradients lead to different diffusion condition across all the stage of laser power variation and formation of δ-phase, primary MC carbides and Laves phase. δ-phase of all samples did not change the shape and characterized only by plate-like shape.

Over 21% Efficiency Stable 2D Perovskite Solar Cells.

Owing to their insufficient light absorption and charge transport, 2D Ruddlesden-Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the light absorption. Optical spectroscopy shows that FA cations substantially increase the portion of 3D-like phase to 2D phases, and X-ray diffraction (XRD) studies reveal that FA-based 2D perovskite possesses an oblique crystal orientation. Nevertheless, the ultrafast interphase charge transfer results in an extremely long carrier-diffusion length (≈1.98µm). Also, chloride additives effectively suppress the yellow δ-phase formation of pure FA-based 2D PSCs. As a result, both FA/MA mixed and pure FA-based 2D PSCs exhibit a greatly enhanced power conversion efficiency (PCE) over 20%. Specifically, the pure FA-based 2D PSCs achieve a record PCE of 21.07% (certified at 20%), which is the highest efficiency for low-dimensional PSCs (n≤ 10) reported to date. Importantly, the FA-based 2D PSCs retain 97% of their initial efficiency at 85°C persistent heating after 1500h. The results unambiguously demonstrate that pure-FA-based 2D PSCs are promising for achieving comparable efficiency to 3D perovskites, along with a better device stability.

Surface Treatment to Inhibit Water-induced Decomposition and δ-phase Formation of Perovskite Thin Films

Surface Treatment to Inhibit Water-induced Decomposition and δ-phase Formation of Perovskite Thin Films

Formamide iodide: a new cation additive for inhibiting δ-phase formation of formamidinium lead iodide perovskite

We synthesized formamide iodide (FoAI) and applied as a novel cation additive of perovskite solar cells (PSCs). We found that FoAI additive can improve the performance of FAPbI3 based PSCs by suppressing the formation of non-photoactive phase.

Potassium-containing triple-cation mixed-halide perovskite materials: Toward efficient and stable solar cells

In this paper, potassium based triple cation mixed-halide perovskite films were explored in order to enhance the stability and photovoltaic performance of perovskite based solar cells. It was found that adding potassium (K+) to a double cation mixed halide perovskite (FA0.80MA0.20PbI2.8Cl0.2), structural, morphological and optoelectronic properties of perovskites are improved. The perovskite films were prepared by one-step spin coating method with and without K+ and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The results indicate that potassium incorporation reduces significantly the yellow non-perovskite δ-phase formation and improves the perovskite film quality, thus contributing to the reduction of hysteresis, improves the stability and increases the PCE up to 12.51%. Furthermore, the doped devices exhibit reduced hysteresis and provide remarkable shelf stability by retaining more than 70% of the initial efficiency with low humidity over 850 h.

Microstructure and Fatigue Crack Growth Behavior of Inconel 718 Superalloy Fabricated Via Laser Directed Energy Deposition Additive Manufacturing

This work investigated the microstructure and its influence on room-temperature fatigue crack growth (FCG) behavior of Inconel 718 (IN718) alloy fabricated with laser-based directed energy deposition (L-DED). As-deposited specimens were heat-treated by direct aging (DA), solution treatment + aging (SA) and homogenization + solution treatment + aging (HSA) respectively to investigate the effect of post-heat-treatment. On comparison with as-deposited microstructure, which mainly consists of γ dendrite and Laves phases in the interdendritic regions. Dendritic segregation still appears after DA heat-treatment, which is certified to alter the precipitation kinetics of γ′′/γ′ precipitations of IN718 and induce the formation of more abundant γ′′/γ′ precipitations in interdendritic Nb-rich regions versus the dendritic trunk regions. After SA heat-treatment, Nb released from the dissolution of Laves phase is prone to diffuse to the ambient interdendritic regions, which facilitates the formation of δ-phase and the homogeneous precipitations of γ′′/γ′ phases in dendritic trunk region. After HSA heat-treatment, the Laves phases are eliminated completely, the dispersive γ′′/γ′ precipitates (~30 nm) are distributed within the bimodal recrystallized grains. The results of FCG testing reveal that as-deposited specimen possesses the lowest threshold for fatigue-crack initiation (ΔKth) and the highest FCG rate (FCGR). The ΔKth and FCGR of DA specimen do not have the notable improvement versus those of as-deposited specimen, due to its inferior strength-plasticity mainly caused by heterogeneous γ′′/γ′ phases and Laves particles. The fatigue-crack propagates easily along the precipitate-free zone around short-acicular δ-phase, which causes the crack branching and deflection, enhances the resistance to FCG of SA specimen to some extent. The ΔKth of HSA specimen increases by ~2.6 times compared with as-deposited specimen and reaches 10.8 MPa∙m1/2, due to its decent strength-plasticity (~1130 MPa, ~26.5%). Especially, the ΔKth and FCGR of HSA specimen are comparable to that of the wrought IN718. The FCG behaviors of all specimens predominantly occur in the transgranular ductile fracture mode. Overall, the present study demonstrates that a judicious post-heat-treatment can enhance the damage tolerance of L-DED IN718 by increasing ΔKth and decreasing FCGR.

