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Cold Hydrostatic Sintering: From shaping to 3D printing
Journal of Materiomics 5 (2019) 496e501
Contents lists available at ScienceDirect
Journal of Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/
Cold Hydrostatic Sintering: From shaping to 3D printing Anna Jiang a, Daoyao Ke a, Ludi Xu a, Qiang Xu a, Jiang Li b, Jiabei Wei b, Chunfeng Hu a, **, Salvatore Grasso a, * a
Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China b Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2018 Received in revised form 15 February 2019 Accepted 22 February 2019 Available online 28 February 2019
We developed a novel consolidation technique, Cold Hydrostatic Sintering (CHS), which allows near full densification of silica. The technique is inspired by biosilicification and geological formation of siliceous rocks. Unlike established cold sintering method which is based on uniaxial pressure, CHS employs an isostatic pressure to enable room temperature consolidation of bulks having a complex threedimensional shape. The resulting material is transparent (in line transmittance exceeding 70% in the visible range) and amorphous. After drying, the Vickers hardness was as high 1.4 GPa which half of materials consolidated at 1200 C and it is the highest among all materials processed at room temperature. The CHS method, because of its simplicity, might be suitable for broad range of applications including 3D printing, mould forming and preparation of multi-layered devices. Because of the absence of the firing step, CHS could be directly integrated in the manufacturing of a wide range of hybrid (organic/inorganic) materials for functional and biological applications. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Silica Cold Hydrostatic Sintering (CHS) Shaping 3D printing
1. Introduction The etymology of ceramics refers to the word Keramos which in Greek means “burned stuff” or “burned earth”. Ceramics, in fact, are commonly obtained through the action of fire/heating. Considering that ceramics are inorganic non-metallic materials, in nature there are “ceramics” as diamonds which are formed in presence of heat or opals which are formed at room temperature from repeated silica deposition. Firing of ceramics has been around since early civilizations and has gone almost unchanged since the beginning of the last century. Conventional sintering is a thermally activated process driven by minimization of surface energy of the particles. Bearing in mind sustainability and multi-material integration of organics and inorganics, in the past few years, Uniaxial Cold Sintering (UCS) has emerged as novel and disruptive consolidation technique [1]. Uniaxial Cold Sintering, uses high pressure (several hundreds of MPa) in presence of a liquid, typically water. The technique has
* Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. ** Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail addresses: [email protected] (C. Hu), [email protected] (S. Grasso). Peer review under responsibility of The Chinese Ceramic Society.
some close affinity to the formation of sedimentary rocks, however such geological process lasting millions of years is not transferable to the research on ceramics. Recent research suggests that UCS can accelerate consolidation/ lithification processes and shorten down the processing time from millions of years to a few hours. The concept of “cold sintering’’ itself is not completely new, and already in 1978. Gutmanas et al. published a work entitled “Cold sintering under high pressure” [2,3] where the consolidation was achieved by plastic deformation of ductile metallic powders or salts. Consolidation of ceramics as alumina even under extreme pressure of 8 GPa has been proven difficult [4], this is because of the limited inherent plasticity of the particles and the negligible atomic diffusion at room temperature. On the other side, consolidation under hydrothermal sintering conditions has been proven feasible for silica at 300 C, 190 MPa, 90 min [5], and a-quartz at 350 MPa, 300 C, 90 min [6]. Using as model system spherical glass particles, Hosoi et al. [7] identified the consolidation mechanism occurring during hydrothermal hot press. Attempts to produce SiO2, TiO2 and CaCO3 ceramics at temperatures below 200 C were initiated by Yamasaki et al. starting from 1986 [8]. Recently, UCS [9e16] has opened up novel scenarios in the traditional way of making ceramics “rendering kilns obsolete” [17]. The first report on “cold sintering” (also known as cold sintering process), as a novel consolidation of ceramics, appeared in
https://doi.org/10.1016/j.jmat.2019.02.009 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
A. Jiang et al. / Journal of Materiomics 5 (2019) 496e501
2016. The work described consolidation in presence of a transitory liquid phase at a temperature exceeding the boiling point of the liquid [18]. Silica constitutes 59 wt% of the earth's crust, which corresponds to 95% of the existing rocks. So rather than CaCO3, used in previous studies [1], silica is a more representative system to mimic a geologically inspired processing occurring in sedimentary rocks. Silica is an inorganic material with an endless number of applications including thermal insulation [19], nuclear waste encapsulation and bio-related applications [20]. Silica based products are usually produced either using sintering [9,21] or melt processing. Both methods require high temperature which are energyintensive. Here, we will introduce a Cold Hydrostatic Sintering (CHS) process which differs from established UCS. The proposed CHS surpass the limitations of UCS by making possible the fabrication of bulks with a complex shape. In CHS, pressure is applied isostatically contributing to homogeneously densified materials even having size of few tens of centimeters. Typically, in UCS the size is limited to 10 mm diameter discs, uniaxial pressure contributes to a reduced density at the sample rim because uneven pressure distribution and accelerated drying (occurring at the punch die clearance). The CHS has potentials to replace a firing cycle with a near room temperature compaction, this could lead to i) high energy efficient manufacturing including 3D printing, ii) reproduce at an accelerated rate bio-silicification [22,23] or bio-mineralization occurring in nature, iii) effortless integration of organics and inorganics into hybrid materials to fabricate operational devices. A parallel comparison between conventional sintering, UCS and CHS is presented in Fig. 1. Conventional sintering is thermally activated process driven by diffusion, in UCS atomic diffusion is instead promoted by dissolution within a liquid at a temperature below 300 C and plastic deformation. In CHS, densification is achieved by dissolution promoted by the isostatic pressure followed precipitation reactions (poly-condensation) occurring at room temperature.
