Reactive cold sintering of temperature-stable multi-phase ceramic dielectrics

多层陶瓷电容器（mlcc）在现代技术中无处不在，每年生产30万亿个。例如，随着电动汽车（EV）市场的增长，每辆电动汽车使用100万辆mlcc，需求正在迅速增长。mlcc中使用的电介质中的介电常数需要高度温度稳定，以确保电容器在任何环境下的一致性能，然而这通常只能通过非常仔细的化学控制来实现。最常用的材料钛酸钡的介电常数随温度变化很大，因此目前的工业生产依赖于bbb10不同的稀有掺杂剂，如Dy， Ho, Er，以帮助减少介电常数对温度的依赖。最近，由两种不同性能的电容器材料制成的测试片已经被用来证明每种电容器材料的理想性能都可以在单个器件中被利用。例如，由一层BaTiO3 （BT）和一层（Na,Ba）（Nb,Ti）O3 （NNBT）制成的器件可以利用BT的高室温介电常数和NNBT的良好温度稳定性，其组成不那么复杂，使用比最先进的更容易获得的元素。这种复合方法可以提供一条途径，以确保电容器行业免受资源短缺，不稳定和单一地点供应链的影响。无论是在实验室规模的测试件中，还是使用工业规模的放大技术，实现这种新型双阶段方法所面临的挑战都是重大的。大多数陶瓷材料都有高度特定的烧结温度，在那里发生致密化，每种成分都是独一无二的。这种固态烧结也是高能耗的，需要高温（通常为100 - 1200℃）和长停留时间（100 - 8小时），占制造总能源成本的三分之一以上。为了成功共烧结两层材料，它们必须具有非常相似的烧结温度，以确保完全致密化，致密化时的热膨胀和收缩速率相似，以避免开裂或分层，并且在加热过程中不得相互反应，这大大限制了可用材料的选择。因此，需要一种低温烧结方法来充分利用分层器件所提供的机会。2016年宣布了一种这样的低温方法：冷烧结（CS）使用少量的瞬态溶剂（通常是水）来帮助在大大降低的温度下致密化。类似于使糖碗中的糖随着时间的推移形成固体块的过程，冷烧结利用施加压力来加速可溶性氧化物的过程，在低至120摄氏度的温度下在几分钟内实现致密化。虽然CS有可能彻底改变氧化物致密化，但大多数陶瓷的不溶解性阻碍了它的使用。对于不溶性材料，可以调整溶剂，使其包括填充颗粒之间空隙的反应性中间相，一旦加热到适当的温度，就会产生完全致密的陶瓷。这种“反应性”冷烧结（RCS）在<1000℃的温度下工作，避免了与高温加工相关的常见缺陷：化学相容性、不同的收缩率和不同的烧结温度，具有破坏性的工业应用。这个雄心勃勃的项目将使用RCS创建双/三层和混合复合介电陶瓷测试片，创建具有高介电常数和优化温度稳定性的电容器。这将在不依赖于当前的稀有元素和复杂成分的情况下实现，并开发出一种比当前技术状态更低能量的制造技术。将研究该工艺在目前工业使用的厚膜沉积技术中的缩放和应用，作为向工业转移的手段，为复合陶瓷提供变革和创新的新途径。

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.