Solvothermal

The big thing here is beeing able to dissolve various compounds which are not dissolvable at regular conditions.

  • H\(_2\)O, T>100\(^\circ\)C
  • Other solvents: NH\(_3\), HF, HBr, Cl\(_2\), HCl, CO\(_2\), SO\(_4\), H\(_2\)S, CS\(_2\), C\(_2\)H\(_5\)OH +++
  • NH\(_4\) is common (Ammonothermal)
  • Hydrothermal:
    • Crystallization of large crystals
    • Synthesis of e.g. oxide powders (ex: Zeolites)
    • Leaching e.g. treatment of ores
  • Started 1845
    • Important for big crystals of quartz
      • Grown by the kilotonne in 1900
      • Used for electronics, watches(oscillators), optical properties, laser windows, prisms
    • Other important crystals: ZnO2, Emerald, Calcite

Technical

  • Usually more material dissolves at higher T
  • H\(_2\)O with higher T:
    • Ion product increases
    • Viscosity decreases
    • Polarity (dielectric constant) decreases (but increases with pressure)
  • Synthesis usually in closed vessels
    • T-P-V considerations are CRUCIAL

Two main synthesis types:

  • Isothermal: powder synthesis
  • T-gradient: for larger crystals

Sub vs Supercritical

  • Closed autoclave -> autogeneous pressure
  • H\(_2\)O above these conds is supercritical T: 374.15\(^\circ\)C, P = 220bar.

Autoclave filling

  • 32% filling -> H\(_2\)O fills autoclave at T\(_c\).
    • Higher filling results in full autoclave at lower T, and pressure increase.
    • ex: 80% filling at 245\(^\cric\)C -> autoclave explosion

Mineralizers

  • Used to increase crystallization rates
  • Usually F\(^-\) or OH\(^-\) (alkali metal hydroxides, chlorides..)
  • Quartz synthesis at T-gradient 400-380\(^\circ\)C at 1kbar
    • Solubility too low at these temps, NaOH(or others) can be added

Retrograde solubility

  • When solubility decrease at higher T.
  • Can be due to properties change in solvent or compound
  • Ex: SiO\(_2\) decrease in solubility over 350\(^\circ\)C as long as below 700bar.
    • Also seen for halides, calcium carbonate ..

Hydrothermal

Advantages:

  • Moderate T (100-300\(^\circ\)C) subcritically at autogeneous pressures.
  • Possible to synthesise materials below transformation temperatures (\(\alpha\)-CuI @ 390\(^\circ\)C, \(\alpha-\beta\) transition for Quartz @ 580\(^\circ\)C)
  • Transition metals can be made with unusual ox-states (CrO\(_2\))
  • Prep of metastable phases (GeO\(_2\) with Quarts structure using Quarts seed)
  • Formation of Zeolites + other microporous materials
  • Usually dissolution/Precipitation mechanisms

Leaching:

  • Bayer process -> extract high-grade Al(OH)\(_3\) from bauxite ore
    • Hydrothermal extraction w/dense NaOH solution via reaction to soluble aluminate complex(NaAl(OH)\(_4\))
    • Al(OH)\(_3\) precipitated by cooling, diluting and seeding
    • Heated to corundum for Al metal production.

Eh-Ph diagrams

  • Ox state can be controlled by pH and V.
  • Diagrams change with T and P
    • Lines pushed down and left at higher T

Buffers: Used to control the potential to ensure spesific ox-states. Hydrogen permeable membranes can be used. CuO can buffer to ensure Fe\(^{2+}\) is not made.

Setups

Synthesis in T-gradient

  • Nutrient (polycrystalline powder of precursor) placed in bottom
  • Baffle (perforated disc) separates dissolution and growth zones and reduces particle flow
  • T\(_1\)< T\(_2\) -> convection to transport hot L to GrowthZone.
  • Required: some wt% solubility 0.001-0.1wt% sol diff over 10

Autoclaves

  • Open and closed vessels
  • Morey: Closed, up to 400\(^\circ\)C, 400 bar, simple, autogeneous
  • Tuttle: “Cold seal” outside oven, may be watercooled. Pressure from external source, up to 1100\(^\circ\)C, 5000bar.
logo