Multi-beam Optical Stress Sensor Research Articles

A Comprehensive Review of In Situ Measurement Techniques for Evaluating the Electro-Chemo-Mechanical Behaviors of Battery Electrodes.

The global production landscape exhibits a substantial need for efficient and clean energy. Enhancing and advancing energy storage systems are a crucial avenue to optimize energy utilization and mitigate costs. Lithium batteries are the most effective and impressive energy utilization system at present, with good safety, high energy density, excellent cycle performance, and other advantages, occupying most of the market. However, due to the defects in the electrode material of the battery itself, the electrode will undergo the process of expansion, stress evolution, and electrode damage during electro-chemical cycling, which will degrade battery performance. Therefore, the detection of property changes in the electrode during electro-chemical cycling, such as the evolution of stress and the modulus change, are useful for preventing the degradation of lithium-ion batteries. This review presents a current overview of measurement systems applied to the performance detection of batteries' electrodes, including the multi-beam optical stress sensor (MOSS) measurement system, the digital image correlation (DIC) measurement system, and the bending curvature measurement system (BCMS), which aims to highlight the measurement principles and advantages of the different systems, summarizes a part of the research methods by using each system, and discusses an effective way to improve the battery performance.

Снижение плотности прорастающих дислокаций в темплейтах AlN/c-сапфир, выращенных методом плазменно-активированной молекулярно-пучковой эпитаксии

Multiple-crystal X-ray diffraction and a multi-beam optical stress sensor were used to study AlN/c-sapphire templates grown by plasma-assisted molecular beam epitaxy. The influence of the nucleation and buffer layers growth regimes, temperature, the ratio between Al and N* growth fluxes on the stress generation and the character of the dislocation structure were analyzed. Templates with the best crystal quality with screw and edge threading dislocation densities in a range of 4∙10^8 and 8∙10^9 cm-2, respectively, were obtained at the flux ratio of Al to N* close to 1 by using two-stage temperature regimes.

Opto-electrical properties and internal stress in Al:ZnO thin films deposited by direct current reactive sputtering

In this work we study the effect of the oxygen concentration, deposition temperature, and post-deposition annealing on the optical and electrical properties of Al:ZnO thin films deposited by direct current (DC) reactive sputtering. The internal stress in the thin film is also monitored in-situ during growth with a multi-beam optical stress sensor. This latter technique allows to have an insight on the growth mechanisms. Coupled with an analysis of the discharge curve, the optical transparency, the electrical conductivity, and the chemical composition of the films as a function of the oxygen concentration and temperature during deposition, it allows to estimate what are the optimal deposition conditions to minimize the resistivity and maximize the transparency. The analysis of the internal stress is also of interest because it can be a source of premature fracture or delamination of thin films. The internal stress can also change the intrinsic properties of a thin film. The influence of the stress on the resistivity is therefore studied for samples deposited in the different sputtering regimes.

Suppression of Stark effect in ultra-thin stress-free GaN/AlN multiple quantum well structures grown by plasma-assisted molecular beam epitaxy

We report on suppression of the Stark effect in (1.5-2)-monolayer(ML)-thick (GaN/AlN)100 multiple quantum well (MQW) structures grown on AlN/c-Al2O3 templates by plasma-assisted molecular beam epitaxy. Different stress relaxation mechanisms are revealed in these structures by using a multi-beam optical stress sensor in comparison with the 5ML-MQW structure. The former (with well thicknesses ⩽2MLs) demonstrate the nearly stress-free growth, whereas the latter structure with thicker wells exhibits the slow stress evolution from the high initial compressive stress to the nearly relaxed state with zero stress. Moreover, the former structures demonstrate a bright room-temperature cathodoluminescence (CL) with the single peak at the shortest wavelength 240 nm (1.5ML-QWs), while the latter shows much weaker multi-peak CL spectra in the spectral range of 270-360 nm.