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2. Material and methods 2.1. Samples preparation The starting silica powders were high-purity powder with nano and micrometric size (SiO2, 99.5%, 5 mm and 20 nm purchased from Shanghai Yuanjiang Chemical Co., Ltd, China). High-purity potassium hydroxide powder (KOH, 99.99%) was used to prepare solutions with different molar concentrations (2, 3, 4 and 5 mol/L). Weighed silica powder and KOH solution were poured into a centrifuge tube. The slurry was evenly mixed using a vortex mixer (XH-C, Lianer Corp., China) operating at 2000 rotation per minute, a closed container was used to prevent water evaporation. For the CHS experiments, samples were pressed using isostatic press (Shanxi Technology Co., Ltd) under 300 MPa for 5 min, 20 min, and 60 min at room temperature. Samples processed without pressure were also produced for comparison. The samples were dried using an environmental chamber (LJS-80, Jincheng Hailan Equipment Co., Ltd) using controlled temperature and humidity as detailed in the supplementary section. For comparison, samples were also processed using UCS approach employing steel hollow dies with a 15 mm diameter under 300 MPa. 2.2. Characterization Phase analysis was done using an X-Ray diffraction (XRD) equipment (PA, Almelo, Holland). The samples were inspected using a scanning electron microscopy (FEI, Hillaboro, US) (with energy spectrum). The density of the sample was determined using an electron densitometer (ZMD-2 Series electronic density meter, Nan Tong Precision Instrument Co., Ltd.). The hardness was measured using a Vickers hardness tester (HVS-50 Digital Vickers Hardness Tester, Shanghai Wanheng Precision Instrument Co., Ltd.) with an applied load of 1 kg and dwell time of 15 s. Optical properties of translucent/transparent samples were
Fig. 1. Comparative analysis between Conventional Sintering, Uniaxial Cold Sintering and Cold Hydrostatic Sintering. The thermodynamics parameters (pressure and temperature) suggest clear differences between the sintering techniques.
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tested using a UV-VIS-NIR spectrophotometer (Model Cray-5000, Varian, CA, US). The sample thickness was 1.5 mm and in line transmittance test rate was up to 0.1 nm/s within 200e1200 nm (testing time was about 100 s). 3. Results and discussion A typical (e.g. as reported in the literature) cold sintering run is usually done using a steel die as shown in Fig. 2 (a). In uniaxial cold sintering experiments, the moisturized powder is pressed under 300 MPa and the temperature is raised to 200e300 C. However, because of the high uniaxial pressure, it was difficult to produce dense and homogeneous samples. The sample extraction from the die was particularly difficult. The samples were propense to crack because of the stresses. It is worth to note when using the UCS setup it was difficult to control the dissolution and drying step as they occured simultaneously. The second set of experiments, based on CHS, was done using a cold isostatic pressure as shown in Fig. 2(b). The processing flowchart is presented in Fig. 2 (c). The slurry was poured into a silicone mould (different shapes). To our best knowledge, this kind of approach was not reported before elsewhere. During isostatic pressing, dissolution of silica occurred and
silicic acid formed in the aqueous solution. During the slow drying process lasting one week, condensation occurred and silicic acid formed a network with water being expelled (see Fig. S2). The role of the caustic solution was to accelerate silica dissolution, higher concentration of potassium is expected to be located at the grain boundaries. A photograph of the samples processed using CHS is shown in Fig. 3. As seen, the samples were crack free and their dimension was in the order of a cubic centimeter. The samples were also translucent. It is worth mentioning that UCS produces mostly discs shapes and optical properties were not previously investigated. The scanning electron micrograph image of the fractured sample (Fig. S4) reveals a smooth surface, no defects as macro pores or cracks are visible. The inset shows high magnification image where initial nanoparticles are visible and grain coarsening was rather limited. The sintering mechanisms, based on a transitory liquid phase sintering, were studied by Taveri et al. [22]. Consolidation is driven by a hydrated silica second phase surrounding the starting particles. Table 1 compares density and hardness of silica bodies consolidated using different techniques. As terms of comparison, we included: 1) sintered silica prepared in pressureless mode, 2) using uniaxial cold sintering process described in Fig. 2 (a) under
Fig. 2. Overview of processing routes developed for near room temperature consolidation of silica using: (a) Setup used in Uniaxial Cold Sintering (UCS) which employs a hollow steel die to press wet powders between two punches, heating bands are used to reach temperature below 300 C. Sample dries and sinters simultaneously. (b) Cold Hydrostatic Sintering (CHS) setup using a paste containing silica water/KOH is pressed isostatically at room temperature , consolidation/hardening is achieved during the drying step; (c) a flow chart describing CHS processing steps of silica samples.
A. Jiang et al. / Journal of Materiomics 5 (2019) 496e501
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Fig. 3. Photograph of silica samples produced using CHS with different shapes.
