Hydrothermal-Assisted Cold Sintering Process: A New Guidance for

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Hydrothermal Assisted Cold Sintering Process: A New Guidance for Low Temperature Ceramic Sintering Hanzheng Guo, Jing Guo, Amanda Baker, and Clive A. Randall ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07481 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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ACS Applied Materials & Interfaces

Hydrothermal Assisted Cold Sintering Process: A New Guidance for Low Temperature Ceramic Sintering Hanzheng Guo*, Jing Guo, Amanda Baker, and Clive A. Randall * Materials Research Institute, The Pennsylvania State University, University Park 16802, Pennsylvania, U.S.A. ABSTRACT: Sintering is a thermal treatment process that is generally applied to achieve dense bulk solids from particulate materials below the melting temperature. Conventional sintering of polycrystalline ceramics is prevalently performed at quite high temperatures, normally up to 1000 ºC to 1200 oC for most ceramic materials, typically 50% to 75% of the melting temperatures. Here we present a new sintering route to achieve dense ceramics at extraordinarily low temperatures. This method is basically modified from the Cold Sintering Process (CSP) we developed very recently by specifically incorporating the hydrothermal precursor solutions into the particles. BaTiO3 nano polycrystalline ceramics are exemplified for demonstration due to their technological importance and normally high processing temperature under conventional sintering routes. The presented technique could also be extended to a much broader range of material systems that have previously been demonstrated via a hydrothermal synthesis using water or volatile solutions. Such a methodology is of significant importance, since it provides a chemical roadmap for cost-effective inorganic processing that can enable broad practical applications.

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KEYWORDS: cold sintering process, barium titanate nanoparticles, hydrothermal-assisted sintering, dense ceramics, low-temperature sintering INTRODUCTION Sintering refers to a process that is utilized to form a dense solid typically assisted by thermal energy and/or pressure.1-4 It is a major manufacturing approach that has been widely applied to a diverse range of materials.5-10 Even though sintering has been extensively studied in the modern era of ceramic science for over five decades, and numerous new technologies have been developed,11-15 in the case of conventional thermal sintering of ceramic materials, they are still accomplished at high temperatures, as a rule of thumb, ~ 50%-75% of their melting points.1 Very recently, we developed a Cold Sintering Process (CSP) to achieve dense ceramic solids at incredibly low temperatures (< 200 ºC) across a wide variety of chemistries and composites. 16 , 17 The CSP is basically a low-temperature liquid phase sintering process via utilizing water as a transient solvent under a uniaxial pressure. In a typical CSP, the ceramic powders are uniformly moistened with a small amount of water solution; the solid surfaces decompose and are partially dissolved in water, so that a controlled amount of liquid phase is intentionally introduced at the particle-particle interfaces. This can be accomplished by simply mixing in a few drops of water and/or water-ethanol solutions, and utilizing a high shear mixing process or exposing the powders or powder ensemble to a controlled relatively humid atmosphere. The dissolution of sharp edges and points of solid particles reduces the interfacial areas, and capillary forces aid a rearrangement and consolidation in the first stage. With the assistance of sufficiently high external and capillary pressure, the liquid phase redistributes itself and fills the pores between the particles. Applying a uniaxial pressure, solid particles rearrange rapidly, which collectively leads to an initial densification. The subsequent growth stage, often


