Ehsan Ghafari

Metal oxides for thermoelectric power generation and beyond

AbstractMetal oxides are widely used in many applications such as thermoelectric, solar cells, sensors, transistors, and optoelectronic devices due to their outstanding mechanical, chemical, electrical, and optical properties. For instance, their high Seebeck coefficient, high thermal stability, and earth abundancy make them suitable for thermoelectric power generation, particularly at a high-temperature regime. In this article, we review the recent advances of developing high electrical properties of metal oxides and their applications in thermoelectric, solar cells, sensors, and other optoelectronic devices. The materials examined include both narrow-band-gap (e.g., NaxCoO2, Ca3Co4O9, BiCuSeO, CaMnO3, SrTiO3) and wide-band-gap materials (e.g., ZnO-based, SnO2-based, In2O3-based). Unlike previous review articles, the focus of this study is on identifying an effective doping mechanism of different metal oxides to reach a high power factor. Effective dopants and doping strategies to achieve high carrier concentration and high electrical conductivities are highlighted in this review to enable the advanced applications of metal oxides in thermoelectric power generation and beyond.

Prediction of Fresh and Hardened State Properties of UHPC: Comparative Study of Statistical Mixture Design and an Artificial Neural Network Model

AbstractThe main objective of the research study described herein is to build two analytical models based on artificial neural networks (ANNs) and the statistical mixture design (SMD) method to predict the required performance of ultra-high-performance concrete (UHPC). Two different curing conditions—heat treatment and water storage—were applied to the specimens. To train the neural network, a total set of 53 different mixtures was designed based on the design matrix of SMD. The statistical analysis results showed the adequacy of both models to predict the required performance of UHPC; however, the ANN model could predict the compressive strength (water storage) and slump flow with higher accuracy than the SMD. The optimum combination of the cement, silica fume, and quartz flour was determined to be 24, 9, and 5% by total volume to achieve a flowable mixture with the highest compressive strength. The accuracy of the model was verified with additional experimental tests.

Surface morphology and beta-phase formation of single polyvinylidene fluoride (PVDF) composite nanofibers

AbstractThis study has developed a reliable model to design and engineer PVDF nanofiber in terms of both morphological and fraction of beta-phase content based on response surface methodology (RSM). The model was further used to assess the effect of each individual electrospinning processing parameter as well as their interdependences on the properties of electrospun PVDF nanofibers. Our experimental results highly agreed with the modeling. The results indicated that both morphological and crystalline properties of PVDF are highly affected by electrospinning process parameters, particularly the fraction of beta-phase content. A beadless PVDF nanofiber with the maximum fraction of the beta phase was achieved through a numerical optimization process. Graphical abstract

Nondestructive evaluation of early age properties of concrete using a polymer based piezoelectric sensor (Conference Presentation)

Nowadays, strength monitoring of concrete structures by the nondestructive method has gained more attention. Strength monitoring is not only important to determine the readiness of structures for service, but also to ensure the safety of the structure itself during the construction. The main goal of in-situ monitoring the strength gain in concrete is to obtain reliable information about the quality of concrete which can be further used to assess construction schedule and process of concrete structures, such as determining the optimal traffic opening time. The current methods are unreliable, inefficient, costly and partially destructive. To address these challenges, this paper aims to investigate the feasibility of employing polymer based piezoelectric sensors to characterize the in-situ compressive strength gain of concrete at the early ages. The pitch-catch approach was considered for the active sensing approach. In this approach, a piezoelectric transducer acts an actuator to propagates the Lamb waves while another piezoelectric device works as a sensor to detect the signal. A PVDF sensor was fabricated using electrospinning method, and a commercial PZT transducer was used to generate the Lamb waves. The developed piezoelectric-based set-up has shown a promising technique for strength development of cementitious materials at early ages.