Development of residual stress and uniaxial magnetic anisotropy during growth of polycrystalline Co film

Growth of e-beam evaporated Co films on Si (001)/SiO2 substrate has been studied using real-time multi-beam optical stress sensor (MOSS), four probe resistivity (FPR) and in-situ magneto-optic Kerr effect (MOKE) measurements. While MOSS and FPR provide information about internal stress and film morphology, MOKE provides information about magnetic properties, thus making it possible to correlate the evolution of film stress and morphology with that of uniaxial magnetic anisotropy (UMA) in the film. Film grows via Volmer–Weber mechanism, where islands grow larger to impinge with other islands and eventually coalesce into a continuous film at around 10 nm thickness. Development of tensile stress in the film is found to be associated with the island coalescence process. The film exhibits a well-defined UMA, which varies synchronously with internal tensile stress developed during growth, suggesting that the observed UMA originates from minimization of magnetoelastic and morphological anisotropic energies in the presence of internal stress. Absence of UMA in Co film having Ag underlayer in the form of islands is understood in terms of random short-range stress in the film caused by arbitrary pinning by the Ag islands.

(Invited) Surface Contaminant Effects on the Properties of Pr-Doped Ceria Determined Via Wafer Curvature and X-Ray Diffraction Measurements

Praseodymium doped ceria (PCO) is a model mixed ionic electronic conductor (MIEC) solid oxide fuel cell (SOFC) cathode material. Under SOFC operating conditions, PCO undergoes the reaction (1): ½ O2 + 2Pr’ Ce + VӦ = 2PrCe + Oo x The kinetics of this reaction can be represented by the chemical oxygen surface exchange coefficient (kchem). It has been reported that surface contaminants can affect the kinetics of this reaction, either boosting or deteriorating the oxygen exchange performance of MIECs (2, 3). Hence, in this work, the effect of surface contaminants on the Pr0.1Ce0.9O2-δ (10PCO) thin film properties were investigated. First, YSZ (yttria doped zirconia) or MgO (magnesium oxide) single crystal substrates were coated with (100)-oriented >21 nm average grain sized 10PCO thin films containing various surface contaminants via Pulsed Laser Deposition. The stress-temperature behavior of these samples was then measured using a dual substrate wafer curvature measurement technique utilizing a multi-beam optical stress sensor (MOSS) to determine the 280-700oC in-plane 10PCO thermal-chemical expansion coefficient, Young’s modulus, and oxygen nonstoichiometry, as described in Ref. (4). The curvature relaxation of these samples reacting to abrupt changes in the surrounding oxygen partial pressure from synthetic air (20% O2-80% Ar) and 10 times diluted synthetic air (2% O2-98% Ar), and vice-versa, were measured to determine the high temperature 10PCO kchem. Finally, the 500-700oC films strains in the 10PCO|YSZ samples were analyzed by measuring the 10PCO (200) peak position via high temperature X-Ray diffraction (HTXRD) and compared to the film stresses measured via MOSS to determine the Young’s modulus and thermal-chemical expansion coefficient (assuming no difference between the in-plane and out-of-plane values). The close agreement of the data in Figures 1a and 1b show that the in-plane and out-of-plane bulk properties measured here were identical to each other, within the measurement errors, and similar to some of the previous values reported for bulk 10PCO. Further, no difference in bulk properties were observed for films with various surface contaminants. However, as shown in Figure 2, Si surface impurities significantly reduced kchem while noble metal surface impurities likely increased it. These results 1) will be important for evaluating the real-world tolerance of 10PCO in real-world SOFCs, 2) highlight the benefit of using electrode-free kchem measurement techniques, and 3) help explain the large kchem variation seen in the literature for identical materials at identical conditions.

Correlation between the wafer curvature and fluorescence of pulsed laser deposited ruby thin films stressed to ∼2 GPa

Here, the room temperature piezospectroscopic response of highly-fluorescent, ∼330 nm-thick pulsed laser deposited crystalline ruby (0.05 wt. % Cr3+ doped α-Al2O3) thin films on either (001)-oriented sapphire or (001)-oriented yttria-stabilized zirconia wafers was investigated and calibrated against biaxial film stress measurements obtained from a multibeam optical stress sensor or profilometry-determined wafer curvature measurements. The piezospectroscopic frequency shift from 0 to 1.9 GPa of compressive biaxial stress for the phase-pure (001)-oriented ruby films produced here had the same piezospectroscopic Π11 and Π22 tensor coefficient values as bulk ruby over its previously calibrated 0–0.9 GPa range. This extended calibration may be useful when using ruby to measure the amount of biaxial stress in a variety of multilayer devices and coatings.

