Hydroflux-assisted densification: applying flux crystal growth techniques to cold sintering


Hydroflux-assisted densification (HAD) is introduced as a method for low-temperature ceramic densification. HAD expands upon the cold sintering process, using an inorganic “hydroflux” secondary mass transport phase as opposed to the aqueous acidic, basic, or salt solutions that have been reported previously. Hydrofluxes combine ionic salts with sparing quantities of water to depress their melting points into the cold sintering range. The substantial solubility of many oxides in these hydrofluxes makes them appealing transport phases for an expanded cold-sinterable materials spectrum. This paper focuses on a hydroflux transport phase containing a eutectic mixture of NaOH and KOH (called “NaK”) for which particular nuances and properties are discussed. We demonstrate HAD in the ZnO system, highlighting the importance and impact of processing variables such as pressure, transport phase quantity, and water content. Additionally, we show densification of the oxide binaries Bi2O3, WO3, CuO, and MnO, and the functional ternaries Bi2WO6 and KxNa1-xNbO3 in the 200–300 °C range. This entire set is challenging to cold sinter using aqueous transport phases.

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  • 01 July 2020

    Correction to: Hydroflux-assisted densification: applying flux crystal growth techniques to cold sintering.


  1. 1

    Guo J, Berbano SS, Guo H, Baker AL, Lanagan MT, Randall CA (2016) Cold sintering process of composites: bridging the processing temperature gap of ceramic and polymer materials. Adv Funct Mater 26:7115–7121

    CAS  Article  Google Scholar 

  2. 2

    Zhao X, Guo J, Wang K, Herisson De Beauvoir T, Li B, Randall CA (2018) Introducing a ZnO–PTFE (polymer) nanocomposite varistor via the cold sintering process. Adv Eng Mater 20:1–8

    Google Scholar 

  3. 3

    Maria JP, Kang X, Floyd RD, Dickey EC et al (2017) Cold sintering: current status and prospects. J Mater Res 32:3205–3218

    CAS  Article  Google Scholar 

  4. 4

    Baker A, Guo H, Guo J, Randall C, Green DJ (2016) Utilizing the cold sintering process for flexible-printable electroceramic device fabrication. J Am Ceram Soc 99:3202–3204

    CAS  Article  Google Scholar 

  5. 5

    Guo J, Baker AL, Guo H, Lanagan M, Randall CA (2017) Cold sintering process: a new era for ceramic packaging and microwave device development. J Am Ceram Soc 100:669–677

    CAS  Article  Google Scholar 

  6. 6

    Xie Y, Yin S, Yamane H, Hashimoto T, Sato T (2009) Low temperature sintering and color of a new compound Sn1.24Ti1.94O3.66(OH)1.50F1.42. Solid State Sci 11:1703–1708

    CAS  Article  Google Scholar 

  7. 7

    Berbano SS, Guo J, Guo H, Lanagan MT, Randall CA (2017) Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte. J Am Ceram Soc 100:2123–2135

    CAS  Article  Google Scholar 

  8. 8

    Roy DM, Gouda GR (1973) High strength generation in cement pastes. Cem Concr Res 3:807–820

    CAS  Article  Google Scholar 

  9. 9

    Gouda GR, Roy DM (1976) Characterization of hot-pressed cement pastes. J Am Ceram Soc 59:412–414

    CAS  Article  Google Scholar 

  10. 10

    Yamasaki N, Yanagisawa K, Nishioka M, Kanahara S (1986) A hydrothermal hot-pressing method: apparatus and application. J Mater Sci Lett 5:355–356

    Article  Google Scholar 

  11. 11

    Anagisawa K, Sasaki M, Nishioka M, Ioku K, Yamasaki N (1994) Preparation of sintered compacts of anatase by hydrothermal hot-pressing. J Mater Sci Lett 13:765–766

    Article  Google Scholar 

  12. 12

    Hosoi K, Hashida T, Takahashi H, Yamasaki N, Korenaga T (1997) Solidification behaviour of calcium carbonate via aragonite-calcite wet transformation with hydrothermal hot pressing. J Mater Sci Lett 16:382–385

