Spatial Uniformity Research Articles

Sequential Molecule-Doped Hole Conductor to Achieve >23% Perovskite Solar Cells with 3000-Hour Operational Stability.

Although hole transport layers (HTLs) based on solution-processed doped Spiro-OMeTAD are extremely popular and effective for their remarkable performance in n-i-p perovskite solar cells (PSCs), their scalable application is still being held back by poor chemical stability and unsatisfied scalability. Essentially, the volatile components and hygroscopic nature of ionic salts often cause morphological deformation that deteriorate both device efficiency and stability. Herein, a simple and effective molecular implantation-assisted sequential doping (MISD) approach is strategically introduced to modulate spatial doping uniformity of organic films and fabricate all evaporated Spiro-OMeTAD layer in which phase-segregation free HTL is achieved accompanied with high molecular density, uniform doping composition, and superior optoelectronic characteristics. The resultant MISD-based devices attain a record power conversion efficiency (PCE) of 23.4%, which represents the highest reported value among all the PSCs with evaporated HTLs. Simultaneously, the unencapsulated devices realize considerably enhanced stability by maintaining over 90% of their initial PCEs in the air for 5200h and after working at maximum power point under illumination for 3000h. This method provides a facile way to fabricate robust and reliable HTLs toward developing efficient and stable perovskite solar cells.

Atomic Layer Deposition Route to Scalable, Electronic-Grade van der Waals Te Thin Films.

Scalable production and integration techniques for van der Waals (vdW) layered materials are vital for their implementation in next-generation nanoelectronics. Among available approaches, perhaps the most well-received is atomic layer deposition (ALD) due to its self-limiting layer-by-layer growth mode. However, ALD-grown vdW materials generally require high processing temperatures and/or additional postdeposition annealing steps for crystallization. Also, the collection of ALD-producible vdW materials is rather limited by the lack of a material-specific tailored process design. Here, we report the annealing-free wafer-scale growth of monoelemental vdW tellurium (Te) thin films using a rationally designed ALD process at temperatures as low as 50 °C. They exhibit exceptional homogeneity/crystallinity, precise layer controllability, and 100% step coverage, all of which are enabled by introducing a dual-function co-reactant and adopting a so-called repeating dosing technique. Electronically, vdW-coupled and mixed-dimensional vertical p-n heterojunctions with MoS2 and n-Si, respectively, are demonstrated with well-defined current rectification as well as spatial uniformity. Additionally, we showcase an ALD-Te-based threshold switching selector with fast switching time (∼40 ns), selectivity (∼104), and low Vth (∼1.3 V). This synthetic strategy allows the low-thermal-budget production of vdW semiconducting materials in a scalable fashion, thereby providing a promising approach for monolithic integration into arbitrary 3D device architectures.

Design and Development of a Diagnostic System for a Non-Intercepting Direct Measure of the SPIDER Ion Source Beamlet Current.

Stable and uniform beams with low divergence are required in particle accelerators; therefore, beyond the accelerated current, measuring the beam current spatial uniformity and stability over time is necessary to assess the beam performance, since these parameters affect the perveance and thus the beam optics. For high-power beams operating with long pulses, it is convenient to directly measure these current parameters with a non-intercepting system due to the heat management requirement. Such a system needs to be capable of operating in a vacuum in the presence of strong electromagnetic fields and overvoltages, due to electrical breakdowns in the accelerator. Finally, the measure of the beam current needs to be efficiently integrated into a pulse file with the other relevant plant parameters to allow the data analyses required for beam optimization. This paper describes the development, design and commissioning of such a non-intercepting system, the so-called beamlet current monitor (BCM), aimed to directly measure the electric current of a particle beam. In particular, the layout of the system was adapted to the SPIDER experiment, the ion source (IS) prototype of the heating neutral beam injectors (HNB) for the ITER fusion reactor. The diagnostic is suitable to provide the electric current of five beamlets from DC up to 10 MHz.