Microstructure, precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment

IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.

The Influence of Annealing Temperature and Time on the Formation of δ-Phase in Additively-Manufactured Inconel 625.

This research evaluated the kinetics of δ-phase growth in laser powder bed additively-manufactured (AM) Inconel 625 during post-build stress-relief heat treatments. The temperatures ranged between 650°C and 1050°C, and the times from 0.25 to 168 hours. The presence of δ-phase was verified for each temperature/time combination through multiple techniques. A conventional time-temperature-transformation diagram was constructed from the time-temperature data. Comparison to the growth in wrought IN625 with a similar nominal composition revealed that δ-phase formation occurred at least two orders of magnitude faster in the AM IN625. The results of this study also revealed that the segregated microstructure in the as-built condition has a strong influence on the kinetics of δ-phase formation in AM IN625 as compared to a homogenized material. Since control of the δ-phase growth is essential for reliable prediction of the performance of IN625 components in service, avoiding heat treatments that promote the formation of δ-phase in AM components that are not homogenized is highly recommended. This will be particularly true at elevated temperatures where the microstructural stability and the consistency of mechanical properties are more likely to be affected by the presence of δ-phase.

Formation of the Ni3Nb δ-phase in stress-relieved Inconel 625 produced via laser powder-bed fusion additive manufacturing.

The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 h at 1143 K (870 °C) is investigated. It is found that this industry-recommended heat treatment promotes the formation of a significant fraction of the orthorhombic D0a Ni3Nb δ-phase. This phase is known to have a deleterious influence on fracture toughness, ductility, and other mechanical properties in conventional, wrought Inconel 625; and is generally considered detrimental to materials' performance in service. The δ-phase platelets are found to precipitate within the inter-dendritic regions of the as-built solidification microstructure. These regions are enriched in solute elements, particularly Nb and Mo, due to the micro-segregation that occurs during solidification. The precipitation of δ-phase at 1073 K (800 °C) is found to require up to 4 h. This indicates a potential alternative stress-relief processing window that mitigates δ-phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to δ-phase precipitation increases the possibility for significant degradation of materials properties in service.

Evaluation of microstructural and mechanical properties of Fe-16Cr-1Ni-9Mn-0.12N austenitic stainless steel welded joints

This study has been aimed to find out the proper welding procedure/conditions for a newly developed Fe-16Cr-1Ni-9Mn-0.12N austenitic stainless steel (ASS), which is uniquely modified from 200 series alloy, in order to control the microstructural metastability of the weld metals. The welded joints of 4mm thick sheet were prepared in a square butt configuration by using three modes of metal transfer, i.e., short-circuit (SC), spray (S), and pulse (P) mode of metal transfer and three austenitic filler metals (304L, 308L and 316L) under two shielding gas environments using gas metal arc welding (GMAW) process. It is observed that the variation in modes of metal transfer and filler metals can effectively manipulate the metastability of γ-phase, formation of δ-phase and grain size of the weld metals. Among the modes of metal transfer, pulse mode of metal transfer produces more metastable γ-phase and higher δ-phase fraction responsible for finer grain structure in weld metals irrespective of filler metals used. Again, higher microstructural metastability was obtained with the 304L filler metal irrespective of modes of metal transfer used. It is evident that the weld metals having higher microstructural metastability also improved mechanical properties (i.e. hardness, strength and toughness). However, weak fusion boundary (FB) region of welded joint limits the beneficial effect of metastability and creates a weakest link in the joint. On the other hand, high temperature heat affected zone (HTHAZ) having a coarse austenite grain surrounded by grain boundary precipitates is governed only by the modes of metal transfer.

