Formation of a two dimensional conducting interface between two perovskite insulators was first reported in 2004. This unexpected result was related to internal polarization, defect structures and polarization discontinuity. In 2006 it was reported for the first time that the conductivity of the hetero-interface could be switched between two states by application of an external field. This memory effect opened the pathway for revolutionary new technologies that depend on different physical effects than semiconductor based memories. Our research focus is on identifying the effects of defects on the properties of interfaces. The five major sections of investigation will be; (i) processing and film characterization, (ii) electrical characterization of the interfaces, (iii) first-principles modeling, (iv) high temperature effects and (v) proof of concept applications. The focus is on seven different parameters: (i) Composition of the film, (ii) strain, (iii) interface composition, (iv) anisotropy, (v) surface conditions, (vi) electrodes, and (vii) temperature. These parameters are not isolated from each other and cross-cutting effects are also investigated.

Our research approach aims to not only identify the origins of charge at hetero-interfaces but also define the quantifiable role of structural and compositional factors, such as defects, dipoles, domain boundaries and strain, on the electrical properties of the interface. Quantitative identification of each parameter is required to understand high temperature behavior, as well as to develop devices based on hetero-interfaces operational at extreme environments.

Collaborators:

Walter Lambrecht (Physics Department - Case Western Reserve University)

Xuan Gao (Physics Department - Case Western Reserve University)

Marie-Helene Berger (Ecole De Mines - Paris, France)

The Electroceramics Group is interested in two-dimensional oxides due to their interesting physical, chemical, and electronic properties. Via wet chemical methods, various layered metal oxides (which have strong in-plane bonding, but weak interplane bonding) can be exfoliated to form freestanding atomically thin nanosheets. Like any material, the properties associated with the bulk can drastically change when transforming to 0D, 1D, or 2D nanomaterials. These nanomaterials have been of much interest in the research community leading to many new and interesting properties and applications. Intriguingly, when working with oxides in two-dimensions, unique properties begin to develop due to the high surface area of oxygen. The properties of these two-dimensional materials can also be tuned based off of defects, or morphology. For example, 2D-MoS₂’s band gap can be tuned from indirect to direct depending on the thickness of the material. Due to the limited research conducted on oxide materials in two-dimensions, reproducible routes for exfoliation have not been established. Our work focuses on producing high quality, reproducible oxide nanosheets while maintaining a control over composition and morphology. To fully understand the applications of these exfoliated nanosheets, processing-structure-property relationships are being investigated. From this, the properties of these freestanding two-dimensional nanosheets can be applied for possible uses in energy generation and storage, catalysis, sensors, and optoelectronics to name a few.

We are currently working on developing high temperature piezoelectrics for a broad range of applications including energy convertors for thermo-acoustic engines. The technological focus is converting the acoustic waves into electrical energy using piezoelectrics, creating a solid state engine. These systems, aimed to power instrumentation for deep space science missions, can also be commercialized for power generation applications for terrestrial applications. Additional applications for the high temperature piezoelectrics include fuel modulation in combustion engines by increasing efficiency and reducing green-house emissions, as well as efficient ultrasonic drilling for oil deeper into the earth’s crust.

Our research focus has been on the development of high temperature piezoelectrics based on Pb-, and Bi-containing systems. We have two main approaches to develop high temperature piezoelectrics: (i) microstructural control and (ii) compositional design of ternary and quaternary perovskite systems. The purpose is to introduce the effects of lower solubility perovskites and still take advantage of proximity to the MPB.

With global energy consumption approaching 50 TW/yr, new power generation strategies are needed to meet the demand. Waste heat is an abundant source that is underutilized as an energy resource. Thermoelectric (TE) technology provides direct conversion of heat to electric power by utilizing the Seebeck effect. Thermoelectrics are solid-state convertors and therefore they are extremely reliable. Due to such reliability, NASA has used them to power their deep space probes such as Cassini; operational over 14 years with no maintenance, and most currently, Curiosity. However, commercial applications of thermoelectric technology have been limited due to low conversion efficiency and cost. Over the last decade, progress in higher conversion efficiency has been achieved by implementation of nano-technology. This created a renewed interest in thermoelectrics from industry, especially the automotive industry. Electrical power generation from waste heat will help reduce fossil fuel consumption but also improve system efficiency. However, successful commercialization of thermoelectric technology will be dependent upon conversion efficiency, material cost and environmental toxicity. Currently the thermoelectrics with largest conversion efficiencies contain Tellerium, a rare and relatively toxic material.

