Showing papers by "Hamid Garmestani published in 2020"

Ultra-high selectivity of H2 over CO with a p-n nanojunction based gas sensors and its mechanism

Abstract: The main disadvantage of metal oxide semiconductor sensors is their poor selectivity to different gases having similar (reducing or oxidizing) nature. Taking two strong interference homogeneous gases CO and H2 as example, it is difficult for the sensor to accurately detect their concentrations when CO and H2 coexist because of the cross sensitivity between the two homogeneous gases. Thus far, there has been no effective method to selectively detect specific gas without the cross sensitivity of another homogeneous gas. In this paper, the n-SnO2/p-Co3O4 composite nanoparticles (NPs) have been prepared for the sensing materials. By means of controlling the p-n nanojunction and holes (h+)-electrons (e−) concentration, the n-SnO2/p-Co3O4 NPs sensor material with the Sn/Co molar ratio of 1:0.15 successfully and selectively detects H2 without the cross sensitivity of CO. This makes a great breakthrough in solving the poor selectivity. Most important, the mechanism of the excellent selectivity of the sensor to H2 against CO has been explained based on the series of characterization results. This provides a theoretical guidance and technical solution for solving the problem of poor selectivity of this type of sensors.

Conductivity and mixed conductivity of a novel dense diffusion barrier and the sensing properties of limiting current oxygen sensors

Abstract: First-principles calculations were used to explore the effect of various Y-doping levels on the electrical conductivity of SrTiO3. Herein, we prepared ((Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ)/x/3Co3O4 (x = 0.1, 0.2, 0.3)) composites using a solid state reaction method. The properties of these sensing materials and the fabricated sensors including crystal phase composition, microstructures, oxygen ionic conductivity, total conductivity and sensor performance were investigated in detail. XRD demonstrates the formation of a highly cubic perovskite structure. The introduction of Co3O4 promotes remarkably the electronic conductivity of the Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ/x/3Co3O4 composites due to the formation of n-type CoO and p-type Co2O3. A limiting current oxygen sensor based on (Y0.07Sr0.93Ti0.6Fe0.4-xO3-δ)/x/3Co3O4 as a dense diffusion barrier shows excellent sensing performance. The recovery time is less than the response time, indicating that Co2O3 promotes the gas desorption reaction which results in a shorter recovery time. The obtained results demonstrate a direct relationship between limiting current (IL) and oxygen content.

Y-doped CaZrO3/Co3O4 as novel dense diffusion barrier materials for a limiting current oxygen sensor

Abstract: Herein, we illustrate a feasible strategy to strengthen the gas sensing of Y-doped CaZrO3 (YxCa1-xZr0.7O3-δ (x = 0.05, 0.06, and 0.07))/0.1Co3O4 used as sensing materials. This compound was prepared via a solid-state reaction technique. The structural, morphological, electrical, and sensing features such as phase identification, microstructure, ionic conductivity, total conductivity and sensitivity of the fabricated sensors were evaluated via X-ray diffraction, scanning electron microscopy, electron-blocking method, electrochemical impedance spectroscopy and cyclic voltammetry. In addition, the influence of the Y-dopant on the properties of YxCa1-xZr0.7O3-δ/Co3O4 was thoroughly studied. XRD results revealed the formation of the orthorhombic perovskite phase of YxCa1-xZr0.7O3-δ. Moreover, the obtained results from the electrical properties elucidated high electronic and low ionic conductivities, and small polaron conduction of YxCa1-xZr0.7O3-δ/Co3O4. Furthermore, the results confirmed an excellent limiting current plateau for the fabricated oxygen sensor based on YxCa1-xZr0.7O3-δ/Co3O4. In particular, experimental observation indicates that Y-doping at the Ca site and/or Zr site might be difficult.

Thermo-mechanical modeling of thermal stress in metal additive manufacturing considering elastoplastic hardening

Abstract: The build-up of thermal stress induced by a high-temperature gradient in an additively manufactured part during laser metal additive manufacturing provides significant limitations to the adoption of this process since thermal stress may induce high tensile residual stress and part distortion in the additively manufactured parts. Herein, a thermomechanical analytical model is proposed to predict the in-process elastoplastic hardening thermal stress and strain for single-track scan strategy. The thermal model is validated using experimental results of melt pool geometry. Also, finite element simulation is performed to validate the proposed elastoplastic hardening thermal stress model of the same problem.

Analytical modeling of in-process temperature in powder feed metal additive manufacturing considering heat transfer boundary condition

Abstract: This paper presents a physics-based predictive model to estimate the in-process temperature in powder feed metal additive manufacturing (PFAM) based on the absolute coordinate with a stationary origin. Quasi-analytical solutions are developed without resorting to FEM or any iteration-based simulations. Heat transfer boundary condition, laser power absorption, scanning strategy, and latent heat are considered in the prediction of time-dependent thermal profiles. The temperature rise due to a moving laser heat source is predicted using a moving point heat source solution. The temperature drop due to convection and radiation at the part boundary is predicted by a heat sink solution, which is derived by modifying the heat source solution with an equivalent power for heat loss and zero velocity. The final temperature solution is constructed from the superposition of the heat source solution and the heat sink solution. Temperature profiles are predicted in multiple layers using the presented model in PFAM of Ti-6Al-4 V for a thin wall structure. Molten pool evolution is investigated with respect to laser travel distance from its starting point. The stabilized molten pool dimensions in multiple layers are obtained from predicted temperatures and agreed well with experimental measurements in the literature. In addition, the increasing molten pool dimensions were observed from predictions with increasing wall thickness, which confirms the finding reported in the literature. With benefits of high computational efficiency of the developed solution, consideration of heat transfer boundary conditions, and absolute coordinate, the presented model can be used for temperature analysis for a dimensional part in real applications.