Table 1 Hardness and density of silica bodies produced using state of the art techniques and CHS. Size
Label
Water solid ratio Concentration (mol/L) Time (min), Pressure (MPa) Vickers hardness 1 kg load (GPa) Density (g/cm3)
State of the art nano- particle pressure-less 1:1.4 UCS 1:1.4 SPS [24] (1200 C) e Present work
3 3 e
20, 0 20,300 7, 50
0.08 ± 0.14 0.19 ± 0.02 3.44 ± 0.16
1.59 1.81 2.43
5 mm particle CHS1 CHS2
1:1.2 1:1.2
3 3
5, 300 20, 300
1.16 ± 0.03 1.19 ± 0.11
2.01 2.05
nano- particle CHS3 CHS4 CHS5 CHS6
1:1.4 1:1.4 1:1.4 1:1.4
4 3 4 3
20, 300 5, 300 5, 300 20, 300
0.78 ± 0.13 1.12 ± 0.08 0.65 ± 0.10 1.38 ± 0.12
2.04 2.02 2.02 2.03
300 MPa for 20 min, and using spark plasma sintering at 1200 C under 50 MPa for 7 min as reported in Ref. [24]. The experiments carried out in pressure-less mode resulted in a porous sample which was opaque (white color). By using uniaxial pressure of 300 MPa the density increased from 1.59 to 1.81 g/cm3. Instead, when using spark plasma sintering at 1200 C, density approaching theoretical value was obtained. Clearly the techniques are hardly comparable in terms of energy demand for the consolidation. When using CHS, the effect of processing parameters on density and hardness are summarized in Table 1. The highest hardness was obtained from sample CHS6 prepared weight ratio of silica powder to water of 1.4 using a 3 mol/L KOH solution under 300 MPa for 20 min. Comparing CHS 6 to CHS 2, larger particles resulted in a drop in hardness from 1.38 to 1.19 GPa. The density of CHSed samples was around 2 g/cm3, as discussed in the previous section no porosity was apparent. The discrepancy in the density values can be attributed to the presence of chemically bonded water as discussed in Ref. [22]. Taking as term of comparison the density opal (1.9 and 2.3 g/cm3) which contains (SiO2$nH2O) a variable amount of water, CHSed sample with density of z2.05 corresponding to a 5 wt% chemically bonded water [25] within the sample. Fig. 4 is an Ashby plot mapping hardness of wide range of materials against processing temperature. Among all engineered materials processed at room temperature, CHS samples resulted in the highest hardness. Materials taken as term of comparison are polymers, bio-minerals (Dentin [26] 0.64 ± 0.04 GPa, Fibula diaphysis (bone) [27] and cements (0.46 GPa). Silica processed using Hydrothermal Hot Press at room temperature led to almost no densification, dense materials were only obtained by increasing the temperature up to 300 C [28]. Fig. 5 shows the in line transmittance of different samples processed by CHS under 300 MPa at 25 C. For comparison, Fig. 5(a) also plots data for commercial fused silica sample. Samples
produced using micrometric particles (CHS1 and CHS2) were almost completely opaque, instead of nanometric powders (CHS 3e6) resulted in transparent/translucent samples. The sample CSP produced using uniaxial pressure (UCS) and pressureless (not shown) were completely opaque. Sample (CHS3) had a 70% in line transmittance at wavelength above 600 nm. The developed CHS, because of the room temperature processing offers an unprecedented versatility, it is expected to be integrated with a wide range of manufacturing techniques. Possible uses of CHS are shown in Fig. 6. After isostatic pressing the resulting paste could be applied to: 1) 3D printing technology using extrusion, 2) mould forming as reported in this work, 3) deposit multi-
Fig. 4. Ashby diagrams mapping processing temperature versus hardness of different class of engineered materials. As seen CHSed silica has the highest hardness among most of the materilas processed at room temperature.
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Fig. 5. a) In line transmittance plots of different silica bodies with CHS at 300 MPa at 25 C. CHS1, CHS2, CHS3, CHS4, CHS5 CHS6 and UCS. For comparison transmittance of a 4 mm thick fused silica sample is also plotted. b) Photograph of 1.5 mm thick silica sintered by CHS at 300 MPa (see Table 1 for processing conditions).
Fig. 6. Potential uses of CHS. It is expected that CHS because of its room temperature processing could be readily integrated within existing manufacturing techniques including 3D printing, mould forming and multilayered material deposition.
layered materials for application to different fields (capacitors, batteries and thermoelectrics). Future work should verify this and the applicability of CHS to other materials. 4. Conclusions We propose a novel consolidation technique, Cold Hydrostatic Sintering (CHS), which allows near full densification of silica. The technique has some affinities with biosilicification and geological formation of siliceous rocks. CHS differs from the state of the art cold sintering (based on uniaxial pressing) as isostatic pressure enables room temperature consolidation of object with a complex three-dimensional shape. The resulting material are transparent (in line transmittance exceeding 70% in the visible range) and amorphous. After a slow drying process, the Vickers hardness was as high 1.4 GPa which is half of materials consolidated at 1200 C and is the highest among all materials processed at room temperature. The CHS method because of its simplicity might be suitable for broad range of applications including 3D printing, mould forming and preparation of multilayered devices. The absence of the firing step could enable multi material integration of organics and inorganics which is currently needed for the manufacturing of a wide range of functional devices. Conflicts of interest The authors declare no conflict of interest.