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referred to as “dissolution-precipitation”, is created through water evaporation that enables a supersaturated state of the liquid phase at a low temperature right above 100 ºC, triggering a large chemical driving force for the solid and liquid phases to reach high levels of densification. After a careful analysis of the chemistries we have accomplished, we noticed that a congruent dissolution in water at low temperature is the key to enable a fast “dissolutionprecipitation” process, which is determinant to the ceramic densification. Unfortunately, incongruent dissolution is prevalent in a large number of multicomponent materials, among which they also have limited solubility in water, especially for the close-packed structures in which the atoms/molecules/ligands have strong chemical bonds. A well-known example is BaTiO3, which is not thermodynamically stable in aqueous environment of pH < 12.18,19 As the BaTiO3 particle reacts with water, Ba ions are preferentially leached out of the surface area, resulting in a Ba depleted passivated layer that is Ti-rich and amorphous.18,20 This amorphous layer is detrimental for the precipitation process, since it physically separates the saturated solution and crystal surface with the active sites. It therefore has a barrier that impedes crystal growth from the supersaturated solution, limiting the mass transport to the surface for epitaxial growth. Also, our initial experiments indeed verified that applying just a water solution to the powders, as typically enables CSP, did not enable densification of the BaTiO3 ceramics. In order to successfully extend the CSP to materials with incongruent dissolution, here we discuss a modified CSP approach that limits the incongruent dissolution process and permits a possibility of low temperature densification. BaTiO3 in the form of nanocrystalline ceramic is employed for the demonstration due to the following reasons: (1) BaTiO3 is unarguably one of the most important functional electroceramic materials, particularly as the basic material for the multilayer ceramic capacitor (MLCC) industry, where over 3x1012 devices are manufactured



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each year and underpin electrical systems in our modern society;21-24 (2) dense BaTiO3 ceramic is generally obtained at ~1200-1400 ºC by conventional thermal sintering;25,26 and (3) compared to micrometer-sized powders, BaTiO3 nanoparticles are generally more chemically reactive, due to their high surface energy. The strategies we adopted are summarized as follows: (1) high quality BaTiO3 nanoparticles are employed as starting powders; our transmission electron microscope (TEM) study suggests that these nanocrystallites are well crystallized without noticeable amorphous phase on their surfaces (Supporting Information Figure S1), and the chemical species are uniformly distributed as well (Supporting Information Figure S2); (2) the liquid phase is always maintained in a supersaturated state with a high enough amount of the Ba ion source so that the dissolution of Ba from BaTiO3 surface is largely inhibited; and (3) as inspired by the hydrothermal synthesis of BaTiO3, Ti source is also added to the liquid phase in order to form BaTiO3, since extensive hydrothermal syntheses studies have clearly suggested that formation of BaTiO3 could be achieved at low temperatures, from room temperature to 300 ºC, by utilizing simple compounds of Ba and Ti.27-32 In this study, the densification stage is outlined with a number of processing variables, and the nature of the microstructure is discussed. We then considered additional thermal processes to design in the dielectric properties of BaTiO3 nanocrystalline ceramics.

EXPERIMENTAL SECTION Ceramic Processing. BaTiO3 nanoparticles (99.9%, 50nm with cubic phase) were purchased from a commercial source (US Research Nanomaterials, Inc.). The Ba(OH)2/TiO2 suspension was made by mixing corresponding chemicals with deionized water. The molar ratio of Ba(OH)2:TiO2 was 1.2:1 to balance the overall stoichiometry, since the quantification analyses of our EDS mapping results on the raw nanoparticles suggested an 1.2:1 ratio of Ti:Ba. The


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concentration of Ba(OH)2 was 0.1 mol L-1. To form the ceramic pellet, 0.14 - 0.15 g Ba(OH)2/TiO2 suspension was added to 0.56 g BaTiO3 nanoparticles; the mixtures were ground using a high shear process, using a pestle and mortar. The mixture was then poured into a die and uniaxially pressed under 430 MPa, first at room temperature (25 ºC) for 10 min, and then the temperature was increased to 180 ºC with a rate of 9 ºC min-1. The temperature was isothermally kept for 1 min to 3 h to obtain a series of samples. The as-prepared ceramic pellets (approximate ½ inch in diameter and ~ 1 mm in thickness) were first baked at 200 ºC overnight to remove possible water residue, and then further annealed at 700 – 900 ºC for 3 h with a temperature ramp rate of 5 ºC min-1 in air. The densities were measured by Archimedes’ method, using acetone as a liquid media. Characterization. The phase structures were checked by X-ray diffraction (Panalytical, X’Pert PRO) with Cu-Kα radiation. For dielectric measurements, platinum was sputtered as electrodes, and dielectric properties were measured at 1 kHz – 1 MHz by an LCR meter (HP4284A, Agilent Technologies) during cooling from 200 ºC to room temperature at a 2 ºC min-1 rate. Thermogravimetric-Mass Spectrum (TGA-MS Q50, TA Instrument) analysis was performed in a helium atmosphere from 30 to 900 ºC at 10 ºC min-1. Ceramic powders crushed from sintered pellets were used. Before heating up, the samples were kept at 30 ºC for 1 h. Transmission electron microscopy (TEM) specimens were prepared via standard procedures, including mechanical thinning, polishing, and ion milling. The specimens were polished down to ~30 µm thick and then mounted on molybdenum grids. The foils were further thinned with an Ar-ion mill (Gatan, PIPS II) until electron transparent perforations were formed. A cryogenic stage was used to cool the specimen to liquid N2 temperature during ion milling so as to minimize structural damage and artifacts. Microstructural and chemical studies were performed