Optical properties of N2/Ar plasma generated by electron beams with silicon nitride transmission window at high pressures

In this paper, we report the generation of nitrogen/argon electron beam plasmas with a 0.4 μm-thick silicon nitride transmission window. The measured transmittance of the electron beam is about 92% when the incident energy of electron beam is 40 keV. The optical properties of the electron beam plasma are investigated at high pressures. The luminescence intensity distribution of nitrogen and argon electron beam plasmas shows a good similarity with changing the gas pressure, suggesting that electron beam ionization is independent of background gas. At the same gas pressure, the nitrogen plasma with conical structure has a longer length along the incident direction of the electron beam and a smaller divergence cone angle compared with the argon plasma. In the nitrogen discharge, it is found that the intensities I380.4, I399.7, and I391.4 corresponding to characteristic lines at 380.4, 399.7, and 391.4 nm decrease along the incident direction of the electron beam, which is correlated with the reduction of the electron density along the incident direction of the electron beam. There is no change in the ratio of I399.7 to I380.4, indicating the spatial uniformity of the electron temperature. When the pressure increases, the ionization process induced by primary electron is enhanced, generating more plasma electrons, which is responding to the increase in the characteristic line intensity. At the same time, the neutral particle collision processes are enhanced and the electron temperature decreases with the increase in the pressure, which is believed to contribute to the observed reduction of the I399.7/I380.4 ratio.

Thermal perception of non-uniform and dynamic air-conditioned environment in the elderly

Due to the decline of the elderly's thermophysiology, the elderly is more sensitive to changes in airflow. Traditional thermal sensation equation was a steady-state model only based on the average of surrounding thermal environment parameters, which cannot describe the thermal perception to the spatial uniformity and temporal fluctuation of the parameters. To characterize the thermal perception of non-uniform and dynamic air-conditioned environments in the elderly, this study determined the characteristics of thermal environment at twenty-four monitoring points (three heights and eight directions) around the elderly through laboratory test. Seven elderly participated in the experiment and were asked to fill in the questionnaire to obtain thermal perception. Different typical airflow with 26 °C set temperature of air-conditioner were set as the measuring conditions. Uniformity and fluctuation indexes were proposed to qualify the spatial and temporal distribution. The results showed that the average of the parameters was basically in the acceptable comfort zone. Through establishing the thermal sensation model based on the average, uniformity, and fluctuation indexes, thermal sensations were only significantly affected by fluctuation of temperature, velocity uniformity, turbulence, and average relative humidity. Compared to traditional model, established model can not only depict the elderly's strong perception of temperature change by temperature fluctuation index, but also refine the elderly's relatively weak perception of static average velocity into a strong perception of the velocity uniformity and turbulence. The R square of this model was 0.153 greater than that of traditional model.

The turbulent flame structure in a steam diluted H2/Air micromix flame

In order to safely organize hydrogen flame with low NOx emission in traditional gas turbines and industrial burners, an attractive combustion technology is the steam diluted micromix combustion technology. The effects of equivalence ratio (φ), steam dilution ratio (D) and nozzle diameter (d) on the turbulent flame structure in a steam diluted H2/air micromix flame were investigated experimentally with a 10 kHz high-speed OH∗ chemiluminescence imaging system to detect the instantaneous flame zone. With the increase of equivalence ratio (φ) or the decrease of steam dilution ratio (D), the overall OH∗ signal intensity and the area of OH∗ signal increase. The maximum OH∗ signal intensity along the axial direction (Z) is distributed far from the nozzle outlet with the increase of nozzle diameter (d). Two types of flame were found for time-averaged images, namely the anchored flame and the lifted flame. And the increase of hydrogen velocity could lead to the changing of flame from anchored to lifted due to lower uniformity of the fuel-air mixture at the nozzle outlet. The instantaneous flames for larger nozzle diameter appear as “M” or “V” shape, and the changing of flame shape indicates an unstable combustion. This was further discussed with the OH∗ spatial distribution uniformity (SDU) and its variation determined by the coefficient of variation (CV) parameter.