Acknowledgements This work is supported by Thousand Talents Program of China and Sichuan Province, the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (17kffk01), Outstanding Young Scientific and Technical Talents in Sichuan Province (2019JDJQ0009), and the Natural Sciences Foundation of China (No. 51741208). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.02.009. References [1] Bouville F, Studart AR. Geologically-inspired strong bulk ceramics made with water at room temperature. Nat Commun 2017;8:14655. https://doi.org/ 10.1038/ncomms14655. [2] Goldman DB, Gutmanas EY, Zak D. Reduction of oxides and cold sintering of water-atomized powders of nickel, Ni-20Cr and Nimonic 80A. J Mater Sci Lett 1985;4:1208e12. https://doi.org/10.1007/BF00723460. [3] Gutmanas EY, Rabinkin A, Roitberg M. Cold sintering under high pressure. Scripta Metall 1979;13:11e5. https://doi.org/10.1016/0036-9748(79)90380-6. [4] Liao SC, Chen YJ, Kear BH, Mayo WE. High pressure/low temperature sintering of nanocrystalline alumina. Nanostruct Mater 1998;10:1063e79. [5] Ndayishimiye A, Largeteau A, Mornet S, Duttine M, Dourges MA, Denux D, et al. Hydrothermal sintering for densification of silica. Evidence for the role of water. J Eur Ceram Soc 2018;38:1860e70. https://doi.org/10.1016/ j.jeurceramsoc.2017.10.011.
A. Jiang et al. / Journal of Materiomics 5 (2019) 496e501 [6] Ndayishimiye A, Largeteau A, Prakasam M, Pechev S, Dourges MA, Goglio G. Low temperature hydrothermal sintering process for the quasi-complete densification of nanometric a-quartz. Scripta Mater 2018;145:118e21. https://doi.org/10.1016/j.scriptamat.2017.10.023. [7] Hosoi K, Kawai S, Yanagisawa K, Yamasaki N. Densification process for spherical glass powders with the same particle size by hydrothermal hot pressing. J Mater Sci 1991;26:6448e52. https://doi.org/10.1007/BF02387828. [8] Yamasaki N, Yanagisawa K, Nishioka M, Kanahara S. A hydrothermal hotpressing method: apparatus and application. J Mater Sci Lett 1986;5:355e6. https://doi.org/10.1007/BF01748104. [9] Guo H, Guo J, Baker A, Randall CA. Hydrothermal-assisted cold sintering process: a new guidance for low-temperature ceramic sintering. ACS Appl Mater Interfaces 2016;8. https://doi.org/10.1021/acsami.6b07481. 20909e15. [10] Guo H, Baker A, Guo J, Randall CA. Protocol for ultralow-temperature ceramic sintering: an integration of nanotechnology and the cold sintering process. ACS Nano 2016;10:10606e14. https://doi.org/10.1021/acsnano.6b03800. [11] Baker A, Guo H, Guo J, Randall C, Green DJ. Utilizing the cold sintering process for flexibleeprintable electroceramic device fabrication. J Am Ceram Soc 2016. https://doi.org/10.1111/jace.14467. [12] Guo J, Berbano SS, Guo H, Baker AL, Lanagan MT, Randall CA. Cold sintering process of composites: bridging the processing temperature gap of ceramic and polymer materials. Adv Funct Mater 2016;26:7115e21. https://doi.org/ 10.1002/adfm.201602489. [13] K€ ah€ ari H, Teirikangas M, Juuti J, Jantunen H. Dielectric properties of Lithium Molybdate ceramic fabricated at room temperature. J Am Ceram Soc 2014;97: 3378e9. https://doi.org/10.1111/jace.13277. [14] K€ ah€ ari H, Teirikangas M, Juuti J, Jantunen H. Improvements and modifications to room-temperature fabrication method for dielectric Li2MoO4 ceramics. J Am Ceram Soc 2015;98:687e9. https://doi.org/10.1111/jace.13471. [15] Guo H, Guo J, Baker A, Randall CA. Cold sintering process for ZrO2-based ceramics: significantly enhanced densification evolution in yttria-doped ZrO2. J Am Ceram Soc 2017. https://doi.org/10.1111/jace.14593. [16] Guo H, Baker A, Guo J, Randall CA. Cold sintering process: a novel technique for low-temperature ceramic processing of ferroelectrics. J Am Ceram Soc 2016. https://doi.org/10.1111/jace.14554. [17] March P, Oney BF. Environmentally-friendly method of manufacturing ceramics may reduce carbon footprint, render kilns obsolete 2017;96. 12e12. [18] Randall CA, Guo J, Baker A, Lanagan M, Guo H. Cold sintering ceramics and composites. 2017. [19] Xu J, Li Q, Yang L, Zeng W, Zhou C, Yuan C, et al. Effects of thermal and electrical histories on structure and dielectric behaviors of (Li0.5Nd0.5)2þmodified (Bi0.5Na0.5)TiO3-BaTiO3ceramics. J Mater 2017;3:121e9. [20] Otzen D. The role of proteins in Biosilicification 2012;2012:22. [21] Berbano SS, Guo J, Guo H, Lanagan MT, Randall CA. Cold sintering process of Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 solid electrolyte. J Am Ceram Soc 2017. https:// doi.org/10.1111/jace.14727. [22] Taveri SG G. Bio-inspired hydro-pressure consolidation of silica. 2018. p. 1e22. 1805794. [23] Aizenberg J. New nanofabrication strategies: inspired by biomineralization. MRS Bull 2010;35:323e30. [24] Porwal H, Tatarko P, Saggar R, Grasso S, Kumar Mani M, Dlouhý I, et al. Tribological properties of silica–graphene nano-platelet composites. Ceram Int 2014;40:12067e74. https://doi.org/10.1016/j.ceramint.2014.04.046. M, Christy AA, Kluvanec D, Ill L. State of water molecules and [25] Bobon asova silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia. Phys Chem Miner 2011;38:809e18. [26] Chun KJ, Lee JY. Comparative study of mechanical properties of dental
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Anna Jiang obtained her Bachelor degree at the school of Materials Science and Engineering in Yancheng Institute of Technology, Jiangsu province. She started her master degree at Southwest Jiaotong University in 2017. Her research current research is on the room temperature consolidation of ceramics.
