A review of cold sintering processes

Figures & data

Figure 1. Branches of science underpinning ULES. The combination of four components: liquid, powder, heat and pressure, requires a multi-disciplinary approach covering physics, chemistry, geoscience and biology. A synergistic knowledge-base, built up from a wide range of disciplines, ranging from material science and engineering to palaeontology [5], could aid the understanding of these techniques to support their technological and scientific impact.

Figure 2. (a) Wollaston Press developed in 1829 for compaction of loose platinum mud into a firm pellet [7]. (b) Extract from early work by Brill and Melczynski dated 1964 [8] on hydrothermal sintering. At the time, the authors did not intend to continue studies because they could not find any useful application. (c) Setup developed by Turba and Rump [9] for carrying out ‘cold sintering’ experiments: a systematic correlation between strength of barite pellets and amount of water was found.

Figure 3. Road map tracing milestones in near room temperature consolidation (T < 300 °C) over the past four decades. Several techniques have been developed, based on dry (top) and wet (bottom) ULES [11–74].

Figure 4. Experimental setups used for ULES. The main differences are presented in Table 1. (a) CS, (b) CSP, (c) HHP, and (d), rHLPS.

Figure 5. Number of publications on ULES per year, as of June 2019: (a) in dry conditions using CS; (b) in the presence of a liquid using HHP and CSP.

Table 1. Comparative analysis of ULES techniques.

	Gutmans’ Group, 1979, Cold Sintering, CS [12], Israel	Yamasaki’s Group 1984, Hydrothermal Hot Pressing, HHP [27,75], Japan	Riman’s Group, 2007, reactive Hydrothermal Liquid-Phase Sintering, rHLPS [76], USA	Randall’s group, 2016 Cold Sintering or Cold Sintering Process (CSP) [77], USA
Key features	No heating and uniaxial pressure (Figure 4(a))	Hydrothermal conditions and uniaxial pressureFigure 4(c)	Hydrothermal reactive sintering (no uniaxial external pressure), Figure 4(d). Sample pre-compaction/firing is needed prior to rHLPS.	Heating and uniaxial pressure (not hydrothermal) Figure 4(b)
Pressure	0.5 to 4 GPa	<300 MPa	Hydrothermal	<1 GPa
Temperature	Room temperature + annealing	<350 °C	<250 °C	<250–300 °C above the boiling point of the liquid
Pressing time	15 s, 1 min sintering + extraction (whole cycle, pressing and extraction)	10–60 mins dwelling time	<70 h hydrothermal reaction time	<60 min
Dies design	Piston-cylinder apparatus from sintered carbides. Figure 4(a)	Consolidation under uniaxial pressure and within a hydrothermal sealed chamber. Figure 4(c)	Cold pressing + hydrothermal reactions. Figure 4(d)	Piston-cylinder die. Figure 4(b)
Sample diameter	Diameter 5–20 mm	10–30 mm	10–30 mm	5–15 mm
Hydrothermal conditions (Y/N), presence of liquid	Done in dry conditions	Yes, hydrothermal reactor chamber (acidic or caustic solutions)	Yes, reactive solvent to produce a desired compound	No, acidic or caustic solutions and organic solvents
Pressure built up between solid (particle) and liquid during sintering (capillarity excluded).	NA	Hydrothermal pressure	Hydrothermal pressure	No pressure between liquid and solid, liquid is free to escape if porosity is open. Inter-particles liquid film might be instead subjected to pressure.
Second phases employed	Diamond	Resin [45], biomaterials, radioactive waste, metal ceramic composites	Not attempted	Wide range of polymers 2D materials and nanomaterials
Materials	Metals including refractory metals, inorganics as NaCl, CdTe, RbI	Oxides, carbonates, bromides, fluorides, chlorides and phosphates see	Oxides, carbonates, fluorides, chlorides and phosphates.	Oxides, carbonates, bromides, fluorides, chlorides and phosphates their composites
Remark	Not applicable to brittle materials	Some materials might require further heat treatment to complete consolidation and/or remove excess of water		The evaporation step is included in the processing as the sintering chamber is not sealed
Sintering mechanism	Plastic deformation of ductile particles	Transient liquid phase sintering		Transient liquid phase sintering, plastic deformation or surface/grain boundary diffusion activated by the liquid reaction with the powder
Final density	Close to theoretical	70–98%, full density achieved with a post treatment	70–90%	80-100%
Typical Advantages	Avoids grain growth or thermal degradation in metastable and nanostructured materials [78]	Low temperature sintering for better replication of bio-mineralisation Low temperature nuclear waste encapsulation Recycling of waste into artificial rocks [79] Crystallisation accelerated by hydrothermal conditions [80]		Single step for sintering consolidation near to full density Integration of Ceramics with polymers [81] improved functional properties possible integration in 3d printing

