Lattice Strain Relaxation Research Articles

An in-situ synchrotron diffraction study of stress relaxation in titanium: Effect of temperature and oxygen on cold dwell fatigue

There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. To better understand the material characteristics that control or exacerbate this behaviour, evaluation of the time dependent plasticity of the main prismatic and basal slip systems is critical. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem. In this work, characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75 °C, 145 °C and 250 °C) was performed during stress relaxation using synchrotron X-ray diffraction. Key parameters that govern the dislocation motion were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses. From the temperatures assessed, 75 °C was found to be the worst-case scenario, where the macroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems. As the temperature increases, the contribution of thermal energy becomes greater than mechanical energy for dislocation glide. Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses. This effect becomes more significant in high oxygen content commercially pure Ti.

Dislocation generation mechanisms in heavily boron-doped diamond epilayers

Doping diamond layers for electronic applications has become straightforward during the last two decades. However, dislocation generation in diamond during the microwave plasma enhanced chemical vapor deposition growth process is still not fully understood. This is a truly relevant topic to avoid for an optimal performance of any device, but, usually, it is not considered when designing diamond structures for electronic devices. The incorporation of a dopant, here boron, into a lattice as close as that of diamond, can promote the appearance of dislocations in the epilayer. The present contribution analyzes the different processes that can take place in this epilayer and gives some rules to avoid the formation of dislocations, based on the comparison of the different dislocation generation mechanisms. Indeed, competitive mechanisms, such as doping atom proximity effect and lattice strain relaxation, are here quantified for heavily boron-doped diamond epilayers. The resulting growth condition windows for defect-free heavily doped diamond are here deduced, introducing the diamond parameters and its lattice expansion in several previously published critical thickness (hc) and critical doping level relationships for different doping levels and growth conditions. Experimental evidence supports the previously discussed thickness-doping-growth condition relationships. Layers with and without dislocations reveal that not only the thickness but also other key factors such as growth orientation and growth parameters are important, as dislocations are shown to be generated in epilayers with a thickness below the People and Bean critical thickness.

Synergistic Effects of Cation and Anion in an Ionic Imidazolium Tetrafluoroborate Additive for Improving the Efficiency and Stability of Half‐Mixed Pb‐Sn Perovskite Solar Cells

AbstractNarrow‐bandgap mixed Pb‐Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all‐perovskite tandem solar cells, in combination with wide‐bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb‐Sn PSCs still lags behind lead‐based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF4) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF4) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under‐coordinated Pb2+ or Sn2+ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF4 anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF4 additive greatly enhance the optoelectronic performance of half‐mixed Pb‐Sn perovskites, leading to much longer carrier lifetimes. The best‐performing half‐mixed Pb‐Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen‐filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.

(Invited) Sn Based Perovskite Solar Cells on Glass Substrate and Plastic Substrate

