An official website of the United States government
Cold sintering densification and coarsening mechanisms are considered from the perspective of the nonequilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolutionprecipitation creep). This is an important mechanism of densification and deformation in many geological rock formations in the Earth’s upper crust, and although very slow in nature, it is of direct relevance to the cold sintering process. In cold sintering, we select particulate materials and identify experimental processing parameters to significantly accelerate the kinetics of dissolution-precipitation phenomena, with appropriate consideration of chemistry, applied stress, particle size and temperatures. In the theory of pressure solution, pressure-driven densification is considered to involve the consecutive stages of dissolution at grain contact points, then diffusive transport along the grain boundaries towards open pore surfaces, and then precipitation, all driven by chemical potential gradients. In this study, it is shown that cold sintering of BaTiO3, ZnO and KH2PO4 (KDP) ceramic materials proceeds by the same type of serial process, with the pressure solution creep rate being controlled by the slowest kinetic step. This is demonstrated by the values of activation energy (Ea) for densification, which are in good agreement with the existing literature on dissolution, precipitation, or coarsening. The influence of pressure on the morphology of ZnO grains also supports the pressure solution mechanism. Other characteristics that can be understood qualitatively in terms of pressure solution are observed in the in systems such as BaTiO3 and KDP. We further consider activation energies for grain growth with respect to the precipitation process, as well as evidence for coalescence and Ostwald ripening during cold sintering. For completeness we also consider materials that show significant plastic deformation under compression. Our findings point the way for new advances in densification, microstructural control, and reductions in cold sintering pressure, via the use of appropriate transient solvents in metals and hybrid organic-inorganic systems, such as the Methylammonium lead bromide (MAPBr) perovskite.
more »
« less
Anisothermal densification kinetics of the cold sintering process below 150 °C
Cold sintering is an emerging non-equilibrium process methodology that densifies ceramic powders at significantly reduced temperatures. This study proposes a fundamental framework to investigate its densification kinetics. By controlling four densification process variables including the transient chemistry, sintering temperature, uniaxial pressure and dwell time, the anisothermal sintering kinetics of highly densified ZnO is identified and phenomenologically modeled for its relative activation energetics.
more »
« less
- Award ID(s):
- 1728634
- PAR ID:
- 10149613
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- Volume:
- 8
- Issue:
- 17
- ISSN:
- 2050-7526
- Page Range / eLocation ID:
- 5668 to 5672
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Cold sintering is an unusually low-temperature process that uses a transient transport phase, which is most often liquid, and an applied uniaxial force to assist in densification of a powder compact. By using this approach, many ceramic powders can be transformed to high-density monoliths at temperatures far below the melting point. In this article, we present a summary of cold sintering accomplishments and the current working models that describe the operative mechanisms in the context of other strategies for low-temperature ceramic densification. Current observations in several systems suggest a multiple-stage densification process that bears similarity to models that describe liquid phase sintering. We find that grain growth trends are consistent with classical behavior, but with activation energy values that are lower than observed for thermally driven processes. Densification behavior in these low-temperature systems is rich, and there is much to be investigated regarding mass transport within and across the liquid-solid interfaces that populate these ceramics during densification. Irrespective of mechanisms, these low temperatures create a new opportunity spectrum to design grain boundaries and create new types of nanocomposites among material combinations that previously had incompatible processing windows. Future directions are discussed in terms of both the fundamental science and engineering of cold sintering.more » « less
-
In binder jetting, shrinkage and deformation occur during the sintering step, both of which are affected by the green density of the binder jetted materials. The study innovatively introduces a cost-effective, practical, and in-process monitoring system for visualizing shrinkage and deformation on larger samples than conventionally observed using small-scale specimens in dillatometry equipment. The powder characteristics and binder jet printing process itself influence the initial green density. The comprehensive analysis of powder flowability and packing density, densification behavior, and shrinkage reveals that the consolidated parts using virgin powder (with a green density of 55%) can achieve a relative density above 99.9% with an anisotropic shrinkage in the Z>X>Y direction. In contrast, the used or recycled powder exhibits a lower green density of ∼48%, higher shrinkage rate in all three dimensions, and a decreased degree of anisotropy. Using in-process imaging and experimental data on the grain size attained through optical microscopy and electron backscatered diffraction imaging, the material's shear and bulk viscosities were determined. The formation of delta-ferrite and its impact on densification were discussed in the context of solid-state and supersolidus liquid phase sintering. The model relied on the continuum sintering theory formulated by Skorohod and Olevsky. The strain evolution from the in-situ imaging of sintering process is correlated with porosity based on the used feedstock and applied sintering temperatures. The outcomes of this study offer valuable perspectives on anisotropic sintering mechanisms, bridging the knowledge gap regarding the relationships between structures produced through binder jetting and subsequent sintering of materials.more » « less
-
Abstract Ceramics such as lead zirconate titanate (PZT) tend to dissolve incongruently, and thus pose a challenge in the cold sintering process. Moist lead nitrate has previously been shown to enable a cold sinter‐assisted densification of PZT by a viscous phase sintering mechanism. In this paper, lead acetate trihydrate is demonstrated to lower the required temperature of the cold sintering step to 200°C. This densification process was described as a two‐step process: cold sintering of PZT with lead acetate trihydrate and post‐annealing the as‐cold sintered PZT ceramics. Unlike in the case of lead nitrate, PZT densification with lead acetate trihydrate occurs by a liquid phase assisted sintering mechanism, leading to an as‐cold sintered relative density of 84% at 200°C. After performing a post‐anneal step at 900°C, >97% relative densities were achieved in samples that were cold sintered with lead acetate trihydrate. This step not only densified PZT but also refined the grain boundaries. In the post‐annealed samples, the room‐temperature relative permittivity at 100 Hz was ~1600, slightly higher than that reported in samples that used lead nitrate as a sintering aid; the loss tangent was about 3.8%. For measurements at 10 Hz, the remanent polarization in both cases was ~28 µC/cm2. Both Rayleigh analysis and aging studies showed that a higher irreversible contribution to the permittivity exists in samples that used lead nitrate as a cold sintering aid.more » « less
-
Cold sintering of surface‐modified iron compacts results in a co‐continuous phosphate interphase between iron particles that provide both enhanced green strength and green density similar to the process that has been successfully introduced in low‐temperature densification of ceramic materials. Relative density as high as 95% along with transverse rupture strength of ≈ 75 MPa, which is almost six times that of conventional powdered metal iron compact and 2.5 times that of warm compacted controls, is achieved. Dilatometry study at different pressures shows a small but significant improvement in densification process during cold sintering relative to the larger densification of warm compacted control. Strength model based on microstructural analysis as well as in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments reveals the nature of the interphase that imparts the large cohesive strength under the cold sintered assisted warm compaction. The process is conducive to produce iron compacts for green machining. Furthermore, the samples when subjected to high‐temperature sintering yield a fully sintered iron compact with density > 7.2 g cm−3and transverse rupture strength as high as 780 MPa. All in all, there are major new opportunities with the cold sintered assisted warm compaction of powdered metals that will also be discussed.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0tc00395f