The Effects of Stochastic Circular Pores on Splitting Tensile Behavior of Concrete Based on the Multifractal Theory

Concrete naturally contains a large number of circular-like stochastic pores which weaken the tensile strength of concrete and change the crack propagation path. This study investigates the influences of the size distribution and the spatial distribution of stochastic pores on the fracture behavior of concrete based on the splitting tensile test. The mesoscale model of concrete containing coarse aggregate, mortar, interface transition zone (ITZ), and circular pores is established to simulate the crack initiation, propagation, and coalescence of concrete. Concrete samples with a single hole are prepared to verify the effectiveness of the numerical simulation method. Numerical tests are conducted on numerous mesoscale concrete samples with various porosities, pore size distributions, and pore spatial distributions. The numerical simulation results indicate that the tensile strength decreases with the increase of pore size at the same porosity. Based on multifractal theory, a quantitative indicator to describe the spatial distribution uniformity of concrete stochastic pores is proposed. There is a positive correlation between the spatial distribution uniformity of stochastic pores and the tensile strength. The stochastic circular pores can have a profound effect on the concrete’s fracture pattern, which results in three typical macro-crack patterns in the numerical simulation of the splitting tensile test. The presented results deepen the understanding of the influence of stochastic circular pores on the tensile mechanical properties of concrete and provide a reference for the design of concrete structures.

Dielectric function and band gap determination of single crystal CuFeS2 using FTIR-VIS-UV spectroscopic ellipsometry

Spectroscopic ellipsometry measurements were performed on antiferromagnetic semiconductor CuFeS2 grown via molecular beam epitaxy. UV/Visible and IR ellipsometry data was merged and modeled to derive the dielectric function of CuFeS2 from 30 meV to 4.5 eV. The CuFeS2 samples were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and cross-section scanning electron microscopy (SEM) which gave the crystal quality, surface roughness and sample film thickness. A critical point analysis revealed a direct band gap of 0.76 eV, while modeling gives a carrier concentration of 8 ± 2 × 1019~cm−3 and an estimate of the indirect band gap of 0.5 eV. Optically active infrared phonons were observed at 319 cm−1 and 350 cm−1 with significant Raman active modes at 85.8 cm−1, 265 cm−1, 288 cm−1, 318 cm−1 and 377 cm−1. The fitted optical constants were then used to characterize the crystal quality and spatial uniformity.

Large Laboratory Simulator of Natural Rainfall: From Drizzle to Storms

Rainfall simulators are versatile research tools that facilitate studying rain events and the many related physical phenomena. This work describes the development and validation of an indoor, large-scale rainfall simulator comprising a rain module installed 10.4 m from ground level, a redistribution screen at an adjustable distance below the rain module, and an ultra-filtered-water recirculation system. The droplet formers installed in the rain module were selected to achieve a wide range of rain intensities. The simulator was calibrated and validated using local natural rainfall data collected with a disdrometer over 30 months. The height of the rain module allows terminal velocity to be reached at ground level. At the same time, the redistribution screen and the droplet formers guarantee the wide variability of simulated rainfall in terms of intensity and the size of the drops. As a result, we show that the rain simulator, with proper calibration of the screen’s position, can reproduce measured natural rainfall over a broad range of intensities with high spatial and temporal uniformity and kinetic energy.

Improvement of the Conformational Stability of 150 mm 4H SiC Wafers

A method for mitigating loss of conformational stability in 150 mm n-type 4H SiC wafers was investigated. Modifications to the physical vapor transport (PVT) process used to grow the parent bulk crystals, combined with post-growth thermal treatment, were examined as means of reducing the internal stresses hypothesized to promote instability. The magnitude of the stresses was analyzed by mechanically thinning sets of wafers produced from each process to determine the critical thickness of stability loss. The average critical thickness was found to be reduced by 13% via growth cell modification, at a reduced level of thermal treatment relative to a control process, with all wafers becoming unstable greater than 30 μm below the minimum recorded production thickness. Assessment of the spatial uniformity of dislocations indicated that lower conformational stability corresponded to elevated densities of basal plane dislocations (BPDs) and threading edge dislocations (TEDs) at the wafer edge relative to the center.