Tuning high power impulse magnetron sputtering discharge and substrate bias conditions to reduce the intrinsic stress of TiN thin films

Ion bombardment during film growth usually induces high compressive stress in compound thin film materials, resulting in rupture and failure of coated tools used in tribological applications. Hence‚ intrinsic stress generated during film growth can drastically limit the industrial appeal of deposition technologies such as high power impulse magnetron sputtering (HiPIMS). This work investigates how to reduce high stress levels by tuning the HiPIMS discharge conditions and selecting the appropriate substrate bias configuration. The strategy is based on optimizing the process discharge parameters, leading to HiPIMS discharges containing fewer multiply charged energetic metal ions, which is combined with pulsed substrate bias synchronized to the HiPIMS pulse to control the chemical nature of the incident ions‚ i.e.‚ inert gas vs. metal ions. The study was performed during growth of TiN thin films, due to their relevance as a protective coating, and the intrinsic stress was measured in situ during film growth using the wafer curvature method and a multi-beam optical stress sensor. The results show that for standard HiPIMS discharges and biased substrates‚ the energetic metal ion bombardment results in very dense, compact microstructures, but highly stressed TiN films. On the other hand‚ when using the here proposed strategy mentioned above‚ the compressive stress was considerably reduced (by a factor 11) while retaining rather compact microstructures compared to direct current magnetron sputtered as well as non-biased HiPIMS samples.

GaN Heteroepitaxy on Strain-Engineered (111) Si/Si1−xGex

The metalorganic chemical vapor deposition growth of GaN on strained Si/Si1−xGex epilayers on (111) Si substrates was investigated. A multi-beam optical stress sensor (MOSS) was used for in situ stress measurements during growth of the entire heterostructure. MOSS was initially used to measure the extent of stress relaxation during the growth of constant-composition Si1−xGex layers on Si. The results compared favorably to that obtained by post-growth high-resolution x-ray diffraction. MOSS was also used to monitor stress during the growth of thin, tensile-strained Si on relaxed Si0.95Ge0.05/compositionally graded epilayers. The tensile-strained Si/Si1−xGex epilayers were then used as virtual substrates for the growth of GaN epilayers using a thin (90 nm) AlN buffer layer. GaN grown on tensile-strained Si exhibited a higher initial compressive stress and reduced final tensile stress compared to films grown directly on (111) Si, resulting in a lower crack density in the GaN along with a reduced density of threading dislocations. These results suggest that strain engineering of the Si surface prior to growth may provide an alternative method to improve the quality of GaN grown on (111) Si.

Using Mechano-Electro-Chemical Coupling to Measure the Thin Film Elastic Constants, Thermo-Chemical Expansion Coefficients, and Oxygen Surface Exchange Coefficients of Praseodymium Doped Ceria Presentation Format: Oral Preferred