Dr. Chunfeng Hu is a full professor of School of Materials Science and Engineering in Southwest Jiaotong University in China. He received Ph.D degree in 2008 in Institute of Metal Research of Chinese Academy of Sciences, and worked in National Institute for Materials Science (NIMS) in Japan for four years and in Drexel University in USA for one year as a postdoctor. Since 2016 he focused on the research of ternary laminar ceramics of MAX and MAB. He has published 111 papers and 22 patents.
Salvatore Grasso Salvatore Grasso is a professor in ceramics at Southwest Jiaotong University Chengdu (CHINA). In 2017 he established FAME (Field Assisted Material Engineering) research group. His doctoral work (2008e2011) at the University of Tsukuba-NIMS (National Institute for material Science) Japan was focused on Spark Plasma Sintering (SPS) and other Electric Current Assisted Sintering Techniques. While at Queen Mary University of London he pioneered the development of Flash Spark Plasma Sintering techniques. His current research is around the use of intense Electric (www.flashsintering. com) and Magnetic (www.magmat.uk) fields to develop novel processing techniques to design unprecedented materials properties. Explorative research is performed along with other groups located in UK, Japan, Germany, USA and Italy using custom built equipment and using multiscale simulations.List of publication available https:// www.researchgate.net/lab/Salvatore-Grasso-Lab
Contents lists available at ScienceDirect
Journal of Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/
Cold Hydrostatic Sintering: From shaping to 3D printing Anna Jiang a, Daoyao Ke a, Ludi Xu a, Qiang Xu a, Jiang Li b, Jiabei Wei b, Chunfeng Hu a, **, Salvatore Grasso a, * a
Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China b Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2018 Received in revised form 15 February 2019 Accepted 22 February 2019 Available online 28 February 2019
We developed a novel consolidation technique, Cold Hydrostatic Sintering (CHS), which allows near full densification of silica. The technique is inspired by biosilicification and geological formation of siliceous rocks. Unlike established cold sintering method which is based on uniaxial pressure, CHS employs an isostatic pressure to enable room temperature consolidation of bulks having a complex threedimensional shape. The resulting material is transparent (in line transmittance exceeding 70% in the visible range) and amorphous. After drying, the Vickers hardness was as high 1.4 GPa which half of materials consolidated at 1200 C and it is the highest among all materials processed at room temperature. The CHS method, because of its simplicity, might be suitable for broad range of applications including 3D printing, mould forming and preparation of multi-layered devices. Because of the absence of the firing step, CHS could be directly integrated in the manufacturing of a wide range of hybrid (organic/inorganic) materials for functional and biological applications. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Silica Cold Hydrostatic Sintering (CHS) Shaping 3D printing
1. Introduction The etymology of ceramics refers to the word Keramos which in Greek means “burned stuff” or “burned earth”. Ceramics, in fact, are commonly obtained through the action of fire/heating. Considering that ceramics are inorganic non-metallic materials, in nature there are “ceramics” as diamonds which are formed in presence of heat or opals which are formed at room temperature from repeated silica deposition. Firing of ceramics has been around since early civilizations and has gone almost unchanged since the beginning of the last century. Conventional sintering is a thermally activated process driven by minimization of surface energy of the particles. Bearing in mind sustainability and multi-material integration of organics and inorganics, in the past few years, Uniaxial Cold Sintering (UCS) has emerged as novel and disruptive consolidation technique [1]. Uniaxial Cold Sintering, uses high pressure (several hundreds of MPa) in presence of a liquid, typically water. The technique has
* Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. ** Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail addresses: [email protected] (C. Hu), [email protected] (S. Grasso). Peer review under responsibility of The Chinese Ceramic Society.