TE materials with figure of merit (ZT) ~ 1 are adequate for waste heat recovery if the cost is low ($/W). To maximize the figure of merit a material has to have high Seebeck coefficient and electrical conductivity and low thermal conductivity. Introduction of nano-precipitates with coherent interfaces has been successful in decreasing the thermal conductivity with no adverse effect on electrical conductivity. However, the stability of these nano-precipitates at high temperatures is a current problem for power generation. Quantum well super- lattices have recently been obtained for metal oxide systems by alternating hetero-interface layers. These film structures have demonstrated, at the laboratory scale, the potential to achieve higher conversion efficiency (ZT=2.4). This increase was due to coherent interfaces that scattered phonons but had no effect on electrical charge carriers. However, the mass production of super-lattices is a big challenge to date. Our research focuses on duplication of these thin film hetero-structures for bulk materials by using self-assemble processes, e.g., spinodal decomposition, directional solidification of eutectic structures and nano-checkerboard structures. Self-assembled nano-structures are usually controlled by composition and temperature, and provide a simple process to fabricate nano-structures in bulk solids for commercialization. These thermodynamically stable nano-structures can be obtained in systems that are environmentally friendly and abundant (i.e., W-Si/Ge, SnO2/TiO2).

I) Electrical characterization

1) Impedance analyzers:

a) Agilent 4991 – 1MHz : 3 GHz

b) Agilent 4294A (2 of them) – 40 Hz : 115 MHz. These measurements can be done up to 1600°C in controlled atmosphere with a customized furnace.

c) Solarton – 1Hz : 1MHz. These measurements can be done up to 1000°C in controlled atmosphere.

2) Ferroelectric analyzers:

a) aixACCT – allows unipolar and bipolar ferroelectric, piezoelectric, Pyroelectric, small voltage capacitance and leakage measurements. The measurements can be done up to 250°C and sample holders can handle bulk materials, thick films and thin films.

b) Radiant – allows unipolar and bipolar ferroelectric, piezoelectric, leakage measurements. The measurements can be done up to 250°C.

3) Piezoelectric measurements:

a) PolyTek Laser Dopplermeter: allow large working distance sub-nano-level displacement measurements coupled with aixACCT.

b) Photonic Sensor: allows displacement measurements using white light.

c) Berlincourt d33 meter: measures direct piezoelectric coefficient at room temperature.

4) Hall measurements (LakeShore): Allows measurement of sign, density and mobility of charge carriers up to 500°C.

5) Thermal Power measurements (ULVAC ZEM 3): Allows measurement of Seebeck coefficient and four-point DC resistivity up to 1000°C.

6) Thermal Conductivity Measurements (Anter): Allow measurements of thermal diffusivity and conductivity from liquid nitrogen temperatures up to > 2000°C.

II) Processing:

1) Conventional ceramic processing: Numerous ball mills, freeze mill, uniaxial press, manual and automatic machining saws, polishing wheels, slip casting molds, drying ovens and several furnaces.

2) Single crystal growth or directional solidification:

a) Float zone laser melting

b) Vertical Bridgman Furnace

3) Thin film growth:

4) Pulse Laser Deposition with RHEED gun that allows inspection of atomic layer by layer growth characteristics.

5) Coherent pico-second (multi-wavelength) pulse laser for ablation and deposition

6) Physical Vapor Deposition (Kurt Lesker): Allow deposition of up to three materials simultaneously, including magnetic targets.

III) Other:

1) Optimol SRV: Tribology measurement instrument. The option to perform the measurements in vacuum makes it one of the two with such ability in the world (other one is in China).

2) Gas mass flow controller system (Tes-Sol): Allows mixing of nitrogen and oxygen sweeping from pure oxygen to 10-8 parts O2.