A Multi-beam Optical Stress Sensor (MOSS) is a curvature measurement platform which is commonly used to measure the film stress in bilayer samples. It has been widely used as an in-situ technique to measure the film stress during deposition.1 However, when combined with the dual substrate method proposed by Zhao et al,2 in situ curvature measurements can be used to measure Young’s moduli and thermo-chemical expansion coefficients simultaneously as a function of temperature. Using the curvature relaxation (κR) technique developed recently,3-5 oxygen surface exchange coefficients (kchem) can also be measured as a function of temperature using in situ curvature measurements. In this work, the Young’s moduli, thermo-chemical expansion coefficients and kchem values of praseodymium doped ceria (PCO) were measured as a function of temperature using a MOSS. First, phase pure Pr0.1Ce0.9O1.95 (PCO) powder was prepared through glycine nitrate combustion and subsequent calcination at 1100oC in air. This powder was then pressed in a stainless-steel die and fired to 1450oC to produce a pulsed laser deposition (PLD) target. In preparation for PLD, (001) oriented 9.5% yttria doped zirconia (YSZ) and (001) oriented magnesium oxide (MgO) substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours to remove residual stress within them. PCO PLD was then conducted at 680oC for 20 min, with a 10-2 torr oxygen partial pressure and 350 mJ power density. After deposition, the PCO bilayers were re-equilibrated with air by firing them in air at 1000oC for 1 hour. For dual substrates measurements, stress vs. temperature data for PCO|YSZ and PCO|MgO were collected with a 1oC/min heating rate and a 0.2oC/min cooling rate. The slopes of the stress vs. temperature curves can be expressed by: dσPCO|YSZ/dT = MPCO(αYSZ-αPCO) (1) dσPCO|MgO/dT = MPCO(αMgO-αPCO) (2) where is the stress of bilayer sample, T is the temperature, M is the biaxial modulus of the film, is the thermo-chemical expansion coefficient. With two unknowns and two equations, and were then extracted as a function of temperature. The Young’s moduli were then calculated from assuming a Poisson’s ratio of 0.33 as has been done previously for 6. For κR measurements, relaxation data were recorded at 650~725oC with 25oC increments. The oxygen partial pressure was switched between synthetic air (20%O2-80%Ar) and 10% diluted synthetic air (10% synthetic air-90%Ar). Figure 1 shows the Young’s modulus and thermo-chemical expansion coefficients measured here compared to other literature studies.6-8 In contrast to other studies the present study produced PCO Young’s moduli over a complete range of temperatures. In addition, the PCO Young’s moduli started to decrease significantly once the PCO started to become nonstoichiometric (as indicated by an uptick in chemical expansion in Figure 1b). The PCO kchem values (not shown) were in good agreement with the kchem values measured by other electrode-free techniques, such as optical relaxation.9 Figure 1

Asymmetric Intermixing and the Stress Buildup in Ni/Al-typed Nanomultilayer with Different Characteristic Scales

In order to detect scale-dependent interfacial evolution of metallic heterostructure during the deposition at room temperature, Ni/Al-typed nanomultilayers were prepared as a function of the periodicity and Ni:Al modulated ratio. Combined with X-ray diffraction, real-time plate curvature measurements by multi-beam optical stress sensor (MOSS) were employed to study the stress buildup so as to speculate interfacial characteristics during the growth process. Results show that with anisotropic nanocrystalline structure within the sub-layers, the multilayers possess asymmetrical interfaces, which is a result of dissymmetrical diffusion of Ni to Al lattice near the interface. Specially, for the smallest periodicity with the lowest Ni:Al ratio, above asymmetric intermixing behaviors turns to be aggravated by a promotion effect.

Stress in (Al, Ga)N heterostructures grown on 6H-SiC and Si substrates byplasma-assisted molecular beam epitaxy

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total)threading dislocation (TD) densities down to 1.7·108 and 2·109 cm-2, respectively, in a 2.8-μm-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2·109 and 7.4·109 cm-2, respectively, were achieved in a 1-μm-thickGaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It is demonstrated that a 1-μm-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-μm-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.

In Situ Stress, Elastic Constant, Oxygen Surface Exchange Coefficient, and Thermo-Chemical Expansion Coefficient Measurements on Praseodymium Doped Ceria Thin Films

A Multi-Beam Optical Stress Sensor (MOSS) provides a contact-free, in-situ platform to measure thin film stress based on the curvature of a bilayer sample. Originally, this instrument was used to measure thin film stress during deposition. However, when combined with a specially designed chemical reactor, it can be used to measure the oxygen surface exchange coefficient of thin film materials [1,2, 3]. In this work, the oxygen exchange coefficient (kchem), stress, biaxial modulus, thermal expansion coefficient (CTE) and chemical expansion coefficient (CCE) were measured as a function of temperature via the wafer curvature technique. Phase pure Pr0.1Ce0.9O1.95 (PCO) powder was prepared through glycine nitrate combustion and subsequent calcination at 1100oC in air. This powder was then pressed in a stainless steel die and fired to 1450oC to produce a pulsed laser deposition (PLD target). In preparation for PLD, (001) oriented 9.5% yttria doped zirconia (YSZ) and (001) oriented magnesium oxide (MgO) substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours to remove residual stress within them. PCO PLD was then conducted at 680oC for 30 min, with a 10-5 torr oxygen partial pressure. After deposition, the PCO bilayers were re-equilibrated with air by firing them in air at 1000oC for 1 hour. To measure kchem, a solution of Fick’s second law was fit to the bilayer curvature response of PCO|YSZ samples responding to abrupt oxygen partial pressure changes (0.21 – 0.021), as shown in Fig. 1 (a). Then the data was fitted to to get kchem values. To measure the biaxial modulus, CTE and CCE, curvature values for PCO|YSZ and PCO|MgO were measured from 25oC to 700oC with a 0.5oC/min ramping rate, converted to stress using Stoney’s equation, and determined using the dual substrate method [4]. As shown in Fig. 1 (b), the PCO thin film kchem values measured here were comparable with other literature PCO thin film kchem values measured using electrode-free techniques such optical relaxation [5, 6]. However, the PCO kchem values measured here were 1-2 orders lower than those measured via impedance spectroscopy (which uses electrodes) [5, 7, 8]. Additional experiments to measure the PCO biaxial modulus, CTE and CCE are underway. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.