some close affinity to the formation of sedimentary rocks, however such geological process lasting millions of years is not transferable to the research on ceramics. Recent research suggests that UCS can accelerate consolidation/ lithification processes and shorten down the processing time from millions of years to a few hours. The concept of “cold sintering’’ itself is not completely new, and already in 1978. Gutmanas et al. published a work entitled “Cold sintering under high pressure” [2,3] where the consolidation was achieved by plastic deformation of ductile metallic powders or salts. Consolidation of ceramics as alumina even under extreme pressure of 8 GPa has been proven difficult [4], this is because of the limited inherent plasticity of the particles and the negligible atomic diffusion at room temperature. On the other side, consolidation under hydrothermal sintering conditions has been proven feasible for silica at 300 C, 190 MPa, 90 min [5], and a-quartz at 350 MPa, 300 C, 90 min [6]. Using as model system spherical glass particles, Hosoi et al. [7] identified the consolidation mechanism occurring during hydrothermal hot press. Attempts to produce SiO2, TiO2 and CaCO3 ceramics at temperatures below 200 C were initiated by Yamasaki et al. starting from 1986 [8]. Recently, UCS [9e16] has opened up novel scenarios in the traditional way of making ceramics “rendering kilns obsolete” [17]. The first report on “cold sintering” (also known as cold sintering process), as a novel consolidation of ceramics, appeared in
https://doi.org/10.1016/j.jmat.2019.02.009 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
A. Jiang et al. / Journal of Materiomics 5 (2019) 496e501
2016. The work described consolidation in presence of a transitory liquid phase at a temperature exceeding the boiling point of the liquid [18]. Silica constitutes 59 wt% of the earth's crust, which corresponds to 95% of the existing rocks. So rather than CaCO3, used in previous studies [1], silica is a more representative system to mimic a geologically inspired processing occurring in sedimentary rocks. Silica is an inorganic material with an endless number of applications including thermal insulation [19], nuclear waste encapsulation and bio-related applications [20]. Silica based products are usually produced either using sintering [9,21] or melt processing. Both methods require high temperature which are energyintensive. Here, we will introduce a Cold Hydrostatic Sintering (CHS) process which differs from established UCS. The proposed CHS surpass the limitations of UCS by making possible the fabrication of bulks with a complex shape. In CHS, pressure is applied isostatically contributing to homogeneously densified materials even having size of few tens of centimeters. Typically, in UCS the size is limited to 10 mm diameter discs, uniaxial pressure contributes to a reduced density at the sample rim because uneven pressure distribution and accelerated drying (occurring at the punch die clearance). The CHS has potentials to replace a firing cycle with a near room temperature compaction, this could lead to i) high energy efficient manufacturing including 3D printing, ii) reproduce at an accelerated rate bio-silicification [22,23] or bio-mineralization occurring in nature, iii) effortless integration of organics and inorganics into hybrid materials to fabricate operational devices. A parallel comparison between conventional sintering, UCS and CHS is presented in Fig. 1. Conventional sintering is thermally activated process driven by diffusion, in UCS atomic diffusion is instead promoted by dissolution within a liquid at a temperature below 300 C and plastic deformation. In CHS, densification is achieved by dissolution promoted by the isostatic pressure followed precipitation reactions (poly-condensation) occurring at room temperature.
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2. Material and methods 2.1. Samples preparation The starting silica powders were high-purity powder with nano and micrometric size (SiO2, 99.5%, 5 mm and 20 nm purchased from Shanghai Yuanjiang Chemical Co., Ltd, China). High-purity potassium hydroxide powder (KOH, 99.99%) was used to prepare solutions with different molar concentrations (2, 3, 4 and 5 mol/L). Weighed silica powder and KOH solution were poured into a centrifuge tube. The slurry was evenly mixed using a vortex mixer (XH-C, Lianer Corp., China) operating at 2000 rotation per minute, a closed container was used to prevent water evaporation. For the CHS experiments, samples were pressed using isostatic press (Shanxi Technology Co., Ltd) under 300 MPa for 5 min, 20 min, and 60 min at room temperature. Samples processed without pressure were also produced for comparison. The samples were dried using an environmental chamber (LJS-80, Jincheng Hailan Equipment Co., Ltd) using controlled temperature and humidity as detailed in the supplementary section. For comparison, samples were also processed using UCS approach employing steel hollow dies with a 15 mm diameter under 300 MPa. 2.2. Characterization Phase analysis was done using an X-Ray diffraction (XRD) equipment (PA, Almelo, Holland). The samples were inspected using a scanning electron microscopy (FEI, Hillaboro, US) (with energy spectrum). The density of the sample was determined using an electron densitometer (ZMD-2 Series electronic density meter, Nan Tong Precision Instrument Co., Ltd.). The hardness was measured using a Vickers hardness tester (HVS-50 Digital Vickers Hardness Tester, Shanghai Wanheng Precision Instrument Co., Ltd.) with an applied load of 1 kg and dwell time of 15 s. Optical properties of translucent/transparent samples were
Fig. 1. Comparative analysis between Conventional Sintering, Uniaxial Cold Sintering and Cold Hydrostatic Sintering. The thermodynamics parameters (pressure and temperature) suggest clear differences between the sintering techniques.
