High performance CMOS-compatible perovskite oxide memristors: compositional control and nanoscale switching characteristics

Nanoscale memristive devices have been dubbed as one of the main contenders for the next generation nonvolatile memories (NVM) and alternative logic architectures. Passive two-terminal metal-insulator-metal (MIM) memristive crossbar configurations based on functional transition metal-oxides (e.g. TiO2, SrTiO3) offer great potential for ultimate integration in contemporary electronic industry. This thesis focuses on the realization and nanoscale characterization of high performance CMOS-compatible memristive devices utilizing functional perovskite oxides. A PVD based synthesis route for the realization of functional perovskite oxides with control over their composition and structure has been established. Utilizing the synthesis approach, first realization of memristive devices based on oxygen deficient amorphous SrTiO3 (a-STO) oxides has been demonstrated and their resistive switching performance has been studied in detail utilizing micro-scale crossbar MIM arrays and a sophisticated conductive nano-contact technique based on in situ electrical nanoindentation. RF magnetron sputtering has been used in this work to synthesis perovskite oxide thin films on conventional silicon substrates. Firstly, a lead-free ferro/piezoelectric perovskite oxide (KxNa1‑xNbO3) was chosen to study the effects of sputtering parameters and post-deposition treatments on the composition and the structure of sputtered thin films. This study demonstrates that the crystal orientation, thickness and the elemental composition of the thin films sputtered from the same ceramic target can be effectively and reliably controlled via tuning the sputtering parameters (process gas, substrate temperature, etc.) and the oxide structure and secondary phases can be engineered through post-annealing treatments. The same procedure was employed for the synthesis of SrTiO3 thin films as a reliable resistive switching perovskite oxide. A low temperature synthesis of amorphous SrTiO3 (a-STO) thin films with precise control over the thickness, oxygen deficiency and A‑site/B-site dopants has been demonstrated for the first time. The switching characteristics of a-STO cross-point devices suggest the possibility of fine tuning the memristive performance through tailoring the oxide composition and device structure. Outstanding switching performance (high switching ratios, excellent endurance and retention) is demonstrated in oxygen deficient a-STO devices. Also, it is shown that niobium doping through low temperature co-sputtering of Nb: a-STO result in significant improvements in device energy requirements. Furthermore, nanoscale conduction and resistive switching mechanisms of these devices have been studied in detail utilizing a sophisticated in situ electrical nanoindentation technique, capable of forming nano‑contacts with controlled size and mechanical force. To this end, a unique empirical model has been developed that allows for a complete characterization of the electrical properties of the load controlled nano‑contact and therefore yields quantified insights into the conduction and switching mechanisms of a‑STO based memristive device at nanoscale. The results exhibit ultimately scalable and isolatedly controllable switching characteristics in these devices and also suggest the possibility of mechanically modulated nanoscale resistive switching in a‑STO based devices. Overall, this thesis highlights a‑STO based devices as strong candidates for the ongoing development of the alternative memory technologies as well as applications in MEMS/NEMS devices

In situ nanoindentation: probing nanoscale multifunctionality

Nanoindentation is the leading technique for evaluating nanoscale mechanical properties of materials. Consistent developments in instrumentation and their capabilities are transforming nanoindentation into a powerful tool for characterization of multifunctionality at the nanoscale. This review outlines the integration of nanoindentation with real-time electron imaging, high temperature measurements, electrical characterization, and a combination of these. In situ nanoindentation measurements have enabled the real-time study of the interplay between mechanical, thermal, and electrical effects at the nanoscale. This review identifies previous reviews in this area, traces developments and pinpoints significant recent advances (post-2007), with emphasis on the applications of in situ nanoindentation techniques to materials systems, and highlighting the new insights gained from these in situ techniques. Based on this review, future directions and applications of in situ nanoindentation are identified, which highlight the potential of this suite of techniques for materials scientists from all disciplines

On the microstructure and mechanical properties of an Fe-10Ni-7Mn martensitic steel processed by high-pressure torsion.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process

Explaining the goals of the curriculum based on teacher-researcher training at Farhangian University