In-situ curvature monitoring and X-ray diffraction study of InGaAsP/InGaP quantum wells

The use of InGaAsP/InGaP quantum well structures is a promising approach for subcells in next generation multi-junction devices due to their tunable bandgap (1.50–1.80eV) and for being aluminum-free. Despite these potentials, the accumulation of stress during the growth of these structures and high background doping in the quantum well region have previously limited the maximum number of quantum wells and barriers that can be included in the intrinsic region and the sub-bandgap external quantum efficiency to less than 30.0%. In this paper, we report on the use of in-situ curvature monitoring by multi-beam optical stress (MOS) sensor measurements during the growth of this quantum well structure to monitor the stress evolution in these thin films. A series of In0.32Ga0.68AsP/In0.49Ga0.51P quantum wells with various arsine to phosphine ratios have been analyzed by in-situ curvature monitoring and X-ray diffraction (XRD) to obtain nearly strain-free lattice matched structures. Sharp interfaces, as indicated by the XRD fringes, have been achieved by using triethyl-gallium and trimethyl-gallium as gallium precursors in InGaAsP and InGaP, respectively, with constant flows of trimethyl-indium and phosphine through the entire quantum well structure. The effect of the substrate miscut on quantum well growth was compared and analyzed using XRD, photoluminescence and time resolved photoluminescence. A 100 period quantum well device was successfully grown with minimal stress and approximately flat in-situ curvature.

In Situ Stress, Elastic Constant, Oxygen Surface Exchange Coefficient, and Thermo-Chemical Expansion Coefficient Measurements on Praseodymium Doped Ceria Thin Films

A Multi-beam Optical Stress Sensor provides a noncontact, in situ method for measuring thin film stress based on the curvature of a multilayer sample. Originally, this technique was used to measure film stresses during thin film deposition. However, as demonstrated here using praseodymium doped ceria, when combined with an optically accessible tube furnace, wafer curvature measurements can be used as an in situ, electrode-free platform for measuring film stress, biaxial modulus, thermal expansion coefficient, chemical expansion coefficient, and oxygen surface exchange coefficient.

In-Situ Monitoring of Stress Developments during Electrochemical Lithiation/Delithiation of Amorphous Silicon Films in Presence/Absence of ‘Buffer’ Interlayers

