Protocol for Ultralow-Temperature Ceramic Sintering: An Integration of

Jul 29, 2016 - The sintering process is an essential step in taking particulate materials into dense ceramic materials. Although a number of sintering...
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Protocol for Ultralow-Temperature Ceramic Sintering: An Integration of Nanotechnology and the Cold Sintering Process Hanzheng Guo, Amanda Baker, Jing Guo, and Clive A. Randall* Materials Research Institute, Material Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The sintering process is an essential step in taking particulate materials into dense ceramic materials. Although a number of sintering techniques have emerged over the past few years, the sintering process is still performed at high temperatures. Here we establish a protocol to achieve dense ceramic solids at extremely low temperatures (90% relative density) is accomplished.22,42−44 In the case of BaTiO3, it is normally sintered at 1200−1400 °C for several hours.14,17,42−44 However, in a significant contrast to this conventional route, it is surprising to notice that dense BaTiO3 ceramics of ∼95% relative density can be achieved at an extraordinarily low temperature (180 °C) in a short time period (∼30 min) by employing the cold sintering process. For a clear demonstration, the density evolution of cold-sintered BaTiO3 as a function of sintering time is displayed in Figure 1b, together with the corresponding densities measured after annealing at 10607

DOI: 10.1021/acsnano.6b03800 ACS Nano 2016, 10, 10606−10614

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ACS Nano

Figure 2. XRD spectrum of cold-sintered bulk BaTiO3 ceramic and corresponding spectra obtained after annealing at 700−900 °C: (a) full spectra and (b) magnified parts within specific ranges.

900 °C, indicating a predominance of the tetragonal phase, which is due to a more thorough cubic-to-tetragonal phase transformation. Corresponding to the phase structure and densification evolution, the microstructural insight is systematically demonstrated in Figure 3 via transmission electron microscopy (TEM) imaging. In the cold-sintered ceramic, a composite-like configuration is manifested with the sub-micrometer-sized crystallites being embedded into the nanoparticles, and such a micrograph is contrasted by the particle size difference. Further looking into the nanoparticle “matrix” reveals a hierarchical structure within which the nanoparticles, typically of round shape, are found to be separated/surrounded by a glass phase. Further energy dispersive spectroscopy (EDS) elemental mapping (Supporting Information Figure S1) analysis reveals that (1) the glass phase is carbonate-rich, and this result is consistent with the mass spectrum (shown latter); (2) elements of Ti, Ba, and O are homogeneously distributed in the glass phase without any noticeable aggregations/agglomeration; that means the constituent particles in the Ba(OH) 2 /TiO 2 suspension indeed dissolve in the solution. This observation seems to suggest that the hydrothermal reaction under the presented cold sintering condition adopts the “dissolution− precipitation” mechanism, which proposes that TiO2 particles first dissolve in the aqueous solution to form amorphous hydroxytitanium complexes [Ti(OH)n−] and then react with dissolved barium to precipitate BaTiO3 homogeneously from the solution/glass environment.47 Once the annealing is performed, an apparent precipitation process is triggered, yielding an epitaxial growth of nanoparticles, as well as a crystal growth process, as manifested by the coalescence of nanoparticles into sub-micrometer-sized crystallites. As exemplified in Figure 3e−h for the ceramics after annealing at 800 °C, the nanoparticles are found to remain almost the same size as in their cold-sintered state, but the original round shape gradually evolves into a polygonal configuration with defined facets. Since BaTiO3 is recrystallized from the glass phase, it leads to an epitaxial growth of the nanoparticles and further forms welldefined grain boundaries once the local content of the glass phase is completely consumed. Even though some portion of the glass phase still remains in the whole specimen owing to the incompleteness of the recrystallization, the majority of the glass content has precipitated onto the nanoparticles. Correspond-

900 °C. In the case of cold-sintered ceramics, a relatively low density (1200 °C.14,17,43,44 On the basis of the outlined thermodynamics principles, ceramics with finely tailored microstructures may be expected by appropriately mediating the sizes of nanoparticles and coarse grains through manipulating the surface energy ratio sn̅ m/sμ̅ m at certain sintering temperatures. Together with the thermodynamic considerations, the enhanced kinetic process, as we mentioned at the beginning, should also be accounted for in the densification process. In addition to the liquid phase creep at the solid−solid contacts,33,34 two other types of mechanisms, the Marangoni effect at the liquid−liquid interface and diffusiophoresis at the solid−liquid interface,51−53 might also contribute to an enhanced mass transport. In the case of the Marangoni effect, the mass transport of the solute ions could take place at the liquid−liquid interfaces driven by the concentration gradient, since a local nonuniform solute distribution is triggered between the grain contacts via the pressure dissolution. Also, such a concentration gradient may yield a pressure gradient across the particles due to particle−fluid interactions, such as repulsive steric exclusion or attractive van der Waals interaction, consequently leading to a diffusiophoresis transport of the colloid particles.53 Finally, rather than this being a specific case just for BaTiO3, we have successfully demonstrated the feasibility of applying this methodology to other ceramic materials, such as MgO, which is an important refractory material, and its dense ceramic form is normally sintered above 1400 °C through conventional

