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Low-Temperature Fabrication of Ceramics by the Cold Sintering Process

Sintering transforms a shaped powder compact into a dense solid by applying high temperatures, typically >1000 °C. It is a vital step in the fabrication of ceramics, which dictates the microstructure, and hence, the final structural and functional properties of the sintered parts. However, the application of high temperatures poses several challenges for ceramic manufacturing including: I) high energy consumption and carbon footprint, II) loss of volatile elements, resulting in the degradation of functional properties, III) grain growth and limited microstructural control, and IV) limited materials integration possibilities for composite systems. Therefore, novel sintering techniques that can lower the sintering temperature have attracted the focus of recent research. The recently developed Cold Sintering Process (CSP) (Fig. 1) has successfully enabled ultralow-temperature densification of ceramics at temperatures below 350 °C by utilising chemical pathways and applied pressure.

Introduction
Sintering is one of the oldest materials manufacturing methods with historical evidences dating back to about 26 000 years ago. Sintering is a powder-based processing method, which densifies a particulate material into a monolithic dense solid by applying thermal treatment at temperatures around 2/3 of the material’s melting point. Since ceramic materials have a high melting point, they require sintering temperatures typically above 1000 °C to bond the particles together through solid-state diffusion and remove porosity. Sintered materials exhibit high densities, up to 99,9 % of theoretical density, improved structural integrity and dimensional stability. However, such high temperatures may lie outside the stability window of the compounds and have detrimental effects on the functional material properties, as well as negative environmental impact of the ceramic manufacturing industry. From a materials engineering point of view, high processing temperatures pose several challenges, such as loss of volatile elements reaction and interdiffusion between different phases in composite systems, and very importantly limited integration possibilities between different materials. In particular, the wide difference in the processing temperature between ceramics, metals and polymers, precludes their integration into single composite material due to oxidisation, decomposition and/or chemical reaction effects during sintering. Nowadays, sintering is carried out on the industrial scale in batch or continuous furnaces which consume high amount of energy associated with the need to heat a large thermal mass (the furnace and materials to be sintered) to high temperatures and hold these temperatures for long dwell times (several hours). This makes the sintering process not only energy-intensive but also a significant contributor to global carbon emissions, especially caused by firing with natural gas [1]. These challenges have incentivised research toward developing alternative, energy-efficient sintering techniques [2]. It is generally recognised that high temperatures are required to activate atomic diffusion which is the key for sintering to occur. Therefore, increasing the efficiency has been targeted by directly applying heat into the green body at fast heating rates, and thereby, minimising heat losses. Examples for such techniques are microwave sintering, ultrafast high-temperature sintering and photonic sintering. On the other hand, lowering the sintering temperature is possible by enhancing the driving force for sintering and accelerating densification, which can be achieved by applying external pressure or adding a liquid phase as in hot pressing and liquid phase sintering. Field Assisted Sintering (FAST/SPS) enhances densification by a combined action of direct heating through electric field and pressure application. Although much progress has been achieved in reducing the sintering temperature, all these techniques still operate at relatively high temperatures (>700 °C) above those required for co-sintering of some complex composite materials. The recently developed CSP has enabled a dramatic reduction in the sintering temperature of various ceramic systems to below 350 °C and can achieve high densification in less than 1 h [3]. Even though at this temperature range solid-state diffusion is unlikely to occur, CSP utilises pressure and chemical pathways to promote atomic diffusion. CSP has unlocked new design strategies and materials combinations which were previously unfeasible, such as co-integration of ceramics with polymers, nanomaterials and metals. Owing to the significantly low sintering temperatures, the CSP has the potential to decarbonise the ceramic industry and open the door to innovative new materials development. 

The principle
The basic driving forces remain the same as in all forms of sintering in that a compaction of powders have high surface energies. This surface energy can be reduced by coarsening of powders, i.e. grain growth process, and/or by the formation of lower energy interfaces in the form of grain boundaries, driving densification and pore elimination. CSP utilises a chemical route and applied pressure, in addition to the driving forces related to the excessive surface energy, to enable densification at low temperatures. Even if the exact mechanisms are not completely elucidated and may depend on the selected material/powder, similarities with the so-called pressure solution creep mechanism can be highlighted [4]. This mechanism enables the geological solidification of sedimentary rocks; however, these are formed naturally over hundreds of thousands to millions of years. In CSP, a careful selection of chemistries, particle size distribution, and use of sufficiently high pressures enables densification of powders at low temperatures in short time cases that are from minutes to a few hours. The selection of the transient chemical phase is a critical aspect, as it must permit dissolution at the stressed particle-particle contacts, as well as precipitation of the dissolved species. First, compaction should be promoted by particle rearrangement, which is mediated by the lubricating effect of the transient liquid phase allowing easier rotation and sliding under pressure. Second, the modified surfaces and interfaces (especially at stressed particle-particle contacts) should facilitate the transport of atoms from the grains/grain boundaries to the pores, leading to further densification. Third, the transient phase should disappear in the final sintered state. These three stages must work in harmony to enable the CSP to take place effectively and form high density materials. The transient chemical phases must be carefully considered with respect to the system that is to undergo CSP. It can be in the form of aqueous solutions of acidic or basic character, chelating agents or hydrated solids such as acetates, and hydroxides [5]. 