Background and Objectives: The purpose of architecture education is to prepare graduates for general proficiency in this field of study. However, many graduates of architecture who are employed at the job market observe a profound gap between what they have learned in college and what is being done in professional work in architecture. This study -by examining the method of architecture education at undergraduate level and the job market situation in Iran- evaluates the relationship between the education and professional work in architecture to answer the following questions: "1. What are the problems of undergraduate architecture education in Iran that cause the inefficiency of architecture graduates in professional environment?", "2. Which features in the professional environment are necessary for a graduate of architecture?" and "3. How can we make a more effective connection between and the professional architectural environment?" Methods: The present study is an applied one and its method is field survey. In addition, the method of data analysis is analytical-descriptive. Data collection has been done through handing questionnaires and holding interviews with senior undergraduate architecture students as the statistical population who are supposed to enter the architectural job market soon, professors of architecture schools as individuals who are teaching students in this field, and professional architects as individuals who are professionally active in the field of architecture. The students and professors who were studied in this research were selected from 3 Iranian universities at the intermediate level, and professional architects were also selected from the managers of architectural companies. Being a veteran architect was the criterion for selecting professors and architects. The criteria for professors to be a veteran architect have been the experience and years of work, and the academic degree. The criteria for selecting architects have been experience and years of work, the significant number of the projects implemented, and winning awards. Findings: According to the common viewpoints of the three groups who were investigated in this research, the problems of the architecture educational environment that have created a gap between architecture education and architectural profession and the low efficiency of the architecture graduate in the professional environment have a wide range from the beginning of admission of architecture students up to after their graduation. The main problems are: 1. Method of student admission, 2. Admission of too many students, 3. Incompatibility of educational topics with the job market needs, 4. The separation of university from the society, 5. Lack of professional ethics training, 6. Lack of interactive skills training, 7. Lack of professional experience of professors, and 8. General problems of the society including economic issues. Therefore, the characteristics that are necessary for a graduate student of architecture in a professional environment are: 1. Familiarity with the topics required by the labor market, such as having a good command of various architectural software, familiarity with rules and regulations, and so on, 2. Having professional ethics, and 3. Having interactive skills. Conclusion: Based on the benefits of participatory education, it is proposed to turn architecture schools into "participatory education" environments in order to reduce the gap between education and professional work and decrease the low efficiency of architecture graduates in the professional environment. In this regard, it is possible to use the simultaneous training of students of different semesters in architecture courses and the presence of professional architects as expert individuals in some of the class sessions. In this way, students can benefit from each other's participation in design, critique of works, and improvement of skills related to the field of architecture, and the presence of professional architects in order to gradually acquire the skills required by the architectural job market. It is also possible to reduce the gap between education and professional work by establishing "NGOs" that are responsible for linking the university to architecture offices, job market and employers. ===================================================================================== COPYRIGHTS  ©2021 The author(s). This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, as long as the original authors and source are cited. No permission is required from the authors or the publishers.  ====================================================================================

Alkali ratio control for lead-free piezoelectric thin films utilizing elemental diffusivities in RF plasma

High performance piezoelectric thin films are generally lead-based, and find applications in sensing, actuation and transduction in the realms of biology, nanometrology, acoustics and energy harvesting. Potassium sodium niobate (KNN) is considered to be the most promising lead-free alternative, but it is hindered by the inability to control and attain perfect stoichiometry materials in the thin film form while using practical large area deposition techniques. In this work, we identify the contribution of the elemental diffusivities in the radio frequency (RF) plasma in determining the alkali loss in the KNN thin films. We have also examined the effect of the substrate temperature during the RF magnetron sputtering deposition on the crystal structure of the substrate and KNN thin films, as well as the effect of the postannealing treatments. These results indicate the need for well-designed source materials and the potential to use the deposition partial pressure to alter the dopant concentrations

Correlation between nanomechanical and piezoelectric properties of thin films: an experimental and finite element study

Piezoelectric materials perform mechanical-to-electrical energy conversion and have unique and enhanced properties, particularly in the thin film form. In this work, the nanomechanical properties of strontium-doped lead zirconate titanate thin films are investigated by nanoindentation. Finite element modeling of the nanoindentation process allows for a comparison of the simulated and experimentally measured load-displacement curves. New insights correlating the nanomechanical and piezoelectric properties of thin films have been obtained which will enable more efficient design of piezoelectric energy harvesting devices

Shape memory characteristics of a nanocrystalline TiNi alloy processed by HPT followed by post-deformation annealing

A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19’ phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19’ phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition

Transparent functional oxide stretchable electronics: micro-tectonics enabled high strain electrodes

Fully transparent and flexible electronic substrates that incorporate functional materials are the precursors to realising nextgeneration devices with sensing, self-powering and portable functionalities. Here, we demonstrate a universal process for transferring planar, transparent functional oxide thin films on to elastomeric polydimethylsiloxane (PDMS) substrates. This process overcomes the challenge of incorporating high-temperature-processed crystalline oxide materials with low-temperature organic substrates. The functionality of the process is demonstrated using indium tin oxide (ITO) thin films to realise fully transparent and flexible resistors. The ITO thin films on PDMS are shown to withstand uniaxial strains of 15%, enabled by microstructure tectonics. Furthermore, zinc oxide was transferred to display the versatility of this transfer process. Such a ubiquitous process for the transfer of functional thin films to elastomeric substrates will pave the way for touch sensing and energy harvesting for displays and electronics with flexible and transparent characteristics

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

Effect of Cu on Amorphization of a TiNi Alloy during HPT and Shape Memory Effect after Post‐Deformation Annealing.

A ternary TiNiCu memory alloy was subjected to high‐pressure torsion (HPT) followed by post‐deformation annealing (PDA) to study the effect of Cu (5 at%) on amorphization after HPT processing and also the microstructural evolution and shape memory effect (SME) after PDA. The results show that even after 20 revolutions the ternary alloy contains nanocrystalline areas and the microstructure is not fully amorphous. An easier martensite to austenite transformation and minor remaining austenite in the ternary alloy are responsible for suppressing amorphization. PDA at 673 K provides nanocrystalline microstructures containing an R‐phase with a minor martensitic B19' phase in the ternary alloy. The SME of this alloy after PDA is not as satisfactory as for the binary alloy processed through similar conditions because of the existence of a high volume fraction of the R‐phase. Nevertheless, the total recovered strain of the ternary alloy after PDA for 30 min has a maximum value of 6.5%