(1)

or

μ=

dG ds = μ0 + γ ̅ dn dn

(2)

Equation 2 can be rewritten as

μ = μ0 +

M ds γ̅ ρ dV

(3)

where G = Gibbs free energy, μ = chemical potential, γ ̅ = mean free surface energy (or interface energy) of the interface and is assumed to be independent of particle size, M = formula molar weight, ρ = density, V = volume of a single particle, n = number of moles, and s = surface area of a single particle. The volume and surface area of a single particle of given shape are denoted by V = αd3 and s = βd2, where d denotes a characteristic dimension of the particle size, and α and β are geometry coefficients. Therefore, ds 2s = dV 3V

(4)

Given that the molar surface s ̅ = NAs and the molar volume V̅ = NAV = (M/ρ), where NA is Avogadro’s constant, eq 3 can be readily rewritten as 2 μ = μ0 + γ ̅ s ̅ (5) 3 or ΔG =

2 γ̅s 3 ̅

(6)

Equation 6 quantifies the Gibbs free energy gain (or reduction) of a system when a coarse particle is subdivided into finely divided clusters with molar surface areas (or vice versa). If replacing coarse particles by nanoparticles, the total free energy change of this system is hence given by 2 2 ΔG = γ ̅ Δ s ̅ = γ ̅( snm ̅ − sμ̅ m) (7) 3 3 10612

DOI: 10.1021/acsnano.6b03800 ACS Nano 2016, 10, 10606−10614

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ACS Nano

ASSOCIATED CONTENT

thermal-activated processes. Our preliminary results suggest that dense MgO ceramics of greater than 90% relative density could be achieved at temperature less than 200 °C by mixing the nanoparticles and coarse particles in an appropriate ratio, with the CSP-assisted approach.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03800. Detailed information regarding tables, STEM micrographs, and EDS mapping (PDF)

CONCLUSIONS In summary, we establish a sintering methodology to accomplish highly dense ceramics at an exceptionally low temperature. This technique is an integration of the cold sintering process and nanotechnology where the large surfaceto-volume ratio of the nanoparticles is effectively utilized to benefit the precipitation and Ostwald ripening process. BaTiO3 ceramic is exemplified to demonstrate the feasibility of this method. In conjunction with a comprehensive experimental investigation regarding the processing−structure−property relationships, underlying principles of the thermodynamic driving force are rationalized. More than a case study of BaTiO3, this method could also serve as a more general guidance for ultralow-temperature-sintering processing of a wide range of materials. Such a technique is of significant impact since it suggests a feasible route for cost-effective and energy-sustainable ceramic manufacturing processing and integration that enables a broad range of scientific applications.

AUTHOR INFORMATION Corresponding Author

*E-mail: [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 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.

METHODS

REFERENCES

Ceramic Processing. BaTiO3 nanoparticles (99.9%, 50 nm with cubic phase) were purchased from a commercial source (US Research Nanomaterials, Inc.); sub-micrometer-sized BaTiO3 powders (BT-04) were purchased from Sakai Chemical Industry Co. with a median particle size of 400 nm. 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, and the concentration of Ba(OH)2 was 0.1 mol L−1. To form the ceramic pellet, BaTiO3 μm/nm-sized powders were batched with a 1:1 weight ratio, and the 25 wt % Ba(OH)2/TiO2 suspension mentioned above was added; the mixtures were ground using a pestle and mortar. The mixture was uniaxially pressed under 430 MPa first at room temperature (25 °C) for 10 min, and then the temperature was ramped up to 180 °C with a rate of 9 °C min−1. The temperature was isothermally kept for 1 min to 3 h to obtain a series of samples. The as-prepared ceramic pellets were first baked at 200 °C overnight to further remove possible water residue and then further annealed at 700−900 °C for 3 h with a temperature ramp rate of 5 °C min−1 in air. The densities were measured by Archimedes’ method using acetone as a liquid media. Characterizations. 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 the dielectric properties were measured at 1 kHz to 1 MHz by an LCR meter (HP4284A, Agilent Technologies) during cooling from 200 °C to room temperature at a 2 °C min−1 rate. Thermogravimetric-mass spectrum (TGA-MS Q50, TA Instrument) analysis was performed in a helium atmosphere from 30 to 900 °C at 10 °C min−1. Ceramic powders crushed from the sintered pellets were used. Before heating, the samples were kept at 30 °C for 1 h to reach an equilibrium state. 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 an electron transparent perforation was formed. A cryogenic stage was used to cool the specimen to the liquid N2 temperature during ion milling to minimize structural damage and artifacts. Microstructural and chemical studies were performed on a Talos (FEI, Talos) microscope equipped with an EDS system operating at an accelerating voltage of 200 kV.

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DOI: 10.1021/acsnano.6b03800 ACS Nano 2016, 10, 10606−10614