Note: Key features, thermodynamic processing parameters and composition are taken into account. Sealing of sintering chamber, presence of a liquid, processing under hydrothermal or atmospheric conditions are the main distinctive features.

Table 2. Primary bond characteristics in dry and wet conditions. A similar table, proposed by Alberts et al. in a biology textbook [167], is useful to understand the formation of primary bonds in ULES.

Type of bonds	Metallic	Ionic	Covalent bonding
Bond strength in vacuum/dry conditions kJ mol^–1	110–350	335-1050	63-1070
Bond strength in water kJ mol^–1	Almost unaffected. Corrosion to be expected	Reduced by more 80 times due to higher permittivity in water (ϵr = 111 formamide, ϵr = 80.1 water, ϵr = 1 air)^a	Almost unaffected
Solubility in water	Insoluble (corrosion)	Generally soluble, governed by solubility product (Kps)^b	Insoluble*
Directionality	Non directional, ions surrounded by free electrons	Non directional, complete electron transfer from metal non-metal atoms	Directional, sharing of electrons in the valence shell between non-metals or metalloids
Electronegativity difference	N.A.	>1.7	mixed bonding: 0.5–1.7; pure covalent <0.5

^aCoulomb law (the electrostatic force, $F$) with permittivity ε_r (1) $F=\frac{q\cdot q^{\mathrm{\prime }}}{4\pi \cdot {\epsilon }_0\cdot {\epsilon }_r\cdot r^2}$(1)

^bIonic compounds are in general highly soluble in water or polar solvents. However, some salts are insoluble (concentration < 0.01 M) due to their low solubility product constant (K_ps). For example NaCl solubility in water at 50 °C is ∼370 g L^–1 while AgCl is ∼5 mg L^–1 [168,169].

Table 3. Processing route, solubility, ionic species.