Rational Strategy for Space-Confined Atomic Layer Deposition

Atomic layer deposition (ALD) offers excellent controllability of spatial uniformity, film thickness at the Angstrom level, and film composition even for high-aspect-ratio nanostructured surfaces, which are rarely attainable by other conventional deposition methodologies. Although ALD has been successfully applied to various substrates under open-top circumstances, the applicability of ALD to confined spaces has been limited because of the inherent difficulty of supplying precursors into confined spaces. Here, we propose a rational methodology to apply ALD growths to confined spaces (meter-long microtubes with an aspect ratio of up to 10 000). The ALD system, which can generate differential pressures to confined spaces, was newly developed. By using this ALD system, it is possible to deposit TiOx layers onto the inner surface of capillary tubes with a length of 1000 mm and an inner diameter of 100 μm with spatial deposition uniformity. Furthermore, we show the superior thermal and chemical robustness of TiOx-coated capillary microtubes for molecular separations when compared to conventional molecule-coated capillary microtubes. Thus, the present rational strategy of space-confined ALD offers a useful approach to design the chemical and physical properties of the inner surfaces of various confined spaces.

Characterization of the chemical kinetics of dimethyl ether (DME) in a controlled trajectory - rapid compression and expansion machine (CT-RCEM)

A numerical study of the chemical kinetics of dimethyl ether (DME)-air mixture is presented when compressed in a controlled trajectory - rapid compression and expansion machine (CT-RCEM). CT-RCEM accurately steers the piston trajectory, allowing a broad operating condition range and an easy operation without needing hardware intervention. This feature enables us to modify the thermodynamic route of compression/expansion by choosing the appropriate piston trajectory. Based on the computational fluid dynamics (CFD) data, DME-air’s chemical kinetics is examined via reaction pathway analysis to understand the effect of the thermodynamic path of compression on the ignition delay. This work first shows how the creviced piston head effectively suppresses roll-up vortices, resulting in spatial uniformity of temperature, species, and, ultimately, a single dominant global reaction pathway of C, H, and O elements of DME. Next, this study shows that for a given compression ratio, the longer compression time, i.e., the longer residence time for more chemical reactions to progress by the end of compression (EOC), does not necessarily have higher reaction progress at the EOC, and in turn, lengthens the ignition delay. This counter-intuitive effect is due to the higher average core temperature of the shorter compression time piston trajectory right before EOC for sufficient time, which triggers the reactions much faster than the longer compression time piston trajectory and shortens the ignition delay. Hence, CT-RCEM’s novel feature, i.e., the ability to attain the same EOC thermodynamic state with different thermodynamic paths of compression, leading to different species build-up by EOC and thereby affecting ignition delay, provides essential information for a deeper understanding of chemical kinetics.

Facile synthesis of large-scale TiO2 nanorod arrays decorated with Ag nanoparticles as sensitive 3D SERS substrates

A facile and green synthesis strategy is employed to prepare uniform three-dimensional (3D) nanostructure as surface-enhanced Raman scattering (SERS) sensors by decorating Ag nanoparticles (AgNPs) on TiO2 nanorod arrays. The results of XPS, SEM and TEM analyses indicate that AgNPs are distributed homogeneously on the surface of TiO2 nanorods. The 3D hybrid substrate presents sensitive SERS signals of rhodamine 6 G (R6G). The relative standard deviation (RSD) for the main Raman vibration modes (611, 776, 1189 and 1651 cm−1) is less than 7%. Further UV–vis absorption spectra and FDTD simulation jointly illustrate that the interaction between AgNPs with symmetrical distribution in three dimensions generated intensive hot spots, which greatly improved the performance and spatial uniformity of SERS. The substrate also exhibits excellent SERS ability in detecting adenine molecule. The results show that the two-steps synthesis is a promising strategy for preparing 3D hybrid nanostructure substrate for SERS.

Quantitative Evaluation of Maize Emergence Using UAV Imagery and Deep Learning

Accurate assessment of crop emergence helps breeders select appropriate crop genotypes, and farmers make timely field management decisions to increase maize yields. Crop emergence is conventionally quantified by manual calculations to quantify the number and size of seedlings, which is laborious, inefficient, and unreliable and fails to visualize the spatial distribution and uniformity of seedlings. Phenotyping technology based on remote sensing allows for high-throughput evaluation of crop emergence at the early growth stage. This study developed a system for the rapid estimation of maize seedling emergence based on a deep learning algorithm. The RGB images acquired from an unmanned aerial vehicle (UAV) were used to develop the optimal model for the recognition of seedling location, spacing, and size, and the prediction performance of the system was evaluated in three stations during 2021–2022. A case study was conducted to show the evaluation of the system for maize seedlings and combined with TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) analysis. The results show that the system has good prediction performance for maize seedling count with an average R2 value of 0.96 and an accuracy of 92%; however, shadows and planting density influence its accuracy. The prediction accuracy reduces significantly when the planting density is above 90,000 plants/ha. The distribution characteristics of seedling emergence and growth were also calculated based on the average value and variation coefficient of seedling spacing, seedling area, and seedling length. The estimation accuracies for the average value of seedling spacing, the coefficient of variation of seedling spacing, the average value of the seedling area, the coefficient of variation of the seedling area, and the average value of the seedling length were 87.52, 87.55, 82.69, 84.51, and 90.32%, respectively. In conclusion, the proposed system can quickly analyze the maize seeding growth and uniformity characteristics of experimental plots and locate plots with poor maize emergence.