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tested using a UV-VIS-NIR spectrophotometer (Model Cray-5000, Varian, CA, US). The sample thickness was 1.5 mm and in line transmittance test rate was up to 0.1 nm/s within 200e1200 nm (testing time was about 100 s). 3. Results and discussion A typical (e.g. as reported in the literature) cold sintering run is usually done using a steel die as shown in Fig. 2 (a). In uniaxial cold sintering experiments, the moisturized powder is pressed under 300 MPa and the temperature is raised to 200e300 C. However, because of the high uniaxial pressure, it was difficult to produce dense and homogeneous samples. The sample extraction from the die was particularly difficult. The samples were propense to crack because of the stresses. It is worth to note when using the UCS setup it was difficult to control the dissolution and drying step as they occured simultaneously. The second set of experiments, based on CHS, was done using a cold isostatic pressure as shown in Fig. 2(b). The processing flowchart is presented in Fig. 2 (c). The slurry was poured into a silicone mould (different shapes). To our best knowledge, this kind of approach was not reported before elsewhere. During isostatic pressing, dissolution of silica occurred and
silicic acid formed in the aqueous solution. During the slow drying process lasting one week, condensation occurred and silicic acid formed a network with water being expelled (see Fig. S2). The role of the caustic solution was to accelerate silica dissolution, higher concentration of potassium is expected to be located at the grain boundaries. A photograph of the samples processed using CHS is shown in Fig. 3. As seen, the samples were crack free and their dimension was in the order of a cubic centimeter. The samples were also translucent. It is worth mentioning that UCS produces mostly discs shapes and optical properties were not previously investigated. The scanning electron micrograph image of the fractured sample (Fig. S4) reveals a smooth surface, no defects as macro pores or cracks are visible. The inset shows high magnification image where initial nanoparticles are visible and grain coarsening was rather limited. The sintering mechanisms, based on a transitory liquid phase sintering, were studied by Taveri et al. [22]. Consolidation is driven by a hydrated silica second phase surrounding the starting particles. Table 1 compares density and hardness of silica bodies consolidated using different techniques. As terms of comparison, we included: 1) sintered silica prepared in pressureless mode, 2) using uniaxial cold sintering process described in Fig. 2 (a) under
Fig. 2. Overview of processing routes developed for near room temperature consolidation of silica using: (a) Setup used in Uniaxial Cold Sintering (UCS) which employs a hollow steel die to press wet powders between two punches, heating bands are used to reach temperature below 300 C. Sample dries and sinters simultaneously. (b) Cold Hydrostatic Sintering (CHS) setup using a paste containing silica water/KOH is pressed isostatically at room temperature , consolidation/hardening is achieved during the drying step; (c) a flow chart describing CHS processing steps of silica samples.
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Fig. 3. Photograph of silica samples produced using CHS with different shapes.
Table 1 Hardness and density of silica bodies produced using state of the art techniques and CHS. Size
Label
Water solid ratio Concentration (mol/L) Time (min), Pressure (MPa) Vickers hardness 1 kg load (GPa) Density (g/cm3)
State of the art nano- particle pressure-less 1:1.4 UCS 1:1.4 SPS [24] (1200 C) e Present work
3 3 e
20, 0 20,300 7, 50
0.08 ± 0.14 0.19 ± 0.02 3.44 ± 0.16
1.59 1.81 2.43
5 mm particle CHS1 CHS2
1:1.2 1:1.2
3 3
5, 300 20, 300
1.16 ± 0.03 1.19 ± 0.11
2.01 2.05
nano- particle CHS3 CHS4 CHS5 CHS6
1:1.4 1:1.4 1:1.4 1:1.4
4 3 4 3
20, 300 5, 300 5, 300 20, 300
0.78 ± 0.13 1.12 ± 0.08 0.65 ± 0.10 1.38 ± 0.12
2.04 2.02 2.02 2.03
300 MPa for 20 min, and using spark plasma sintering at 1200 C under 50 MPa for 7 min as reported in Ref. [24]. The experiments carried out in pressure-less mode resulted in a porous sample which was opaque (white color). By using uniaxial pressure of 300 MPa the density increased from 1.59 to 1.81 g/cm3. Instead, when using spark plasma sintering at 1200 C, density approaching theoretical value was obtained. Clearly the techniques are hardly comparable in terms of energy demand for the consolidation. When using CHS, the effect of processing parameters on density and hardness are summarized in Table 1. The highest hardness was obtained from sample CHS6 prepared weight ratio of silica powder to water of 1.4 using a 3 mol/L KOH solution under 300 MPa for 20 min. Comparing CHS 6 to CHS 2, larger particles resulted in a drop in hardness from 1.38 to 1.19 GPa. The density of CHSed samples was around 2 g/cm3, as discussed in the previous section no porosity was apparent. The discrepancy in the density values can be attributed to the presence of chemically bonded water as discussed in Ref. [22]. Taking as term of comparison the density opal (1.9 and 2.3 g/cm3) which contains (SiO2$nH2O) a variable amount of water, CHSed sample with density of z2.05 corresponding to a 5 wt% chemically bonded water [25] within the sample. Fig. 4 is an Ashby plot mapping hardness of wide range of materials against processing temperature. Among all engineered materials processed at room temperature, CHS samples resulted in the highest hardness. Materials taken as term of comparison are polymers, bio-minerals (Dentin [26] 0.64 ± 0.04 GPa, Fibula diaphysis (bone) [27] and cements (0.46 GPa). Silica processed using Hydrothermal Hot Press at room temperature led to almost no densification, dense materials were only obtained by increasing the temperature up to 300 C [28]. Fig. 5 shows the in line transmittance of different samples processed by CHS under 300 MPa at 25 C. For comparison, Fig. 5(a) also plots data for commercial fused silica sample. Samples
produced using micrometric particles (CHS1 and CHS2) were almost completely opaque, instead of nanometric powders (CHS 3e6) resulted in transparent/translucent samples. The sample CSP produced using uniaxial pressure (UCS) and pressureless (not shown) were completely opaque. Sample (CHS3) had a 70% in line transmittance at wavelength above 600 nm. The developed CHS, because of the room temperature processing offers an unprecedented versatility, it is expected to be integrated with a wide range of manufacturing techniques. Possible uses of CHS are shown in Fig. 6. After isostatic pressing the resulting paste could be applied to: 1) 3D printing technology using extrusion, 2) mould forming as reported in this work, 3) deposit multi-
Fig. 4. Ashby diagrams mapping processing temperature versus hardness of different class of engineered materials. As seen CHSed silica has the highest hardness among most of the materilas processed at room temperature.