Lead-based perovskite solar cells (PSCs) with small cell area have achieved a conversion efficiencies of higher than 25 %. Because of the Pb ion concern, various Pb-free PSCs also have been explored such as Bi, Ge, Ag, and Sn-PSCs. Among then, Sn-PSCs are is the most promising candidate. In this presentation, to begin with, the effect of the photo-voltaic performance caused by Ge ion doping, A site cation replacement, and surface passivation are reviewed briefly from the view point of the carrier concentration, carrier life time, crystal lattice strain, and band alignment. Recently, We reported the correlation between lattice strain relaxation and the power conversion efficiency of Sn-PSCs by substituting the A site with cations of various ionic radii(1). From the study, we knew the unique behaviors of ethylammonium iodide (EAI) replacement at A site cation, giving the efficiency over 13 %. After the brief review, especially, the effect of EAI addition on the enhanced efficiency is discussed. There is some works previously on EAI replacement at A site(2).The formulation with Ge ion doped Sn-perovskite with an inverted structure is used as the discussion. The lattice disorder induced by the A site cation size has detrimental effects on the charge carrier dynamics in perovskite semiconductors. Among them, EAI promoted crystal growth towards preferred crystal orientation and caused the crystal lattice expansion, suggesting that bigger EA+ cation is incorporated in the perovskite crystal structure. The addition of EAI decreased the lattice strain, shifted the band edge absorption towards lower wavelength, and caused blueshift of the PL, showing that the introduction of EAI makes band gap a little wider. PL lifetime became longer from 12.9 ns to 20.1 ns after the addition of EAI. Efficiency of solar cells consisting of FTO/PEDOT:PSS/Sn-PVK/C60/BCP/Ag/Au was enhanced from 9.20 % ( JSC of 23.38 mA cm-2, VOC of 0.56 V and FF of 0.70) to 9.86 % ( JSC of 23.41 mA cm-2, VOC of 0.59 V and FF of 0.71) after EAI addition. After optimization, the efficiency reached a high value of 10.80 % with a JSC of 23.05 mA cm-2, a VOC of 0.62 V and a FF of 0.75. The increased in the device performance with EAI addition was attributed to the significant increase in the VOC. We have already published a report regarding this phenomenon in which we attribute this to the lattice strain relaxation of the perovskite crystal(1). The ideality factor is decreased from 1.47 to 1.26. By using open-circuit voltage decay (OCVD) method, we proved that the addition of EAI minimizes the charge carrier recombination process and as a result showed longer carrier lifetime. Further optimization of the best device is performed by surface passivation using Lewis base following our previous report(3). This helped to increase the efficiency even further to an unprecedented efficiency of 13.24 % with a JSC of 20.32 mA cm-2, FF 0.78 of and VOC of 0.84 V. The cause is explained by using the results of Hall effect measurement and Mott-Schottkey plot from the view point of carrier concentration, mobility, and built in potential. The carrier diffusion length increased from 88.7 nm, to 348 nm after addition of EAI. This justifies the higher VOC. This agrees with the trend obtained from Hall effect measurement. XPS study proved that by the introduction of EA cation, the oxygen content has been reduced compared to the pristine perovskite film. Tuning the A site cation not only has a significant effect on the crystal stability but also the lattice strain of the perovskites. This work is expected to establish a trend on fabricating tin halide PSCs that is able to compete with lead-based PSCs.

Cold creep of titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was performed to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shifts from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τcbasal = 252 MPa and τcprism = 154 MPa) and activation energy (ΔFbasal= 10.5×10−20J = 0.65 eV andΔFprism = 9.0×10−20J = 0.56 eV). The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.

Minimizing Defect States in Lead Halide Perovskite Solar Cell Materials

In order to reach the theoretical efficiency limits of lead-based metal halide perovskite solar cells, the voltage should be enhanced because it suffers from non-radiative recombination. Perovskite materials contain intrinsic defects that can act as Shockley–Read–Hall recombination centers. Several experimental and computational studies have characterized such defect states within the band gap. We give a systematic overview of compositional engineering by distinguishing the different defect-reducing mechanisms. Doping effects are divided into influences on: (1) crystallization; (2) lattice properties. Incorporation of dopant influences the lattice properties by: (a) lattice strain relaxation; (b) chemical bonding enhancement; (c) band gap tuning. The intrinsic lattice strain in undoped perovskite was shown to induce vacancy formation. The incorporation of smaller ions, such as Cl, F and Cd, increases the energy for vacancy formation. Zn doping is reported to induce strain relaxation but also to enhance the chemical bonding. The combination of computational studies using (DFT) calculations quantifying and qualifying the defect-reducing propensities of different dopants with experimental studies is essential for a deeper understanding and unraveling insights, such as the dynamics of iodine vacancies and the photochemistry of the iodine interstitials, and can eventually lead to a more rational approach in the search for optimal photovoltaic materials.