Inhibition of stress corrosion cracking in 304 stainless steel through titanium ion implantation

Abstract: The effects of titanium ion implantation on the stress corrosion cracking (SCC) behaviour of 304 austenitic stainless steel were studied. Slow strain rate tests (SSRTs) were conducted on 304 steel ...

Analytical Thermal Modeling of Powder Bed Metal Additive Manufacturing Considering Powder Size Variation and Packing.

Abstract: This work presents a computationally efficient predictive model based on solid heat transfer for temperature profiles in powder bed metal additive manufacturing (PBMAM) considering the heat transfer boundary condition and powder material properties. A point moving heat source model is used for the three-dimensional temperature prediction in an absolute coordinate. The heat loss from convection and radiation is calculated using a heat sink solution with a mathematically discretized boundary considering non-uniform temperatures and heat loss at the boundary. Powder material properties are calculated considering powder size statistical distribution and powder packing. The spatially uniform and temperature-independent material properties are employed in the temperature prediction. The presented model was tested in PBMAM of AlSi10Mg under different process conditions. The calculations of material properties are needed for AlSi10Mg because of the significant difference in thermal conductivity between powder form and solid bulk form. Close agreement is observed upon experimental validation on the molten pool dimensions.

High-Throughput Exploration of the Process Space in 18% Ni (350) Maraging Steels via Spherical Indentation Stress–Strain Protocols and Gaussian Process Models

Abstract: Several challenges are encountered in the development of maraging steels with desired combinations of mechanical properties. These include the need to explore a large process design space, the time- and effort-consuming standardized testing protocols for property evaluations, and the lack of a formal design of experiments strategy that guides the selection of the process conditions for the next set of experiments based on a thorough analyses of the previously accumulated data. In this work, we explored the effect of the different combinations of the aging temperature and the aging time on the yield strength of 350-grade maraging steels using high-throughput protocols. For this purpose, a total of 21 small volumes of differently aged samples were produced and studied using spherical nanoindentation stress–strain protocols. Furthermore, the results of these tests were modeled using Gaussian process regression (GPR) to establish data-driven linkages between the yield strengths and the aging parameters. The predicted yield strengths were found to be in reasonable agreement with the experimentally measured ones. It is also demonstrated that the GPR model can provide objective guidance in the selection of the next set of experiments.

Prediction of the deformation behavior of a selective laser-melted Ti-6Al-4V alloy as a function of process parameters

Abstract: Recent studies have shown that the mechanical properties of Ti alloys produced by additive manufacturing (AM) methods are sensitive to AM process parameters. The mechanical threshold stress (MTS) model is capable of predicting the flow stress behavior of materials; however, the parameters needed in the MTS model are affected by the microstructure that originates from the AM process parameters. To find a relationship between the AM process parameters and the MTS parameters, the effect of process parameters on the mechanical properties of selective laser-melted (SLM) Ti-6Al-4V samples was studied. As the MTS model is sensitive to the microstructure, only near fully dense samples were considered.

Analytical Modeling of Residual Stress in Laser Powder Bed Fusion Considering Volume Conservation in Plastic Deformation

Abstract: Residual stress (RS) is the most challenging problem in metal additive manufacturing (AM) since the build-up of high tensile RS may influence the fatigue life, corrosion resistance, crack initiation, and failure of the additively manufactured components. While tensile RS is inherent in all the AM processes, fast and accurate prediction of the stress state within the part is extremely valuable and results in optimization of the process parameters to achieve a desired RS and control of the build process. This paper proposes a physics-based analytical model to rapidly and accurately predict the RS within the additively manufactured part. In this model, a transient moving point heat source (HS) is utilized to determine the temperature field. Due to the high temperature gradient within the proximity of the melt pool area, the material experiences high thermal stress. Thermal stress is calculated by combining three sources of stresses known as stresses due to the body forces, normal tension, and hydrostatic stress in a homogeneous semi-infinite medium. The thermal stress determines the RS state within the part. Consequently, by taking the thermal stress history as an input, both the in-plane and out of plane RS distributions are found from the incremental plasticity and kinematic hardening behavior of the metal by considering volume conservation in plastic deformation in coupling with the equilibrium and compatibility conditions. In this modeling, material properties are temperature-sensitive since the steep temperature gradient varies the properties significantly. Moreover, the energy needed for the solid-state phase transition is reflected by modifying the specific heat employing the latent heat of fusion. Furthermore, the multi-layer and multi-scan aspects of metal AM are considered by including the temperature history from previous layers and scans. Results from the analytical RS model presented excellent agreement with XRD measurements employed to determine the RS in the Ti-6Al-4V specimens.