    CAS  Article  Google Scholar 

  13. 13

    Kähäri H, Teirikangas M, Juuti J, Jantunen H (2014) Dielectric properties of lithium molybdate ceramic fabricated at room temperature. J Am Ceram Soc 97:3378–3379

    Article  Google Scholar 

  14. 14

    Guo J, Guo H, Baker AL, Lanagan MT, Kupp ER, Messing GL, Randall CA (2016) Cold sintering: a paradigm shift for processing and integration of ceramics. Angew Chemie Int Ed 55:11457–11461

    CAS  Article  Google Scholar 

  15. 15

    Kang X, Floyd R, Lowum S, Long D, Dickey E, Maria JP (2019) Cold sintering with dimethyl sulfoxide solutions for metal oxides. J Mater Sci 54:7438–7446

    CAS  Article  Google Scholar 

  16. 16

    Bin HW, Li L, Yan H, Chen XM (2019) Cold sintering and microwave dielectric properties of dense HBO2-II ceramics. J Am Ceram Soc 102:5934–5940

    Article  Google Scholar 

  17. 17

    Grasso S, Biesuz M, Zoli L, Taveri G et al (2020) A review of cold sintering processes. Adv Appl Ceram 119:115–143

    CAS  Article  Google Scholar 

  18. 18

    Floyd RD, Lowum S, and Maria J-P (2020) Cold sintering zinc oxide with a crystalline zinc acetate dihydrate mass transport phase. Submitted for publication.

  19. 19

    Floyd R, Lowum S, Maria J-P (2019) Instrumentation for automated and quantitative low temperature compaction and sintering. Rev Sci Instrum 90:055104

    Article  Google Scholar 

  20. 20

    Floyd RD (2019) Improving the instrumentation and science of cold sintering. PhD Dissertation, North Carolina State University

  21. 21

    Bugaris DE, Zur Loye HC (2012) Materials discovery by flux crystal growth: quaternary and higher order oxides. Angew Chemie 51:3780–3811

    CAS  Article  Google Scholar 

  22. 22

    Elwell D, Scheel HJ (1975) Crystal growth from high-temperature solutions. Academic Press Inc., London

    Google Scholar 

  23. 23

    Chance MW (2014) Hydroflux synthesis: a new and effective technique for exploratory crystal growth. PhD Dissertation, University of South Carolina

  24. 24

    Chance WM, Bugaris DE, Sefat AS, Zur Loye HC (2013) Crystal growth of new hexahydroxometallates using a hydroflux. Inorg Chem 52:11723–11733

    CAS  Article  Google Scholar 

  25. 25

    Maria J-P, Floyd RD, and Lowum S (2019) Hydroflux-assisted densification. US Patent U.S. Provisional Patent Application.

  26. 26

    Mugavero SJ, Gemmill WR, Roof IP, Zur Loye HC (2009) Materials discovery by crystal growth: lanthanide metal containing oxides of the platinum group metals (Ru, Os, Ir, Rh, Pd, Pt) from molten alkali metal hydroxides. J Solid State Chem 182:1950–1963

    CAS  Article  Google Scholar 

  27. 27

    Kang X, Floyd R, Lowum S, Cabral M, Dickey E, Maria JP (2019) Mechanism studies of hydrothermal cold sintering of zinc oxide at near room temperature. J Am Ceram Soc 102:4459–4469

    CAS  Article  Google Scholar 

  28. 28

    Dargatz B, Gonzalez-Julian J, Guillon O (2015) Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering. Sci Technol Adv Mater 16:25008

    Article  Google Scholar 

  29. 29

    Gonzalez-Julian J, Neuhaus K, Bernemann M, Pereira da Silva J, Laptev A, Bram M, Guillon O (2018) Unveiling the mechanisms of cold sintering of ZnO at 250°C by varying applied stress and characterizing grain boundaries by Kelvin Probe Force Microscopy. Acta Mater 144:116–128

    CAS  Article  Google Scholar 

  30. 30

    Dauby C, Glibert J, Claes P (1979) Electrical conductivity and specific mass of the molten NaOH-KOH eutectic mixture. Electrochim Acta 24:35–39