An investigation on structures and strains of gas-ion-implanted and post-implantation-annealed SiC

Silicon carbide (SiC) single crystals were implanted by 5.3-MeV Kr ion, 2.3-MeV Ne ion, and 100-keV He ion with the fluence of 2.0 × 1014, 3.75 × 1015, 3.0 × 1016 ions/cm2, respectively. After ion implantation, SiC specimens were annealed at different temperatures in a high vacuum. The HRXRD measurements were used to characterize the structure damage, strain generation and their recovery processes. HRXRD results reveal that the full width at half maximum (FWHM) of the main diffraction peak broadens and new diffraction peaks appear at lower angles besides the main diffraction peak due to lattice expansions and strain occurrences caused by the implantation. Meanwhile, with increasing annealing temperature, the lattice recovery and strain relaxation occur gradually and several stages of the strain release are presented. Moreover, based on the Arrhenius model, the thermal relaxation activated energy of strains at different temperature ranges was estimated. The mechanisms on strain releases were discussed in detail.

An In-Situ Synchrotron Diffraction Study of Stress Relaxation in Titanium: Effect of Temperature and Oxygen on Cold Dwell Fatigue

There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. To better understand the material characteristics that control or exacerbate this behaviour, evaluation of the time dependent plasticity of the main prismatic and basal slip systems is critical. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem. In this work, characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75 oC, 145 oC and 250 oC) was performed during stress relaxation using synchrotron X-ray diffraction. Key parameters that govern the dislocation motion were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses. From the temperatures assessed, 75 oC is found to be the worse-case scenario, where the macroscopic plastic strain accumulation was the significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems. As temperature increases, the contribution of thermal energy becomes greater than mechanical energy for dislocation glide. Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses. This effect becomes more significant in higher oxygen content commercially pure Ti.

Cold Creep of Titanium: Analysis of Stress Relaxation Using Synchrotron Diffraction and Crystal Plasticity Simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in 'soft' grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring 'hard' grain poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was used to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shift from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. Critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slips modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τc basal=252MPa and τcprism=154MPa) and activation energy (∆F basal=10.5×1020 J=0.65eV and ∆F prism=9.0×1020 J=0.56eV). The prism slip parameters correspond to stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate will have a significant effect on stress redistribution to hard grain orientations during cold dwell fatigue.

Ferroelastic domain motion by pulsed electric field in (111)/(111¯) rhombohedral epitaxial Pb(Zr0.65Ti0.35)O3 thin films: Fast switching and relaxation

Reversible electric-field induced domain switching in ferroelectric thin films gives rise to a large electromechanical coupling. Despite extensive in situ studies confirming a dominant contribution from domain switching, the speed of the domain wall motion had not been discussed enough. In this study, we performed time-resolved measurement of lattice elongation and non-180\ifmmode^\circ\else\textdegree\fi{} domain switching for an epitaxial rhombohedral $(111)/(11\overline{1})$-oriented ($\mathrm{Pb}(\mathrm{Z}{\mathrm{r}}_{0.65}\mathrm{T}{\mathrm{i}}_{0.35}){\mathrm{O}}_{3}$ film under nanosecond electric field pulses by means of synchrotron x-ray diffraction. Both lattice elongation and non-180\ifmmode^\circ\else\textdegree\fi{} domain switching due to a 200-ns electric pulse were directly observed from the shift of the 222 diffraction position toward a lower angle and the change in the integrated intensity ratio of 222 to $22\overline{2}$ peaks, respectively. The non-180\ifmmode^\circ\else\textdegree\fi{} domain switching also results in an increase of the switchable polarization. Following the removal of the electric field, it is seen that the non-180\ifmmode^\circ\else\textdegree\fi{} domain back switching from 222 to $22\overline{2}$ is sluggish compared to the relaxation of the field-induced lattice strain. This is different from the (100)/(001)-oriented tetragonal epitaxial $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ films, in which no obvious delay was detected. These results show the importance of the direct time-resolved response observation of the crystal structure change with the application of a high-speed electric pulse field to understand the frequency dispersion of the ferroelectric and piezoelectric responses of $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ films.

Relationship between Lattice Strain and Efficiency for Sn-Perovskite Solar Cells.