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on a Talos (FEI, Talos) microscope equipped with an Energy Dispersive X-ray Spectroscopy (EDS) system operating at an accelerating voltage of 200 kV.

RESULTS AND DISCUSSION

Figure 1. (a)-(c) XRD patterns of cold-sintered bulk BaTiO3 ceramic, and after annealing at 700-900 ºC. Impurity phase ~24º is outlined by the dash circle in (b). (d) TGA-MS plot for coldsintered BaTiO3 ceramic from 30-900 ºC. Four peaks are marked as P1-P4 on the derivative weight loss curve. (e) Density evolution of cold-sintered and subsequently annealed BaTiO3 ceramics as a function of cold sintering time at 180 ºC. Figure 1a displays the phase structure evolution of cold-sintered BaTiO3 ceramics and those after subsequent post annealing at 700 – 900 ºC. Details within a specific range are also magnified as Figures 1b and 1c for better illustration. In the as-cold-sintered BaTiO3 pellet, an


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impurity phase is identified, as circled by the dash line (Figure 1b). It has been commonly reported that BaCO3 generally appears as a byproduct during hydrothermal synthesis of BaTiO3, since a certain amount of barium species react with CO2 at low temperatures.29,33 To this point, it is reasonable to deduce that the impurity phase (~ 24º) coincided with the {111} peak of the XRD spectrum of BaCO3 is most likely due to the formation of BaCO3 through a chemical reaction between Ba(OH)2 and the CO2 source taken from the atmosphere. To improve the phase purity, a post annealing process, as generally done in the literature,29 is carried out between 700 900 ºC. As expected, the annealing process effectively removes the impurity phase through facilitating the formation of BaTiO3; all spectra profiles after annealing perfectly match with the desired perovskite structure. From the crystal symmetry perspective, the cubic phase seems to be maintained after annealing at the temperatures ≤ 800 ºC, but an apparent cubic-to-tetragonal phase transformation occurs after annealing at 900 ºC, as indicated by a peak splitting ~ 45 º. This crystallographic evolution from cubic to tetragonal symmetry is consistent with the literature.29 Figure 1d illustrates the thermogravimetric property of the cold-sintered ceramic during annealing process. Even though only a total weight loss of ~ 1.8% is observed, sharp changes can still be detected at different temperature stages, and this can be more easily identified when a weight loss derivative with respective to temperature is considered, as marked by peaks P1 - P4. With the assistance of mass spectroscopy, these spectral peaks perfectly correlate with the decomposition and evaporation of two chemical species, the OH- (or H2O) and CO2. Firstly, the water vapor comes off at ~ 100 ºC, which might be attributed to the water detachment from the surface areas of ceramic powders. Upon further heating to ~ 300 ºC, the detection of OHsuggests a decomposition of a hydroxide species. A subsequent heating process leads to a