Unexpected δ-Phase Formation in Additive-Manufactured Ni-Based Superalloy

An as-built and solutionized Ni-based superalloy built by additive manufacturing through a direct metal laser sintering technique is characterized to understand the microstructural differences as compared to the as-wrought alloy. Initially, each layer undergoes rapid solidification as it is melted by the laser; however, as the part is built, the underlying layers experience a variety of heating and cooling cycles that produce significant microsegregation of niobium which allows for the formation of the deleterious δ-phase. The as-built microstructure was characterized through Vickers hardness, optical microscopy, scanning and transmission electron microscopy, electron back-scattering diffraction, x-ray diffraction, and synchrotron x-ray microLaue diffraction. The isothermal formation and growth of the δ-phase were characterized using synchrotron-based in situ small angle and wide angle x-ray scattering experiments. These experimental results are compared with multicomponent diffusion simulations that predict the phase fraction and composition. The high residual stresses and unexpected formation of the δ-phase will require further annealing treatments to be designed so as to remove these deficiencies and obtain an optimized microstructure.

Impression Creep Behaviour of U−6%Zr Alloy: Role of Microstructure

Abstract Creep behavior of U−6%Zr alloy was studied in the temperature range 550−650 °C using impression creep technique. Creep curves at 575 °C for various stresses are not smooth and exhibited some waviness. Sometimes creep curves show a sudden penetration of the punch which is followed by decrease in impression depth with time. The punching stress versus creep rate data for the above temperature also show a random behaviour. A detailed microstructural investigation was carried out with the help of SEM and EDX to understand this phenomenon. At creep testing temperature, δ-UZr2 phase continuously forms and it takes very long time to complete the reaction. Hence, the reason for waviness may be due to the formation of δ-phase coming out from the supersaturated α-phase.

Improved surface morphology and corrosion resistance for galvannealed coatings by pre-electroplating iron

Dual-phase steel was electroplated with Fe prior to reduction annealing to improve surface quality and corrosion characteristics after galvannealing. This pre-electroplating process caused remarkable reduction in surface cracks and induced δ-phase formation over the surface of the galvannealed coating. Moreover, the degree of Fe–Zn alloying significantly increased, resulting in a thicker coating. Consequently, the corrosion potential increased and corrosion current decreased in the highly polarized anodic reaction. Further, spontaneous passivation occurred, resulting in a very low corrosion rate.

Corrosion and wear properties of TiC/Ni–Mo composites produced by direct consolidation during a self-propagating high-temperature reaction

TiC/Ni–Mo composites were prepared by one-step direct consolidation during a self-propagating high-temperature synthesis reaction and their corrosion and wear properties were studied. The composites reached 99.8% of theoretical density after the direct consolidation. Corrosion potentials and rates of the composites in a 50% NaOH solution were −982.7 mV SHE and 3.6×10 −6 A cm −2 for TiC/50% Ni and −1074.8 mV SHE and 8.12×10 −6 A cm −2 for TiC/46.5% Ni–3.5% Mo, respectively. Wear resistance determined by abrasive wear increased with molybdenum addition. Electron probe microanalysis revealed that δ-phase was formed around titanium carbide in the TiC/Ni–Mo composites. Transmission electron microscopy revealed that the dislocation density around the carbide in the composites was high. The improved wear resistance was associated with increases of the interfacial bonding strength due to the formation of δ-phase and the high dislocation density around carbide due to the difference of thermal expansion coefficient between carbide and matrix.

Solid-state nucleation at the interfaces of thin Ni films on CdTe and Hg0.8Cd0.2Te

The interfacial reaction between thin Ni film (deposited by either e beam evaporation or sputtering) and (Cd, Hg0.8Cd0.2)Te has been investigated using transmission electron microscopy and diffraction. Auger depth profiles have been used to determine the distribution of Te, Cd, Hg, and Ni with respect to the interface of Ni-Hg1−x CdxTe. In the Ni-Hg0.8Cd0.2Te system, we observed the δ-phase (NiTe-NiTe2) formation upon deposition of 40- and 80-Å Ni films with either deposition system at the as-deposited temperature. The electron diffraction patterns of the δ phase and some Ni were observed, with the formation of the δ phase located at the interface. The electron diffraction of both the Ni film and δ phase were partially ordered with respect to the substrates. The Auger depth profile indicated that the excess of Te appeared mostly at the interface. Annealing of the Ni-Hg0.8 Cd0.2 Te system in a vacuum and at 100, 200, and 300 °C shows growth of the crystal and increased ordering of the δ phase. In the Ni-CdTe system the generation of Te was observed for both sputter deposition and e beam evaporation. Auger profiling indicated that the amount of Te generated by sputtering was approximately three times the amount of Te generated by e beam evaporation. The generated Te was mostly accumulated at the interface of Ni-CdTe. We observed δ-phase formation upon annealing at 250 °C after sputter deposition, and at 300 °C after e beam evaporation of Ni on CdTe substrates, but the crystallites did not have the orientational relation with CdTe substrates (texturing), as was seen in the Ni-Hg0.8 Cd0.2 Te system.