Geometrical and thermal optimization of cylinder-conical blackbody cavities to uniform radiance temperature profiles in the infrared

Large-area blackbody cylinder conical cavities are generally used as primary radiation sources for calibrating and characterizing radiation thermometers and quantitative thermal imagers in the infrared, both in metrology laboratories or for industrial applications. The radiation temperature of the emitted radiation depends on effective emissivity and surface (contact) temperature. The instrument’s field of view generally only covers the cavity bottom since this is where contact temperature is best measured. A non-uniform effective emissivity profile at the bottom poses a problem if instruments with a different field of view have to be calibrated with the same blackbody. Spatial uniformity depends principally on the cone angle and temperature gradient along the cavity. In this paper, we analyse blackbody optimization in terms of uniform radiance temperature profiles as a function on geometric parameters and temperature. For the isothermal case, we conclude that angles between 160° and 170° are the optimum, depending on intrinsic emissivity and length-to-diameter ratio of the cavity. For non-isothermal cavities, the effect of the temperature gradient on uniformity is relatively small, with only the temperature near the bottom proving significant. The use of multi-zone maintenance furnaces would allow temperature gradients to be designed that can improve uniformity in addition to geometric optimization.

Novel non-Joule heating technique: Externally laser-heated diamond anvil cell

The externally heated diamond anvil cell (EHDAC) conducts high pressure and temperature experiments with spatial uniformity and temporal stability. These are conventionally combined with various spectroscopies and x-ray diffraction measurements. EHDAC techniques perform Joule heating on a heater placed close to or directly in contact with diamond anvils. However, the electrical wiring and heater required for Joule heating complicate EHDAC setups, hindering easy access for the measurement of physical properties. This study proposes an EHDAC technique using laser- instead of Joule-heating. We successfully achieved temperatures reaching 900 K by applying heat to diamond anvils through laser-heating of the gaskets with thermally insulating anvil seats. To test this setup, we measured the melting temperature of H2O ice VII, which was consistent with previous studies. We also measured the high-pressure and temperature impedance of H2O VII and verified the capability of electrical resistivity measurements in this setup. This technique allows various physical property measurements owing to its simple setup required for externally laser-heated diamond anvil cell experiments. The unique characteristics of this heating technique are that (1) no heaters or wiring are required, (2) it exhibits the most efficient heating among EHDAC studies, (3) it maintains the DAC body at room temperature, and (4) diamond anvils do not detach from anvil seats after the EHDAC experiment. This method significantly simplifies the experimental setup, which allows much easier access to various physical property measurements using an EHDAC.

Characterization of the East—West Spatial Uniformity for GOES-16/17 ABI Bands Using the Moon

The Advanced Baseline Imager (ABI) is the primary instrument onboard the NOAA Geostationary Operational Environmental Satellite-R Series (GOES-R) satellites, providing continuous weather imagery over the vast area in the Western Hemisphere. It is imperative to ensure consistent calibration accuracy within the instrument’s field of regard (FOR). This paper characterized the spatial uniformity in the east–west (EW) direction for the six ABI visible and near-infrared (VNIR) bands of the first two GOES-R satellites, GOES-16 (G16) and GOES-17 (G17), using a special collection of lunar chasing images during their post-launch testing and post-launch product testing (PLT/PLPT) periods. The EW response versus scan-angle (RVS) is examined with the normalized lunar irradiance ratios at varying scan angles combined from multiple lunar-chasing events. The impacts of straylight from the Earth were found in some of the B01–B03 lunar images. The straylight, including those scattered into the spacelook scenes near the polar regions and those leaked into space near the Moon, can cause RVS variation up to 1% for B01 and to a lesser magnitude for the other two bands. Straylight correction algorithms are applied for the accurate ABI lunar image irradiance calculation. After the corrections, the RVS variation is reduced to less than 0.3% for all the VNIR bands of both G16/17 in full-disk (FD) images. Results of this study also confirm that the Global Space-based Inter-Calibration System (GSICS) Implementation of the ROLO (GIRO) model has high relative accuracy for the ABI VNIR bands when the lunar images are collected within a relatively short time. The method described in this paper can be applied to validate the EW spatial uniformity for imagers on other geostationary satellites, including the recently launched GOES-18 and the future GOES-U satellites.