    CAS  Article  Google Scholar 

  31. 31

    Janz GJ (1967) Molten Salts Handbook, 1st edn. Academic Press Inc., New York, NY

    Google Scholar 

  32. 32

    Meyer B, Marx D, Dulub O, Diebold U, Kunat M, Langenberg D, Wöll C (2004) Partial dissociation of water leads to stable superstructures on the surface of zinc oxide. Angew Chemie Int Ed 43:6641–6645

    CAS  Article  Google Scholar 

  33. 33

    Wöll C (2007) The chemistry and physics of zinc oxide surfaces. Prog Surf Sci 82:55–120

    Article  Google Scholar 

  34. 34

    Raymand D, van Duin ACT, Spångberg D, Goddard WA, Hermansson K (2010) Water adsorption on stepped ZnO surfaces from MD simulation. Surf Sci 604:741–752

    CAS  Article  Google Scholar 

  35. 35

    Morimoto T, Nagao M, Tokuda F (1968) Desorbability of chemisorbed water on metal oxide surfaces. I. Desorption temperature of chemisorbed water on hematite, rutile, and zinc oxide. Bull Chem Soc Jpn 41:1533–1537

    CAS  Article  Google Scholar 

  36. 36

    Li L, Bin HW, Yang S, Yan H, Chen XM (2019) Effects of water content during cold sintering process of NaCl ceramics. J Alloys Compd 787:352–357

    CAS  Article  Google Scholar 

  37. 37

    Sengul MY, Guo J, Randall CA, van Duin ACT (2019) Water-mediated surface diffusion mechanism enables the cold sintering process: a combined computational and experimental study. Angew Chemie 131:12550–12554

    Article  Google Scholar 

  38. 38

    Varela JA, Whittemore OJ, Longo E (1990) Pore size evolution during sintering of ceramic oxides. Ceram Int 16:177–189

    CAS  Article  Google Scholar 

  39. 39

    Anderson PJ, Morgan PL (1964) Effects of water vapour on sintering of MgO. Trans Faraday Soc 60:930–937

    CAS  Article  Google Scholar 

  40. 40

    Angle JP, Morgan PED, Mecartney ML (2013) Water vapor-enhanced diffusion in alumina. J Am Ceram Soc 96:3372–3374

    CAS  Article  Google Scholar 

  41. 41

    Dargatz B, Gonzalez-Julian J, Bram M, Jakes P et al (2016) FAST/SPS sintering of nanocrystalline zinc oxide-Part I: enhanced densification and formation of hydrogen-related defects in presence of adsorbed water. J Eur Ceram Soc 36:1207–1220

    CAS  Article  Google Scholar 

  42. 42

    Kang X (2017) Hydrothermal cold sintering. PhD Dissertation, North Carolina State University

  43. 43

    Zhen Y, Li JF (2006) Normal sintering of (K, Na)NbO3-based ceramics: Influence of sintering temperature on densification, microstructure, and electrical properties. J Am Ceram Soc 89:3669–3675

    CAS  Article  Google Scholar 

  44. 44

    Tsuji K, Ndayishimiye A, Lowum S, Floyd R, Wang K, Wetherington M, Maria J-P, Randall CA (2020) Single step densification of high permittivity BaTiO3 ceramics at 300°C. J Eur Ceram Soc 40:1280–1284

    Article  Google Scholar 

  45. 45

    Guo H, Guo J, Baker A, Randall CA (2016) Hydrothermal-assisted cold sintering process: a new guidance for low-temperature ceramic sintering. ACS Appl Mater Interfaces 8:20909–20915

    CAS  Article  Google Scholar 

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The authors acknowledge Professor Paul Maggard of North Carolina State University for his helpful suggestions to draw comparisons between cold sintering processes and flux crystal growth processes. The authors would also like to thank members of the Huck Institutes of the Life Sciences’ Microscopy and Cytometry Facility at the Pennsylvania State University for use of their equipment. 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-1841453 and IIP-1841466. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE1255832. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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Lowum, S., Floyd, R. & Maria, J. Hydroflux-assisted densification: applying flux crystal growth techniques to cold sintering. J Mater Sci 55, 12747–12760 (2020). https://doi.org/10.1007/s10853-020-04926-7

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