The combination of greater theoretical Li-capacity (~4200 mAhg-1) and improved safety aspects with respect to graphitic carbon renders silicon a potential candidate for replacement of the presently used graphitic carbon based anode materials in Li-ion batteries. However, enormous volume changes that occur during electrochemical cycling (i.e., Li-insertion/removal) lead to severe stress developments, resulting in mechanical degradation of the Si-based anode materials (cracking/fracture/delamination/loss of electric contacts/and continued SEI formation) and concomitantly drastic fade in capacity [1]. It is widely believed that use of ‘buffer’ interlayers may reduce the stresses or at least the impact of such stresses [1-3]. However, the literature base lacked direct experimental investigation/assessment of the stress developments during lithiation/delithiation of Si in the presence/absence of such ‘buffer’ interlayers. Against this backdrop, we have monitored the stress developments in-situ during electrochemical cycling of continuous amorphous Si (a-Si) films, in the presence and absence of suitable ‘buffer’ interlayers between the active electrode film and the current collector. The effects of presence of such interlayers on the mechanical integrity during the course of lithiation/delithiation cycles have also been thoroughly monitored. Such comprehensive experimental work, along with in-depth analysis, has led to better understandings of the mechanistic aspects related to the effects of the presence of ‘buffer’ interlayers and also the interfacial properties. In the present work, two different types of ‘buffer’ interlayers, viz. CVD-grown well-ordered multi-layered graphene (MLG; ~9-10 layers) and NiTi shape memory alloy (SMA; grown via PLD; 150 nm thick), have been used (sandwiched between ~250 nm thick a-Si and ~100 nm thick Ni current collector films). While interlayer sliding of the individual graphene layers may be expected in the case of MLG in response to the expansion/contraction of the a-Si, NiTi SMA is expected to exhibit pseudo-elasticity (viz., reversible stress induced phase transformation; B2 <=> B19’) [3]. The multi-layered film electrodes, on stiff circular quartz substrates (~500 µm thick) were assembled in custom-made electrochemical cell and cycled galvanostatically against Li-metal (counter/reference electrode). Multi-beam optical stress sensor was used for real-time monitoring of the stress developments in the film electrodes, similar to our previously reported works [1,4-8]. The presence of both the buffer interlayers improved not only the cyclic stability, but also the Li-capacity possibly because of reducing the magnitude of compressive stresses developed during lithiation. In addition to reducing the stress magnitudes (when normalized by the corresponding Li-capacities), the stress profiles recorded in real-times tend to indicate that while MLG supressed the ‘flow’ of a-Si during lithiation, stress development was accommodated by preferential plastic deformation of NiTi. Top-view and cross-section SEM observations at different stages of the cycling indicated that, even though the a-Si films cracked in the absence, as well as in the presence of the buffer ‘interlayers’, the cracked ‘islands’ maintained considerably improved integrity upon further cycling in the presence of the ‘buffer’ interlayers (as compared to directly on Ni current collector). Scratch tests indicated weaker interfacial adhesion strengths in the presence of the ‘interlayers’. Along with the reduced magnitudes of the lithiation induced stresses, engineering of the adhesion strengths, in combination with the observed cracked ‘island’ sizes, concomitant suppression of flow of cracked a-Si ‘islands’ (due to weaker adhesion in the presence of the interlayers), and possible incremental plastic deformation of the Ni current collector in the absence of the ‘interlayers’, were associated with the observed stress profiles and improved mechanical integrity in presence of the ‘buffer’ interlayers. Keywords: amorphous Si, patterned film, multi-layered graphene, NiTi shape memory alloy, in-situ stress monitoring.

Calculating Anisotropic Correlated Chemical Expansion of Oxygen and Lithium Vacancies in Li2MnO3 to Determine Vacancy Concentrations

Lithium-excess layered cathode materials for lithium-ion batteries of the form Li2MnO3-LiMO2 (M=transition metal(s)) show high capacities (>200 mAhg-1) due to the activation of the Li2MnO3 component.1 This activation process occurs during the first cycle at >4.4V, where it is widely accepted that oxygen is removed from the Li2MnO3 component. Thus, Li2-XMnO3-δ is formed upon cycling, creating a structure with both oxygen vacancies (VO) and lithium vacancies (VLi). It has been shown previously that the VLi and VO interact and that it is energetically favorable for VLi to form near VO.2 VO concentration in the Li2-XMnO3-δ is believed to be dilute, although the exact amount of VO(δ) is unclear. To estimate the VO concentrations, calculations of the chemical expansion coefficient for vacancies in Li2MnO3 are combined with experimental stress measurements of a lithium-excess layered cathode material. The coupled VO and VLi have previously been shown to exhibit correlated anisotropic chemical expansion in Li2-XMnO3-δ at dilute concentrations.3 Thus, the chemical expansion coefficients for VO and VLi in the system are broken down into a dilute component for coupled VO and VLi and a component to capture the rest of the VLi which are far away from VO and contribute to the high capacity of these materials. The computational results are used to interpret stress measurements from a multibeam optical stress sensor (MOSS) on Li1.2Mn0.55Ni0.125Co0.125O2. 1. Thackeray, M. M.; et al., Journal of Materials Chemistry 2007, 17(30), 3112-3125. 2. James, C.; et al., Solid State Ionics 2016, 289, 87-94. 3. James, C.; et al., MRS Advances 2016, 1(15), 1037-1042. doi:10.1557/adv.2016.48.

