Gaining A Better Understanding of Material Conductivity to Develop Improved Energy Storage Devices

July 02, 2020 by Gary Elinoff

A new study probes how changes in material nanostructure can affect electrical conductivity.

Investigators at North Carolina State University have developed a computational model that aims to explain how material conductivity is affected by changes in its nanostructure. The research effort focuses on the materials used to construct capacitors, with a the primary goal of enhancing the development of new methods to more efficiently store energy.

NC State’s Doug Irving, an associate professor of materials science and engineering, was the study’s corresponding author. According to Irving, “One of the things that we’re pleased with is that this model looks at multiple spatial scales simultaneously – capturing everything that is happening from the device-level scale to the nanoscale.”


Defects and Grain Boundaries

Irving describes two types of possible material defects. One example is an atom that is missing from its proper place in the material’s structure. The other is an atom resident within the structure that doesn’t belong there. Grain boundaries are noted as the borderlines marking the end of one type of crystalline structure and the beginning of another.

According to Irving, “Because different ways of processing a material can control the presence and distribution of things like defects and grain boundaries, the model gives us insights that can be used to engineer materials to meet the demands of specific applications.” 


Grain size decreasing from microscale to nanoscale.

An illustration of grain size decreasing from microscale to nanoscale — Overlap of space charge (SC) layers. Image credited to North Carolina State University (NC State)


Previous Studies of Single Crystal Strontium Titanate

Single crystal strontium titanates (STO) have been well studied to ascertain best practice doping strategies. However, many applications employ STO polycrystals instead. This is because the use of single-crystal STOs is expensive, and the products derived are subject to long-term degradation. 

For these reasons, the new NC State model studies STO polycrystals to facilitate an understanding of the effects of STO polycrystal microstructure on electrical responses.

Polycrystalline Perovskites

The model developed in this study has enabled prediction of the electrical properties of polycrystalline perovskites with their complex defect chemistries. The model specifically shows the conductivity of polycrystalline acceptor-doped strontium titanate (STO) as the grain size decreases. STO and its alloys are used for many important commercial uses, including capacitors.

It was observed that the changes to conductivity result from the charge transfer occurring from grain boundaries into the grain interiors. It is this forming a space-charge layer near the grain boundary core that perturbs the local defect chemistry. The impact of the grain size on the electrical conductivity and the underlying defect chemistry were also factors.

The model has also opened the door to exploring the electrical response of polycrystalline semiconductor systems with complex defect chemistries. This will be critical to the design of future electronic components.


Reducing the Cost of Capacitors 

The presence and the physical locations of grain boundaries and defects are controlled by how the material is processed. The NC State model can aid designers in the engineering of materials to meet the needs of the specific application at hand. 

As per Irving, “... we’re optimistic that the model can help us keep the cost of future capacitors low while ensuring that they’ll work well and last a long time.”