Material	Process	Solubility in water at 25 °C (g L^–1)	Ionic species	Comments/feature	Ref.
CaCO3 Nano-CaCO3	HHP	1.3 × 10^−2	Ca^2+,${\mathrm{C}\mathrm{O}}_3^{2-}$, ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$	Salt compound – strong base/weak acid	[191,192]
SiO2 – Amorphous	CSP – HHP	3.9 × 10^−2	$\mathrm{S}\mathrm{i}\mathrm{O}(\mathrm{O}\mathrm{H}{)}_3^{-}$,$\mathrm{S}\mathrm{i}\mathrm{O}(\mathrm{O}\mathrm{H}{)}_2^{2-}$	Extensively used in other low temperature processes of densification	[193–196]
SiO2 – Quartz	CSP – HHP	2.9 × 10^−3	$\mathrm{S}\mathrm{i}\mathrm{O}(\mathrm{O}\mathrm{H}{)}_3^{-}$, ${\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{O}\mathrm{H}\right)}_2^{2-}$	processed mostly in alkaline environment	[196–198]
NaCl	CSP	357	$\mathrm{N}\mathrm{a}({\mathrm{H}}_2\mathrm{O}{)}_4^{+}$, Cl^–	Salt compound – strong base/strong acid	[199,200]
NaNO2	CSP	820	$\mathrm{N}\mathrm{a}({\mathrm{H}}_2\mathrm{O}{)}_4^{+}$, ${\mathrm{N}\mathrm{O}}_2^{-}$	Salt compound – strong base/weak acid	[201]
WO3	CSP	insoluble	${\mathrm{W}\mathrm{O}}_4^{2-}$	Almost totally stable in water – it could only be superficially hydrated	[202,203]
ZnO	CSP	6 × 10^−6	$\mathrm{Z}\mathrm{n}({\mathrm{H}}_2\mathrm{O}{)}_6^{2+}$	Highly dissolvable in acetic acid solution or biological buffers	[204–206]
V2O3	CSP	0.1	[VO2(H2O)4]^+	Less stable metal oxide due to the r/s ̴ 8	[207–209]
V2O5	CSP	8	[VO(H2O)5]^2+	Solubility enhances as the valence increases (r/s > 8)	[207–211]
MgO	CSP	6.2 × 10^−5	$\mathrm{M}\mathrm{g}({\mathrm{H}}_2\mathrm{O}{)}_4^{2+}$	Quite stable in water – r/s ∼3	[212–214]
ZrO2	CSP	Negligible	$[\mathrm{Z}\mathrm{r}{\left(\mathrm{O}\mathrm{H}\right)}_2\cdot 4{\mathrm{H}}_2\mathrm{O}{]}_4^{8+},$[HZrF6]^–	Highly stable metal oxide - dissolvable in HF	[215,216]
TiO2	HHP	Insoluble	As for Zr plus $\mathrm{T}\mathrm{i}(\mathrm{O}\mathrm{H}{)}_5^{2-},\mathrm{T}\mathrm{i}(\mathrm{O}\mathrm{H}{)}_6^{-}$	The hydrophobicity renders this material insoluble in water – when hydrophilic it is the most stable metal oxide	[217–219]
BaTiO3	CSP – HHP	Insoluble	Ba^2+, $\mathrm{T}\mathrm{i}(\mathrm{O}\mathrm{H}{)}_5^{2-},$ $\mathrm{T}\mathrm{i}(\mathrm{O}\mathrm{H}{)}_6^{-}$	Titanium hydrates at high temperatures tend to form TiO2 prior reaction with Ba^2+	[83,220]
MoO3	CSP	4.9(28 °C)	MoO^2+, $\mathrm{M}{\mathrm{o}}_2{\mathrm{O}}_4^{2+}$, $\mathrm{M}{\mathrm{o}}_2{\mathrm{O}}_3^{4+}$, MoO^3+, ${\mathrm{M}\mathrm{o}\mathrm{O}}_2^{2+}$	Protonisation produces several cations	[221–225]
Li2MoO4	CSP	82.6	Li^+, ${\mathrm{M}\mathrm{o}\mathrm{O}}_4^{2-}$^-	Reduction (in acid media) produces many molybdate salts	[224,226,227]
Na2Mo2O7	CSP	8.4	Na^+, $\mathrm{M}{\mathrm{o}}_2{\mathrm{O}}_7^{2-}$		[224]
K2Mo2O7	CSP	15.3	K^+, $\mathrm{M}{\mathrm{o}}_2{\mathrm{O}}_7^{2-}$		[224]

Figure 6. (a) Aqueous geochemistry and speciation of ‘Hard’ (also referred to as ‘Type A’) Cations (All electrons removed from outer shell). Extract of periodic table from an Earth Scientist’s Periodic Table [229]. Ionic potentials follow the order F > O>N = Cl > Br > I>S (isolines). (b) Logarithm of the solubility of their oxide in water at 25 °C [229].

Figure 7. Cumulative number of papers of typical materials processed using CS, HHP and CSP. According to Marie et al. [136], more than 50 materials have been processed using CSP. Considering the chemical composition presented in Table 1 and their combinations (metals/ceramics and organics/ceramics) the list of materials processable by ULES could become endless.

Figure 8. An overview of the room temperature consolidation mechanisms in metals and ceramics: (a) Hertizian contact between two sintering particles showing an increased stress at the contact point which might contribute to increased solubility at the neck during ULES; (b) Application of the Panneli and Filho equation applied to several dry pressed materials (adapted from [179]); (c) formation of a hydrated layer during HHP of silica (taken from [244] and re-printed with the permission of the editor); (d) sintering kinetics during CSP of ZnO showing plastic flow sintering (adapted from [245]); (e) and (f) grain growth kinetics in ZnO during CSP, indicating that grain boundary diffusion is activated even at low temperature (adapted from [204]).

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