Stimuli-Responsive Macromolecular Composites: Enhanced Stress Modulation in Polypyrrole with Redox-Active Dopants

Structure and conformation of conducting polymer (CP) films have been widely investigated to understand the variation in electrical, chemical, and mechanical properties in different electrochemical environments. The pH-dependent stress response in films of polypyrrole (pPy) doped with three bulky dianionic dopants (indigo carmine (IC), anthraquinone disulfonate (AQ), and naphthalene disulfonate (NP)) were studied using the multibeam optical stress sensor (MOSS) technique. Based on combined current, stress, and mass analysis, a mechanism of mixed ion dynamics is proposed that explains the electro-chemomechanical behavior of these polymers. By changing the pH of the electrolyte from neutral to acidic, a modulation of the stress response (ΔStress) of these polymers is observed, which was found to be the largest in pPy[IC] (80% decrease) compared to related structural analogues, pPy[AQ] (33%) and pPy[NP] (30%). The decrease in stress response for pPy[IC] and pPy[AQ] is attributed to the simultaneous and oppos...

AlGaN Nanostructures with Extremely High Room-Temperature Internal Quantum Efficiency of Emission Below 300 nm

We present theoretical optimization of the design of a quantum well (QW) heterostructure based on AlGaN alloys, aimed at achievement of the maximum possible internal quantum efficiency of emission in the mid-ultraviolet spectral range below 300 nm at room temperature. A sample with optimized parameters was fabricated by plasma-assisted molecular beam epitaxy using the submonolayer digital alloying technique for QW formation. High-angle annular dark-field scanning transmission electron microscopy confirmed strong compositional disordering of the thus-fabricated QW, which presumably facilitates lateral localization of charge carriers in the QW plane. Stress evolution in the heterostructure was monitored in real time during growth using a multibeam optical stress sensor intended for measurements of substrate curvature. Time-resolved photoluminescence spectroscopy confirmed that radiative recombination in the fabricated sample dominated in the whole temperature range up to 300 K. This leads to record weak temperature-induced quenching of the QW emission intensity, which at 300 K does not exceed 20% of the low-temperature value.

Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient and Film Stress Measurements Via the Curvature Relaxation Technique

Introduction Praseodymium doped ceria (PCO) is a mixed ionic electronic conductor (MIEC) that can be used as a cathode material in solid oxide fuel cells (SOFCs). Under SOFC operating conditions, PCO undergoes the following reaction [1]: ½ O2 + 2Pr’ Ce + VӦ à 2PrCe + Oo x The kinetics of this oxygen exchange reaction are represented by the oxygen surface exchange coefficient (kchem ). In this work, simultaneous kchem and stress measurements on PCO thin films atop yttria-stabilized zirconia (YSZ) substrates were performed from 625-725 oC using the curvature relaxation (κR) technique. Experimental Methods Phase pure Pr0.1Ce0.9O1.95 powder was prepared through glycine nitrate combustion and subsequent powder calcination in air. (001) oriented 9.5% YSZ substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours. PCO thin films were sputtered onto YSZ substrates at room temperature and then sintered for 1 hour at 1100oC. The bilayer structures were tested using a multi beam optical stress sensor (MOSS) from 625oC to 725oC with 25oC increments. The κR technique determines kchem by fitting a solution of Fick’s 2nd Law to the curvature response of mechano-chemically active film | inert substrate bilayers reacting to sudden changes in oxygen partial pressure (synthetic air (21% O2 - 79% Ar) and 10% synthetic air - 90% Ar) [2,3]. Film stress values were calculated using Stoney’s equation [2,3]. In order to compare the kchem values with those in the literature, electrically-measured PCO kq values from the literature [4] were turned into kchem values by multiplying them by the corresponding PCO thermodynamic factor from Ref [5]. Results Figure 1 shows how the kchem value measured via the κR technique compares with literature [4,5] values. Conclusions The PCO kchem and activation energies measured here are significantly different than those reported in the literature. Although additional work is needed before these effects can be explained in full, these discrepancies may be caused by the different synthesis techniques (sputtering vs. pulsed laser deposition) used to produce the two films shown in Figure 1. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.