In the composition of Q0.1(FA0.75MA0.25)0.9SnI3, Q is replaced with Na+, K+, Cs+, ethylammonium+ (EA+), and butylammonium+ (BA+), respectively, and the relationship between actually measured lattice strain and photovoltaic performances is discussed. The lattice strain evaluated by the Williamson-hall plot of X-ray diffraction data decreased as the tolerance factor was close to one. The efficiency of the Sn-perovskite solar cell was enhanced as the lattice strain decreased. Among them, EA0.1(FA0.75MA0.25)0.9SnI3 having lowest lattice strain gave the best result of 5.41%. Because the carrier mobility increased with a decrease in the lattice strain, these lattice strains would disturb carrier mobility and decrease the solar cell efficiency. Finally, the results that the efficiency of the SnGe-perovskite solar cells was gradually enhanced from 6.42 to 7.60% during storage, was explained by the lattice strain relaxation during the storage.

Growth temperature and relaxation of lattice strain in epitaxial Pt films exhibiting diffraction fringes

We fabricated Pt epitaxial films on SrTiO3(111) using a dc-sputtering method. The surface structure, the surface morphology, and the electrochemical properties are discussed. Reflection high-energy electron diffraction (RHEED) observation reveals that epitaxial films are obtained at substrate temperatures higher than 450 °C. The RHEED patterns exhibit clear streaks up to the 1st order Laue zone, which indicates the high crystallinity and the atomically smooth surface. Oscillation in the intensity of the X-ray diffraction pattern is observed near the Pt-111 peak. It is caused by multiple scattering, showing the perfection in the lattice plane stacking along the out-of-plane direction. Analysis using a dynamic theory of diffraction shows that the film is subjected to a compressive in-plane lattice strain. The in-plane lattice parameter decreases by approximately 0.3%. The oscillation amplitude decreases at 600 °C, which suggests that the lattice relaxation occurs caused by misfit dislocation.

Effect of Varying Co Content in PtCo Nanoparticles on the Performance of PEM Fuel Cells

PtCo alloy catalysts are leading cathode candidates for PEMFC because of their relatively high activity and durability. Although it is known that during fuel cell operation the dissolution of Co results in loss of oxygen reduction reaction activity and that the migration of Co2+ into the membrane can have a negative impact on fuel cell high-current-density performance, the detailed mechanisms are unclear. Thus there are opportunities for PtCo alloy optimization to minimize these losses. Here, we focused on dealloyed PtCo catalysts with varying degrees of acid leaching in an attempt to vary the Pt-rich shell content while maintaining the Pt-to-Co core composition. While the specific activity of the catalyst decreased with reducing Co content, as expected, the high current fuel cell performance improved. Post-mortem analyses were done to quantify the amount of Co2+ in the membrane. Mathematical modelling was used to help interpret the voltage losses at high current density. We also employed newly developed nanobeam electron diffraction techniques to measure local crystal structure across the PtCo core-shell nanoparticles at subnanometer resolution. This allows us to correlate lattice strain relaxation and defect sites to the apparent catalyst kinetic activity. This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Cations substitution tuning phase stability in hybrid perovskite single crystals by strain relaxation.

Methylammonium (MA) and formamidinium (FA) are two typical A site cations in lead halide perovskites. Instability of synthesised crystals will degrade the properties of the photoelectrical device constructed by such perovskites. MAPbI3 and FAPbI3 in cubic crystal structure have been demonstrated to be the most stable at room temperature. Herein we synthesised MA(EA)PbI3 and FA(MA)PbI3 single crystals using an inverse-temperature crystallization strategy by partially substituting the methylammonium (MA) with ethylammonium (EA) and the formamidinium (FA) with methylammonium (MA) respectively. The XRD results show that both crystal structures are cubic, which means organic incorporation can stabilize the crystal structure of lead halide perovskites. The lattice distortion decrease and strain relaxation in single crystals were considered to be the reason leading to higher stability. The single crystals of MA(EA)PbI3 and FA(MA)PbI3 with low trap state density exhibit excellent light-absorbing properties, indicating their potential applications in photoelectric devices.

