A study of Tantalum Carbide Crystals Arnab Kundu

Brief Outline Aims & Objectives Description of Tantalum Tantalum Carbide & its applications Structure of Transition metal carbides Experimental work on Tantalum Carbide Micro-hardness test Dislocation etching experiment Recommendation for future study

Introduction

This study is significant because of two most important characteristics of transition metal carbides:  their hardness,  and their highly refractory nature These two properties render the combination of carbides with metals belonging to the transition group highly useful in the industrial world in applications involving high temperature and high strength.

Aims and Objectives  To discuss the physical characteristics exhibited by tantalum carbide at variable temperatures.  To determine the degree of hardness of tantalum carbide crystal at room temperature by means of micro- indentation test.  To study the effect of change in microstructure of the crystal after dislocation itching experiment.

What is Tantalum?

History In Greek, Tantalos: Tantalum was discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. Von Bolton produced the first relatively pure ductile tantalum in 1903.

Occurrence. Tantalum occurs principally in the mineral columbite-tantalite. ·Tantalum ores are found in Australia, Brazil, Mozambique, Thailand, Portugal, Nigeria, Zaire, and Canada. ·Separation of tantalum from niobium requires several complicated steps. ·Several methods are used to commercially produce the element, including electrolysis of molten potassium fluorotantalate, reduction of potassium fluorotantalate with sodium, or reacting tantalum carbide with tantalum oxide.

The essentials Name: Tantalum Symbol: Ta Atomic number: 73 Atomic weight: (1) Group in periodic table: 5 Period in periodic table: 6 Block in periodic table: d-block

Properties ·Tantalum is a grey, heavy, and very hard metal. ·When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminium. ·Tantalum is almost completely immune to chemical attack at temperatures below 150C, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulphur trioxide. ·Alkalis attack it only slowly. ·At high temperatures, tantalum becomes much more reactive. ·The element has a melting point exceeded only by tungsten and rhenium.

Uses · Tantalum is used to make a variety of alloys with desirable properties such as high melting point, high strength, and good ductility. ·Scientists at Los Alamos have produced a tantalum carbide graphite composite material, which is said to be one of the hardest materials ever made. ·The compound has a melting point of 3738C. ·Tantalum is used to make electrolytic capacitors and vacuum furnace parts, which account for about 60% of its use. ·The metal is also widely used to fabricate chemical process equipment, nuclear reactors, and aircraft and missile parts.

What is Tantalum Carbide? It is a metallic powder of a dark light-brown colour. It has a theoretical carbon content of 6.23%. It is only slightly soluble in acids and burns in air with a bright flash. Its melting point ranges between o C.

Applications Though tantalum carbide (TaC) has been proposed for use as wound filaments in the form of wires, it is prohibited due to the low strength of TaC wires. It is of practical importance in the production of cemented multicarbide hard metals. In the industrial world, it can be used in machining-tool materials to reduce the tendency of welding between steel chips and tool material.

Structures of Transition Metal Carbides

Experimental work on TaC The crystal sample used for experimentation was grown at Stanford Research Institute by floating zone melting technique. Experiments performed: 1) Hardness indentation measurement on the sample at room temperature using Knoop indenter. 2) Dislocation etching experiments.

Micro-Hardness Test Knoop Indentations were made on the sample surface using a Leitz micro-hardness tester with 200grams load. The indentations were performed throughout the diameter of the sample at a spacing of 100µ. All indentations were made on the(001) surface of the crystal sample.

Table of measured Knoop Hardness

Knoop hardness at different positions of the sample

Dislocation Etching Experiment After etching, the crystal sample showed 4 distinct different zones which reflect the change in micro-structure. Micro-indentation test revealed the variation in hardness in the 4 zones under observation.

Average Knoop Hardness in 4 Different Zones

Variation of Hardness at 4 Different Zones