Multilayer ceramic capacitors (MLCCs) are found everywhere in modern technology, with >3 trillion produced every year. With the growth of, for example, the electric vehicle (EV) market, with each EV using >30,000 MLCCs, the demand is growing rapidly. Permittivity in the dielectrics used in MLCCs needs to be highly temperature stable to ensure consistent performance of the capacitor in any environment, however this is usually only achieved through extremely careful control of chemistry. The permittivity of most commonly-used material, barium titanate, varies greatly with temperature, so current industrial production relies on >10 different rare dopants e.g. Dy, Ho, Er, in fractional quantities to help reduce dependence of permittivity on temperature.
Recently, test pieces made of two capacitor materials with different properties, have been used to demonstrate that the desirable properties of each can be exploited in a single device. For example a device made of a layer of BaTiO3 (BT) and a layer of (Na,Ba)(Nb,Ti)O3 (NNBT) can exploit the high room temperature permittivity of BT and the good temperature stability of NNBT, with a less complex composition and using more readily-available elements than the state-of-the-art. This composite approach may provide a route to secure the capacitor industry against resource scarcity, and unstable and single-location supply chains.
The challenges associated with implementing this novel bi-phase approach, either in lab-scale test pieces, or using industrial-style scale-up techniques, are significant. Most ceramic materials have highly specific sintering temperatures where densification occurs, unique to each composition. This solid state sintering is also highly energy intensive, requiring high temperatures (often >1200 C) and long dwell times (>8h) and accounting for over a third of the total energy cost of manufacture. To successfully co-sinter two layers of materials, they must have very similar sintering temperatures to ensure full densification, similar rates of thermal expansion and shrinkage on densification to avoid cracking or delamination, and must not react with one another during heating, significantly limiting the choice of materials available. A low temperature sintering method is therefore required to fully exploit the opportunities presented by layered devices.
One such low temperature method was announced in 2016: cold sintering (CS) uses a small amount of transient solvent (usually water) to aid densification at greatly reduced temperatures. Akin to the processes which cause sugar in a sugar bowl to form a solid lump over time, cold sintering uses applied pressure to speed up the process in soluble oxides, creating densification at temperatures as low as 120 C in a few minutes. Whilst CS has the potential to revolutionise oxide densification, the insolubility of most ceramics prevents its use. For insoluble materials, the solvent can be adjusted to include reactive intermediate phases which fill in voids between grains, and once heated to modest temperatures, create a fully dense ceramic. This "reactive" cold sintering (RCS) works at temperatures <1000 C, avoiding usual pitfalls associated with high temperature processing: chemical compatibility, different shrinkage rates, and differing sintering temperatures, with disruptive industry applications.
This ambitious project will use RCS create dual/triple-layer and mixed composite dielectric ceramic test pieces, creating capacitors with high permittivity and optimised temperature stability. This will be achieved without the current reliance on scare elements and complex compositions, and develops a manufacturing technique which is lower in energy than the current state of the art.
The scaling and application of this process to the thick film deposition techniques currently used by industry will be investigated as a means to transfer to industry, providing a transformative and innovative new route to composite ceramics.
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