Effect of long-term thermal aging on magnetic hysteresis for low-alloy pressure vessel steel

Long-term thermal aging experiments have been performed at 290 and 550°C up to ∼ 10000 h for low-alloy pressure vessel steels with low and high Ni contents, in order to elucidate effects of thermal embrittlement on a structure-sensitive property of minor B-H loops. While a minor-loop property obtained from a power-law scaling of minor-loop parameters exhibits a small but steady increase after aging at 290°C, that exhibits a decreasing trend after aging at 550°C. The observations suggest that the evolution of nanometer precipitates plays a crucial role for a magnetic property change at 290°C, while relaxation of lattice strain in a matrix associated with precipitation may dominate at 550°C.

Thermal Evolution of Implantation Damages in Mg-Implanted GaN Layers Grown on Si

Localized p-doping is a keystone in the development of many GaN based devices that need selectively doped pocket area such as MOSFETs or PIN diodes. Electrical activation of implanted Mg has been obtained lately with GaN grown on sapphire substrate using an AlN based protective layer combined with multi cycle of rapid thermal annealing at very high temperature (>1300 °C) proving the feasibility and interest of the method [1]. The use of Si (111) substrate allows a better integration of the process on microelectronic production line. However the growth of GaN on Si (111) is less mature and necessitates a buffer usually composed of a complex staking based on AlN intermediate layers [2]. The difference in lattice parameters and thermal extension coefficient in the stacking may make the samples more unstable during the high thermal budget required for doping activation. In the last few years, several groups focused their research on the prevention of GaN surface degradation at high temperature [3] and we have shown that a combination of in situ epi deposited AlN and SiN thin layers is very efficient to avoid this phenomena [4]. Here, we propose to study the impact of high thermal annealing on the evolution of the damage in Mg-implanted GaN (on Si). Indeed, comprehensive studies of the GaN healing mechanism in this kind of structures are not yet reported. We use samples of GaN layer grown by MOCVD on Si (111) using an AlN based buffer layer and protected by an AlN/SiNx cap layer. These samples are implanted with Mg fluences ranging from 1013 to 1015 at/cm² corresponding to maximum effective concentration between 5.1017 and 5.1019 at/cm3 in the implanted layer. 400 °C -1100 °C anneals are proceeded in controlled N2 atmosphere. X-Ray Diffraction (XRD) and Photoluminescence (PL) measurements point out that ion implantation induced damage are healed thanks to different mechanisms: both lattice strain relaxation and point defect annihilation can occur during annealing with different kinetics. Indeed as we can see on Fig.1a, on samples implanted with low to medium fluence (up to 1014 at/cm²), relatively low temperature anneals (< 450 °C) are enough to obtain a near complete correction of the lattice deformation. However, surprisingly, the PL spectra of 400 °C annealed samples shown on Fig1.b indicates that a lot of non-radiative defects are still present and shutting down the PL intensity. The spectra of the sample annealed at 1100°C exhibits bands characteristic of Mg in Ga site (Mg(Ga)) which proves the necessity of a higher thermal treatment to reduce non radiative defects concentration and promote Mg incorporation in the GaN lattice. As shown on Fig 2.a, an increase of the fluence (1015 at/cm²) leads to an increase of the induced strain but also of the temperature necessary to fully relax it (1100°C). However, the PL spectra (Fig 2.b) indicates that even after such a high thermal treatment, the PL intensity of the bands associated with Mg(Ga) is weaker than for medium fluence despite the higher Mg concentration in the matrix [4]. We assume that this behavior may be due to residual damage. This is supported by Transmission Electron Microscopy images of the layer (not shown here) that evidence remaining defective pockets after a 1 h 1100 °C in N2 annealing. These results give insight on the healing mechanism occurring during the activation anneal of Mg implanted in GaN. They also highlight that damage management is a critical parameter for GaN (on Si) p-doping via Mg implantation. To reach this objective, we will show that it is necessary to implement further specific processes to limit the implantation induced damage and to improve the damage recovery efficiency. 1. Tadjer, M.J., et al., Selective p-type Doping of GaN:Si by Mg Ion Implantation and Multicycle Rapid Thermal Annealing. ECS Journal of Solid State Science and Technology, 2015. 5(2): p. P124-P127. 2. Charles, M., et al., The effect of AlN nucleation temperature on inverted pyramid defects in GaN layers grown on 200 mm silicon wafers. Journal of Crystal Growth, 2017. 464: p. 164-167. 3. El-Zammar, G., et al., Surface state of GaN after rapid-thermal-annealing using AlN cap-layer. Applied Surface Science, 2015. 355: p. 1044-1050. 4. Lardeau-Falcy, A., et al., Capping stability of Mg-implanted GaN layers grown on silicon. physica status solidi (a), 2016. Figure 1