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consecutive releasing of CO2, which is primarily observed at two temperature windows centered ~ 520 ºC and ~ 780 ºC, and the decomposition of a carbonate species. These results suggest that the chemical reactions are almost complete at ~ 900 ºC, and the annealing process will be most likely to affect the density development of the ceramics. To investigate this, Figure 1e displays the density evolution of cold-sintered BaTiO3, as well as corresponding ceramic pellets after annealing at 900 ºC, as a function of cold sintering time. Both curves show a similar trend with two notable stages: the ceramics cold-sintered in less than 30 min exhibit low density; a boost appears once the sintering time is elongated to 30 min, and the density curve keeps an almost plateau configuration after that. It is interesting to notice that the density of BaTiO3 ceramics prepared by the cold sintering process can even reach ~ 5.6 g cm-3 (~ 93% relative density if a theoretical density of 6.02 g cm-3 is adopted) at a surprisingly low temperature (< 200 ºC) but also in a short time period (~ 30 min). These two density evolution curves unambiguously indicate that the cold sintering process is the determinant to the final density, even though the density can be slightly improved ~ 2% by a post annealing at relatively low temperatures of 700 – 900 ºC (see Table S1 in Supporting Information for the density evolution at this temperature range) compared to the conventional thermal sintering temperatures of 1200 – 1400 ºC for BaTiO3.25,26 In response to the structural evolution demonstrated in Figure 1, dielectric properties of the CSP produced ceramics are also investigated in regard to the annealing temperatures, as displayed in Figure 2 during a cooling process starting at the temperature (200 ºC) beyond the well-known Curie transition of BaTiO3 at ~ 120 ºC.25 The as-cold-sintered ceramic shows a lossy dielectric behavior with low relative dielectric permittivity ~ 70 (1 kHz frequency) at room temperature; and the original cubic phase remains, as evidenced by the lack of the dielectric


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anomaly at the phase transition, such as a Curie-Weiss transition peak (Figure 2a). However, the post annealing process triggers a formation of tetragonal phase, as manifested by the emergence of a dielectric abnormality on the dielectric spectra. The diffuse nature of this dielectric abnormality suggests that the cubic-to-tetragonal phase transition is limited by the portion of tetragonal phase developed after annealing at a temperature ≤ 800 ºC. This fact also explains the XRD results showing no obvious peak; splitting on the ceramics after annealing under similar heat treatment conditions. Further annealing at 900 ºC leads to a more completed cubic-totetragonal phase transformation, hence contributing to the appearance of a prominent and sharp dielectric peak (Figure 2d). In addition to the significant influence on the crystal phase evolution, the post annealing process is also effective for the dielectric constant improvement; in the case of the value at room temperature (1 kHz), it is initially increased from ~70 to ~ 480 after annealing at 700 ºC, and then rises to ~ 1800 if further annealed at 900 ºC.



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Figure 2. Temperature dependent dielectric properties of (a) cold-sintered ceramics at 180 ºC, and subsequently annealed at (b) 700 ºC, (c) 800 ºC, and (d) 900 ºC.


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Figure 3. TEM micrographs of the microstructural evolution of BaTiO3 ceramics after (a)-(c) cold sintering at 180 ºC, (d)-(f) annealing at 700 ºC, and (g)-(i) annealing at 900 ºC. Micrographs with 3 different magnifications are provided for each case to show the overall feature and the region around grain boundaries. The grain boundaries (GB) are marked by the bright triangles. To better illustrate the processing-structure-property relationship, the microstructural evolution is further investigated, as shown in Figure 3 through a systematic TEM imaging. Overview of the microstructural characteristics of cold-sintered ceramics are also provided in Figure S3 with SEM and low magnification TEM micrographs. In the case of as-cold-sintered



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ceramic, round-shaped grains are typically manifested and are found to be surrounded by a large amount of glass phase (Figures 3a, 3b). Our chemical mapping analysis (Supporting Information Figure S4) reveals that the glass phase is carbonate-rich, which is consistent with the XRD and TGA-MS results shown in Figure 1. Most importantly, Ti and Ba are found to be uniformly distributed in the glass phase, which means the initial additives of Ba(OH)2 and TiO2 indeed react with each other and/or with the CO2 during the cold sintering process. Within the glass phase, short-range ordered lattices in a few nanometers scale are observed, as outlined by a bright circle in Figure 3c. These tiny crystallites might be owing to the formation of (1) some intermediate compounds, such as BaCO3; or (2) the precipitates of BaTiO3, since the previous literature suggests that nanocrystalline BaTiO3 can be hydrothermally synthesized through mixing Ba(OH)2 and TiO2 in water solution in a temperature range of 25-300 ºC.27-32 As the post annealing is applied, it triggers a recrystallization of BaTiO3 at the expense of the glass phase. The round shape of the grains gradually evolves into a polygonal configuration (Figures 3d, 3e), and the facets become clearer as the annealing temperature increases (Figures 3g, 3h). After annealing at 700 ºC, well-defined grain boundaries are developed (Figure 3f), but some portion of the amorphous phase still occupies the interstitial space among the grains (Figure 3e). Due to the substantial reduction of the amorphous phase, the content of carbonate is significantly suppressed (Supporting Information Figure S5). Further improvement of the microstructure is achieved via the recrystallization process at 900 ºC (Figures 3g, 3h): the microstructure is overwhelmingly dominated by angular-shaped grains, and a well-crystallized grain boundary is also developed (Figure 3i); correspondingly, the glass phase is nearly exhausted, and the carbonate is mostly decomposed (Supporting Information Figure S6). After comparing the microstructures before and after annealing (Figures 3a, 3g), a certain extent of crystal growth is


