krsna nb c
TRANSCRIPT
Synthesis of niobium carbide at low temperature and its use in hard metal
By K.Sairam GoudRoll.No:155577
Materials TechnologyDepartment of Metallurgical and Materials Engineering
Contents
• Introduction• Objective • Motivation for the work• Sample Preparation and Experimental Procedure • Results and Discussion• Advantages and Limitations• Conclusion• References
IntroductionNiobium Carbide
Basic Details• Chemical Formula : NbC• Density : 7.820 g/cm3
• Melting point : 3,490 °C
Why and Where to use NbC ?• High hardness, melting point and good thermal conductivity makes it one of
suitable material to be used in hard metals such as cutting tools,drills, abrasives and wear resistant pieces.
How to produce NbC ?• Commonly adopted method in industries is subjecting the Nb2O5 to carbon black
in the temperature range of 1600°C-1800°C for certain time
Objective
• To synthesize NbC using new method at low temperature
(950 °C)and in a short time (2 h)
• To characterize it by XRD,SEM,TG/DTG and granulometry and
thereby investigate its ability to serve as good WC grain inhibitor
by comparing to a commercial NbC powder
Motivation for work
1. Li et al produced NbC whiskers by heating a mixture of Nb2O5
and carbon black at 1120 °C for 30 min
2. Dal et al produced NbC by heating Nb-C composite at 1000 °C
for 4 h under argon(layered oxides=> large amorphous powder)
3. Kim et al produced NbC by reacting Nb2O5 with a mixture of
CH4 and H2 at 850 °C for 8 h (low free carbon contamination)
4. Teixeira da Silva et al used same reactants as in [3] but at
1100 °C for 42 min (high carbon contamination )
Problem Noticed : Either high ‘T’ or High ‘t’
Sample Preparation and Experimental Procedure
1.Preparation• Niobium Oxolate Complex-Fuse Nb2O5 and KHSO4 in the ratio 1:7
-Ground and dissolve in H2SO4
-Neutralize with ammonium hydroxide and precipitate Nb2O5_ nH2O
-The potassium ions are eliminated by repeated filtration
and washing with warm water
-Dissolution of the hydrated oxide in mixture of oxalic
acid and ammonium oxalate, with a 1:3 ratio between
niobium and oxalate ions, and the controlled evaporation
of the solution yield the niobium – oxalate complex
whose composition is (NH4)3[NbO(C2O4)8]_xH2O
• Hard metal
-Three hard metal alloys using raw materials
WC ,NbC and Co powders are produced by wet
ball milling for 48h ,granulation ,pressing and
sintering
2. Experimental Procedure• Carburization Reaction
-Precursor (niobium – oxalate complex )3g taken in ceramic
crucible and placed in alumina reactor attached to furnace
controlled by a T-programmer at 950 °C with 5 °C /min with
mixture of gases H2 (reducer) and CH4 (carburizer) for 2h
• Characterization
1.The niobium complex was characterized by XRD,
SEM and TG/DTG in nitrogen.
2.The product of carburization was characterized by
XRD, SEM and grain size measurement (laser
scattering).
3.The sintered alloys were characterized with regard
to the structure (qualitative WC grain size
evaluation) and hardness (Vickers hardness)
Reactions Involved
Nb2O5 -> Nb2O -> NbOxCy -> NbC
H2 reduces Nb2O5 to Nb2O and other reactions
by carbon
Results and DiscussionTG/DTG Results
Above curves reveal that • decomposition starts at around 100 °C • the material loses mass in temperature intervals: 100 – 145, 180 – 244, 244 – 400 and 600 – 630
°C .• Gases such as CO, CO2, H2O and NH3 evolve.
Fig. TG/DTG curves of the niobium complex calcination in nitrogen
Granulometry and SEM
Fig:Particle size distribution curve of NbC by laser scattering
Fig.SEM pictures of the carburized product (low and high magnifications)
From both results it can be pointed out that ,• Mean particle size of powder is in 40µm• No change in shape of particles • Thus no alteration of the characterstics of the niobium precursor due to carburization
NbC in Hard Metal
Vickers hardness (HV50) of alloys Exp Nb,Com Nb, and NoNb are 1827,1776 and 1629 respectively
(a) (b) (c)
Fig:SEM pictures of the structures of the alloy (a) Com Nb ,(b) ExpNb and (c) NoNb showing the WC pictures of the granulation.
• ExpNb structure is homogeneous ,slightly finer than that of the alloy
ComNb with mixed NbW mixed carbides precipitation (due to the high
amount of NbC added to alloys) is the same. This indicates that the
large particles of the Lab-P NbC powder were very finely milled and a
good dispersion was achieved. The particles of the Lab-P NbC are
highly porous and porous particles are more brittle than bulky ones
• The particles of the Lab-P NbC are highly porous and porous particles
are more brittle than bulky ones
Advantages and Limitations
Advantages • Low temperature and less
time process• Less contamination• Fine particles with different
shapes is possible
Limitations• Gaseous.atmosphere mixture
maintainence• Low temperature => little
chance to eliminate oxygen completely
• High dependence of powder quality on gaseous mixture ratio
Conclusions
• Successfully NbC is sythesised at low temperature (950 °C )
and short time (2h) from niobium – ammonium – oxalate
complex through a gas – solid reaction
• No alteration in shape and size of its particles due to
carburization
• Well dispersion of NbC into WC by milling process is achieved
• Inhibition of WC grain growth by experimental NbC is slightly
more efficient than the commercial powder.
Authors of Paper
F.F.P. Medeiros, A.G.P. da Silva*, C.P. de SouzaDepartment of Chemical Engineering, Universidade Federal do Rio Grande do
Norte, 59072-970, Natal, RN, BrazilReceived 1 May 2001; received in revised form 1 January 2002; accepted 31
January 2002