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Fig. 5. a) In line transmittance plots of different silica bodies with CHS at 300 MPa at 25 C. CHS1, CHS2, CHS3, CHS4, CHS5 CHS6 and UCS. For comparison transmittance of a 4 mm thick fused silica sample is also plotted. b) Photograph of 1.5 mm thick silica sintered by CHS at 300 MPa (see Table 1 for processing conditions).
Fig. 6. Potential uses of CHS. It is expected that CHS because of its room temperature processing could be readily integrated within existing manufacturing techniques including 3D printing, mould forming and multilayered material deposition.
layered materials for application to different fields (capacitors, batteries and thermoelectrics). Future work should verify this and the applicability of CHS to other materials. 4. Conclusions We propose a novel consolidation technique, Cold Hydrostatic Sintering (CHS), which allows near full densification of silica. The technique has some affinities with biosilicification and geological formation of siliceous rocks. CHS differs from the state of the art cold sintering (based on uniaxial pressing) as isostatic pressure enables room temperature consolidation of object with a complex three-dimensional shape. The resulting material are transparent (in line transmittance exceeding 70% in the visible range) and amorphous. After a slow drying process, the Vickers hardness was as high 1.4 GPa which is half of materials consolidated at 1200 C and is the highest among all materials processed at room temperature. The CHS method because of its simplicity might be suitable for broad range of applications including 3D printing, mould forming and preparation of multilayered devices. The absence of the firing step could enable multi material integration of organics and inorganics which is currently needed for the manufacturing of a wide range of functional devices. Conflicts of interest The authors declare no conflict of interest.
Acknowledgements This work is supported by Thousand Talents Program of China and Sichuan Province, the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (17kffk01), Outstanding Young Scientific and Technical Talents in Sichuan Province (2019JDJQ0009), and the Natural Sciences Foundation of China (No. 51741208). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.02.009. References [1] Bouville F, Studart AR. Geologically-inspired strong bulk ceramics made with water at room temperature. Nat Commun 2017;8:14655. https://doi.org/ 10.1038/ncomms14655. [2] Goldman DB, Gutmanas EY, Zak D. Reduction of oxides and cold sintering of water-atomized powders of nickel, Ni-20Cr and Nimonic 80A. J Mater Sci Lett 1985;4:1208e12. https://doi.org/10.1007/BF00723460. [3] Gutmanas EY, Rabinkin A, Roitberg M. Cold sintering under high pressure. Scripta Metall 1979;13:11e5. https://doi.org/10.1016/0036-9748(79)90380-6. [4] Liao SC, Chen YJ, Kear BH, Mayo WE. High pressure/low temperature sintering of nanocrystalline alumina. Nanostruct Mater 1998;10:1063e79. [5] Ndayishimiye A, Largeteau A, Mornet S, Duttine M, Dourges MA, Denux D, et al. Hydrothermal sintering for densification of silica. Evidence for the role of water. J Eur Ceram Soc 2018;38:1860e70. https://doi.org/10.1016/ j.jeurceramsoc.2017.10.011.
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Anna Jiang obtained her Bachelor degree at the school of Materials Science and Engineering in Yancheng Institute of Technology, Jiangsu province. She started her master degree at Southwest Jiaotong University in 2017. Her research current research is on the room temperature consolidation of ceramics.
Dr. Chunfeng Hu is a full professor of School of Materials Science and Engineering in Southwest Jiaotong University in China. He received Ph.D degree in 2008 in Institute of Metal Research of Chinese Academy of Sciences, and worked in National Institute for Materials Science (NIMS) in Japan for four years and in Drexel University in USA for one year as a postdoctor. Since 2016 he focused on the research of ternary laminar ceramics of MAX and MAB. He has published 111 papers and 22 patents.
Salvatore Grasso Salvatore Grasso is a professor in ceramics at Southwest Jiaotong University Chengdu (CHINA). In 2017 he established FAME (Field Assisted Material Engineering) research group. His doctoral work (2008e2011) at the University of Tsukuba-NIMS (National Institute for material Science) Japan was focused on Spark Plasma Sintering (SPS) and other Electric Current Assisted Sintering Techniques. While at Queen Mary University of London he pioneered the development of Flash Spark Plasma Sintering techniques. His current research is around the use of intense Electric (www.flashsintering. com) and Magnetic (www.magmat.uk) fields to develop novel processing techniques to design unprecedented materials properties. Explorative research is performed along with other groups located in UK, Japan, Germany, USA and Italy using custom built equipment and using multiscale simulations.List of publication available https:// www.researchgate.net/lab/Salvatore-Grasso-Lab