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noticed, but such a ripening process is still limited to nanoscale. Correspondingly, tweed-like structures, as traces of domain wall formation, are observed within relatively larger grains (Figure 3h). It has been generally known that the appearance of ferroelastic domain walls in BaTiO3 is highly crystallite size dependent, which is more favorable to emerge in µm/sub-µm sized grains rather than in a nanocrystallite environment.34,35 Such observation also indicates that a notable portion of tetragonal phase has been formed, which is consistent with the XRD results (Figure 1c) and also contributes to the well-defined Curie transition on the dielectric spectrum.36

Figure 4. Schematic illustration of the primary stages during cold sintering and post annealing processes in the case of BaTiO3 nanocrystalline ceramics preparation. On the basis of the experimental observations shown above, the underlying mechanism and primary stages during cold sintering and relative annealing process in BaTiO3 nanoceramics is summarized, as schematically illustrated in Figure 4. BaTiO3 nanoparticles are first homogeneously wetted with a water suspension containing the constituents for hydrothermal



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synthesis of BaTiO3. With the assistance of external pressure, the liquid phase redistributes itself and fills the pores between the particles, aiding particle compaction and rearrangement. Raising the temperature facilitates the hydrothermal reactions to generate a glass phase. Once the cold sintering is performed at the temperature above the boiling point of water, a non-equilibrium dynamic environment is created and always preserved until the water content is completely consumed. As the water vapor comes off the ceramic ensemble, further compaction proceeds under applied external pressure. As time elapses, the BaTiO3 nanoparticles are tightly glued by the newly formed glass phase, and a dense (~93% relative density comparing to BaTiO3) crystal/glass “composite” is obtained at the end of the cold sintering process.

Once post

annealing is applied, a large chemical driving force is triggered for the crystalline and glass phases to reach an equilibrium state; corresponding ionic species and/or atomic clusters (ligands) in the glass phase precipitate on BaTiO3 crystallites with lower chemical potential, as they are thermodynamically more favorable. When the precipitation process proceeds, the shape of the crystallite accommodates: a round configuration is generally manifested when the glass phase is prevalent, while polyhedra with flat facets are normally developed when the volume of glass phase is significantly reduced. Simultaneously, mass transport during this process minimizes the excess free energy of the surface area and removes surface and porosity; the areas of crystallitecrystallite contacts increase, leading to the formation of a rigid particulate skeletal network, and also resulting in a further improvement of the relative density to ~ 95%. It has been known that the hydrothermal synthesis of BaTiO3 is a complicated process, and the chemical reaction path highly depends on the hydrothermal conditions.37-39 Even though the mechanism for hydrothermal synthesis of BaTiO3 still remains controversial,37-39 two mechanisms have been primarily proposed: the first one is the “in-situ transformation (or


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diffusion reaction) mechanism”,37,38 which assumes that the chemical reaction is initiated at the surface of TiO2 particles and triggers a heterogeneous nucleation process; the dissolved barium diffuses into TiO2, resulting a continuous layer of BaTiO3 until TiO2 is completely consumed. The other one is the “dissolution-precipitation mechanism”,39 which suggests that TiO2 particles first dissolve in an aqueous solution to generate amorphous hydroxytitanium complexes [Ti(OH)n-], and then react with dissolved barium to precipitate BaTiO3 homogeneously from the solution/glass environment. In considering our chemical mapping observations (Supporting Information Figure S4), the Ti element is found to be uniformly distributed in the glass phase. From this point of view, it seems to suggest that the presented cold sintering process most likely takes place via the dissolving-precipitation path aided by an epitaxial growth of the BaTiO3 surfaces.