Different systems and potential applications
Owing to the newly unlocked low-temperature processing window for ceramics and the simple equipment required for CSP, the international research community has expressed increasing interest in exploring novel composite materials with potential applications. Over a hundred different materials have now been densified with the CSP. These materials range from materials for energy conversion and storage, electroceramics, structural materials to refractory materials. Although most of the materials have been oxides, other non-oxides have been successfully sintered including metals. In principle, the CSP should work on a wide range of materials provided that the appropriate transient phase is found. In some cases, final properties might be adjusted by a post-annealing treatment [2]. Another important opportunity for the low sintering temperatures with CSP is the ability to fabricate new composite materials with uniquely designed grain and phase boundaries. There have been several works demonstrating the integration of a few volume percent of dispersed polymers into the grain boundaries of high-density ceramics. Fig. 2 shows an example of integrating a Polytetrafluoroethylene (PTFE) polymer into the grain boundaries of BaTiO3 ceramic, enabled by cold sintering at only 150 °C [6]. Moreover, densification of ceramic composites consisting of 2D materials, such as Buckminsterfullerene C60 (bucky balls) and nano carbon fibres has been demonstrated. Many of these materials would simply decompose or chemically react with the matrix phase at the high temperatures of conventional sintering. These types of nanocomposites with special grain boundary designs are an exciting advance as it can lead to new electrical, thermal, and mechanical properties. Multilayer structures with different types of materials have also been demonstrated in several electronic applications including multilayer thermoelectrics, dielectrics, semiconductors, and all solid-state batteries. In these prototypes the conventional thick film forming techniques of tape casting and screen printing were used prior to the densification with CSP. An example for the fabrication of a multilayer varistor (MLV) with CSP is shown in Fig. 3 [7]. The MLV consists of layers of ZnO ceramic with polyetherimide polymer integrated into the grain boundaries. These active ceramic/polymer composite layers were successfully co-sintered with Cu electrodes using CSP without decomposing the polymer or reacting with the Cu metal electrodes. This shows the potential of CSP in enabling the fabrication of unique materials combinations, providing a common platform for a one step processing of distinct material classes such as ceramics, polymers, and metals. 

Manufacturing prospects
The extremely low sintering temperatures and the related advantages of sustainability and enabling co-sintering of different materials, make the CSP very attractive for industrial implementation. Yet, it will demand increasing the productivity of the process by scaling up the number of parts of various geometries and sizes to be sintered in a single cycle. In a recent work [8], scaling up has been successfully demonstrated by cold sintering multiple discshaped samples simultaneously, reaching high density and mechanical strength, comparable to conventionally sintered advanced ceramics. The quality of cold sintered parts, in terms of densification homogeneity and macroscopic defects detection, was demonstrated through spatially resolved ultrasonic nondestructive testing, which may guide the production as a quality control tool [8]. CSP has been successfully extended to other geometries (Fig. 4). Homogenous densification has been obtained by optimising the chemistry, sintering parameters and tooling quality of the cold sintering setup. Ceramic parts with a diameter of 13 mm and heights ranging between 1 mm and 4,3 cm have been successfully cold sintered to high relative densities approaching 97 % at temperatures as low as 150 °C. Moreover, cold sintering of samples with a diameter of 3 cm and thickness of 2 mm has been proven feasible (Fig. 4). 

Concluding remarks
As a community ranging from academy, industry, national laboratories and government agencies we have to continue to find sustainable manufacturing processes. New innovations continue to explore manufacturing options around the high temperature process of sintering, and this is in the form of hybrid cold sintering and spark plasma sintering approaches that provides more efficient heating and cooling cycles, which would further enhance the efficiency of the process. National research programs around the world covering sustainable manufacturing are starting to integrate cold sintering into their studies, and a number of these shall include development of scalable processing machinery. 

Acknowledgments
Part of the funding for this research was provided by the European Research Council (ERC) excellent science grant CERATEXT through the Horizon 2020 Program under contract 817615. Abdullah Jabr acknowledges the Austrian Marshall Plan Foundation for the financial support during his stay at Penn State University. Clive Randall would like to thank NSF_FMSG (2134643) Program for partial support of this work. 

References
 [1] Cerame-Unie, Ceramic Roadmap to 2050. Continuing Our Path towards Climate Neutrality, The European Ceramic Industry Association, Brussels, Belgium, 2021

[2] Guillon, O.; Rheinheimer, W.; Bram, M.: A perspective on emerging and future Sintering technologies of ceramic materials. Adv. Engin. Mater. (2023) 2201870 

[3] Guo, J.; et al.: Cold sintering: A paradigm shift for processing and integration of ceramics. Angew. Chem. Int. 55 (2016) [38] 11457– 11461 

[4] Ndayishimiye, A.; et al.: Reassessing cold sintering in the framework of pressure solution theory. J. Europ. Ceram. Soc. 43 (2022) [1] 1–13 

[5] Randall, C.A.; et al.: Cold sintering ceramics and composites; US 10,730,803 B2; 2020 

[6] Sada, T.; et al.: High permittivity BaTiO3 and BaTiO3-polymer nanocomposites enabled by cold sintering with a new transient chemistry: Ba(OH)2∙8H2O. J. Europ. Ceram. Soc. 41 (2021) [1] 409–417 

[7] Dursun, S.; et al.: A route towards fabrication of functional ceramic/polymer nanocomposite devices using the cold sintering process. ACS Appl. Electron. Mater. 2 (2020) 1917–1924 

[8] Jabr, A.; et al.: Scaling up the cold sintering process of ceramics. J. Europ. Ceram. Soc. 43 (2023) [12] 5319–5329


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