Improving the spatial control of soil biocementation using indigenous microorganisms: Column experiments and reactive transport modeling

Microbially-Induced Calcite Precipitation (MICP), or biocementation, is a biomediated soil improvement process that uses microbial ureolytic activity to enable the precipitation of CaCO3 on soil particle surfaces and contacts. As the technology advances towards practical adoption, approaches that can modulate the spatial uniformity of biocementation and maximize treatment extent will be critical towards reducing implementation costs and impacts. In this study, treatment strategies capable of enriching indigenous ureolytic microorganisms at controlled ureolytic activities and the effects of these techniques on resulting spatial distributions of MICP were explored using centimeter- and meter-scale soil column experiments and reactive transport simulations. In column experiments, differences in treatment solution supplied yeast extract concentrations were shown to reliably control the enrichment of indigenous ureolytic microorganisms and achieve a wide range of ureolytic activities. Reactive transport simulations further demonstrated that reductions in stimulated ureolytic rates could enable more uniform improvement and increases in the extent of improvement with reduced sensitivity to changes in solution injection velocities. A reaction-to-injection duration ratio (RTIDR) parameter was proposed and captured the collective impacts of changes in injection rates and ureolytic reaction rates on reactive transport conditions thereby unifying outcomes from both simulations and experiments. A nonuniformity area (NA) parameter was also introduced to quantitatively characterize differences in biocementation uniformity independent of length scale and CaCO3 magnitudes. Results from this study collectively demonstrate the utility of changes in stimulated ureolytic activities towards controlling the spatial uniformity and extent of biocementation over meter-scale distances.

Model assessment of synthetic jets for turbulent combustion experiments

Understanding turbulent premixed flames is essential to predict and optimize advanced combustion strategies, but critical capability gaps exist for collecting and validating measurements such as turbulent flame speed. Here, we evaluate synthetic jets as a new, promising turbulence generation device for constant-volume combustion chambers, quantitatively assessing turbulence intensity and spatial uniformity in a hypothetical 4,189-cm3 vessel for various premixture conditions.Graphic

Uniform plasma generation with filament assisted DC discharge in a linear plasma device

Uniform and quiescent (δ n/n < 0.5%) laboratory plasma has been produced in a linear plasma device with a simple filament-assisted DC source without using any magnetic field for plasma confinement. A filament-assisted DC plasma source has been designed, fabricated in-house, and operated successfully to achieve the desired plasma parameters. A stainless steel(ss)-grid is placed in-front of the filament assembly and biased appropriately using a DC-regulated power supply to accelerate thermionically emitted electrons from the heated filaments along the length of the main chamber and facilitate the production of uniform plasma. Heating of the filaments was done by passing a current of ∼3.8 − 4.2 A through it using another DC-regulated power supply. 2-dimensional spatial profiles of plasma density (n e ), electron temperature (T e ) and plasma potential (V P ) obtained from the Langmuir Probe measurements by inserting 4 number of Langmuir Probes inside the plasma from 4 co-linear radial ports of the plasma chamber and scanning them radially with the help of indigenously built probe drive setups reveal spatially uniform plasma generation with n e in the range ∼(1 − 2) × 1015 m−3 and T e ∼ (2.5 − 3.5) eV. Variations of plasma parameters and its spatial uniformity with neutral pressure are also investigated. It is observed that the spatial uniformity of the plasma produced at neutral pressures in the range of ∼(3 − 6) × 10−4 mbar is very good with δ n/n < 0.5%.