CONCLUSIONS In summary, we demonstrate an improved CSP route to achieve dense ceramics with incongruent dissolution issue. Such a method is an integration of recently developed cold sintering process and long-standing hydrothermal synthesis. The feasibility of this method is illustrated in the case of BaTiO3 nanocrystalline ceramics. A dense ceramic solid is successfully obtained at extraordinarily low temperature, in contrast to the traditional thermal sintering generally performed at high temperatures. Our experimental observations suggest that a highly dense crystal/glass compact (~ 93% relative density compared to BaTiO3) is first obtained at a surprisingly low temperature of 180 ºC; then, post heat treatment leads to a thorough crystallization and further improves a relative density to ~ 95%. The processing-structureproperty relationship has been well illustrated, as elaborated by systematic characterizations



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regarding the phase structure, thermal property, density development, dielectric properties, and microstructure/microchemical evolutions. The fundamental mechanism and basic stages of this process is also outlined in the end. Rather than a case study of BaTiO3, the demonstrated process is significantly important in establishing a fundamental methodology to sinter dense ceramic materials at extremely low temperatures via combining the cold sintering process and their hydrothermal synthesis routes.

ASSOCIATED CONTENT Supporting Information Detailed information regarding TEM micrographs, EDS mapping, and supporting figures. This material is available free of charge via the Internet at ACS Publications website. AUTHOR INFORMATION Corresponding Authors * Email: [email protected]; [email protected] Author Contributions C.A.R. and H.G. conceived the idea and designed the experiments. C.A.R. supervised the project. H.G. performed the ceramic processing, characterizations, and thermodynamics analysis. A.B and J.G. conducted the initial experiment with water. A.B. performed the cold sintering for coarse ceramic particles. H.G. and C.A.R. analyzed the data and wrote the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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The authors would like to thank Joanne E. Aller for a proof reading. H. Guo would like to thank Wesley Auker (MCL at The Pennsylvania State University) for the SEM imaging. This material is based upon work supported by the National Science Foundation, as part of the Center for Dielectrics and Piezoelectrics under Grant Nos. IIP-1361571 and 1361503. REFERENCES (1) Rahaman, M.N. Sintering of Ceramics. (CRC Press, Boca Raton, 2008). (2) German, R.M. Sintering Theory and Practice. (Wiley, New York, 1996). (3) Kang, S. J. L. Sintering: Densification, Grain Growth and Microstructures (Elsevier, Oxford, 2004). (4) Gutmanas, E.Y. Cold Sintering under High Pressure-Mechanisms and Application. Powder Metall. Int. 1983, 15, 129-132. (5) Chen, I.-W.; Wang, X.-H. Sintering Dense Nanocrystalline Ceramics without Final-Stage Grain Growth. Nature 2000, 404, 168-171. (6) Liu, G.; Zhang, G.J.; Jiang, F.; Ding, X.D.; Sun, Y.J., Sun, J.; Ma, E. Nanostructured HighStrength Molybdenum Alloys with Unprecedented Tensile Ductility. Nat. Mater. 2013, 12, 344350. (7) Chaim, R.; Shlayer, A.; Estournes, C. Densification of Nanocrystalline Y2O3 Ceramic Powder by Spark Plasma Sintering. J. Eur. Ceram. Soc. 2009, 29, 91-98. (8) Goodridge, R.D., Tuck, C.J.; Hague, R.J.M. Laser Sintering of Polyamides and Other Polymers. Prog. Mater. Sci. 2012, 57, 229-267. (9) Inam, F.; Yan, H.; Jayaseelan, D.D.; Peijs, T.; Reece, M.J. Electrically Conductive AluminaCarbon Nanocomposites Prepared by Spark Plasma Sintering. J. Eur. Ceram. Soc. 2010, 30, 153157.



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Table of Contents Graphic


Hydrothermal-Assisted Cold Sintering Process: A New Guidance for

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