Structure, morphology and photoluminescence attributes of Al/Ga co-doped ZnO nanofilms: Role of annealing time

The influence of annealing time on the structure, morphology and photoluminescence behavior (Al)/(Ga) co-doped ZnO (AGZO). nanofilms are grown on the p-type Si(100) substrate via combined sol-gel, spin coating annealed in air at 500°C at 0–3h. Samples are characterized using XRD, TEM, AFM, FESEM, EDX, (PL) and Raman measurements. XRD pattern confirmed the growth of highly poly-crystalline hexagonal wurtzite structure of ZnO with preferred orientation along (101) direction. At (3h) is found to cause lattice contraction and strain relaxation. TEM images revealed the nucleation of nanoparticles (NPs) and SAED pattern identified the lattice parameter. Raman spectra of AGZO exhibited optical and acoustic modes. FESEM displayed an increase in the particles size and number of nanoflakes with increasing annealing time. EDX detected right elemental traces. PL revealed an intense emission peak centered at 3.23eV, which is continuously shifted toward lower frequency with increasing time.

X-ray diffraction study of laser-driven solid-state diffusional mixing and new phase formation in Ni-Pt multilayers

An in situ optical pump and x-ray probe technique has been utilized to study photoinitiated solid-state diffusion in a Ni-Pt multilayer system. Hard x-ray diffraction has been used to follow the systematic growth of the NiPt alloy as a function of laser intensity and total energy deposited. It is observed that new phase growth can be driven in as little as one laser pulse, and that repeated photoexcitation can completely convert the entire multilayer structure into a single metallic alloy. The data suggest that lattice strain relaxation takes place prior to atomic diffusion and the formation of a NiPt alloy.

CADEM: calculate X-ray diffraction of epitaxial multilayers.

Epitaxial multilayers and superlattice (SL) structures are gaining increasing importance as they offer the opportunity to create artificial crystals with new functionalities. These crystals deviate from the parent bulk compounds not only in terms of the lattice constants but also in the symmetry classification, which renders calculation of their X-ray diffraction (XRD) patterns tedious. Nevertheless, XRD is essential to get information on the multilayer/SL structure such as, for example,out-of-plane lattice constants, strain relaxation and period length of the crystalline SL. This article presents a powerful yet simple program, based on the general one-dimensional kinematic X-ray diffraction theory, which calculates the XRD patterns of tailor-made multilayers and thus enables quantitative comparison of measured and calculated XRD data. As the multilayers are constructed layer by layer, the final material stack can be entirely arbitrary. Moreover, CADEM is very flexible and can be straightforwardly adapted to any material system. The source code of CADEM is available as supporting material for this article.

In-situ neutron diffraction during stress relaxation of a single crystal nickel-base superalloy

The stress relaxation behaviour of a single crystal nickel-base superalloy has been quantified using time-of-flight neutron diffraction analysis for a range of temperatures relevant to casting. A new iterative analysis methodology is described to isolate the lattice strain behaviour of the γ matrix and γ′ precipitate phases from data obtained sufficiently rapidly to help elucidate the microscopic effect of macroscopic stress relaxation. The independent response of γ and γ′ is revealed, showing the temperature sensitivity of lattice strain relaxation. The γ/γ′ response is discussed in the context of thermo-mechanical conditions that may affect the propensity for recrystallisation.