Carbothermal synthesis of nano-structured metal, carbides and nitrides, using solution processed precursors; from precursor to product

• This thesis describes how solution chemical based carbothermal syntheses can be used for the synthesis of nanostructured zirconium carbide and nitride nanophase powders as well as a nickel-metal coating on NbC for use as a sintering agent. The focus is on the development of novel synthesis routes using atomic or small nanoscale level mixing of the metal oxide and carbon source in the starting precursors, enabling the use of lower synthesis temperatures and non-agglomerated nanosized powders. Detailed characterisation of the final materials and those found during the phase-evolution were performed using TG-DTA, dilatometry, XPS, XRD, IR and Raman spectroscopy, ToF-ERDA, PIXE, SEM-EDX/EBSD/TKD, (S)TEM-EDX/HAADF/EELS and Vickers micro-hardness.Starting materials with various carbon-to-zirconia ratios, composed of 2-4 nm sized ZrO2 particles embedded in amorphous carbon, were prepared by hydrolysis of Zr(OPrn)4 and sucrose. Following heating in argon and nitrogen gas yielded nano-phase ZrC and ZrN powders, respectively. Heating in argon gas to 1495 oC, yielded ca. 75 nm sized, loosely agglomerated ZrC particles after 30 minutes annealing when using a sucrose-C:Zr ratio of 5:1. Weakly agglomerated, ca. 30-40 nm sized ZrC particles with a dense 5 nm thick shell of micro-crystalline or amorphous Zr(C,O) and surplus-carbon was obtained after 3 minutes annealing, with a sucrose-C:Zr ratios of 12-20.The same starting components and heat treatment, but with a nitrogen atmosphere, yielded well-crystallised Zr(N,C) particles centred around 40-90 nm in size, with and without a 4-6 nm Zr(N,C,O) shell; the former with 5-6 sucrose-C:Zr and the latter with 7-8 sucrose-C:Zr, together with surplus carbon. The phase-evolution was studied for the sample with sucrose-C:Zr ratio of 8:1. This showed that the nitridation started as early as at 1200 °C and that shell-less c-Zr(N,C) was obtained at 1400 °C.The other part of this thesis describes solution-based nickel metal coating of a fine NbC powder and its sintering into dense NbC-Ni composites. The coating process produced well-distributed dot-like nickel metal coatings, after heating to 500 °C. The nickel dots sizes varied from 6 to 26 nm depending on the nickel loading. Pellets of the 14 wt. % Ni coated NbC powder were sintered at temperatures up to 1500 °C with 30 minutes annealing. The main sintering step started at 1110 °C and ended at 1375 °C, whereupon a final contraction took place ending at 1500 °C. Nearly full-dense composites were obtained after heating to 1375 and 1405 °C whereas a fully dense composite was obtained after heating to 1500 °C, with 30 minutes annealing. The composites were composed of networked, interconnected NbC grains with sizes of ca. 0.5-5 µm for the 1375 and 1405 °C samples. For the corresponding sample heated to 1500 °C, with a 30 minutes annealing, the size-range was 5-30 µm. The binder phases were well-distributed and highly textured.

Ämnesord

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

Nyckelord

Solution-based chemical processing

sol-gel

ceramic powders

transition metal carbides and nitrides

sintering

nickel binder

mechanical properties

Chemistry with specialization in Inorganic Chemistry

Kemi med inriktning mot oorganisk kemi

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