TRANSITION METAL CARBIDE CATHODES

Transition metal carbides are good candidates for durable electron sources that can perform well even in relatively poor vacuum environments. Carbides are characterized by very high melting points, extreme hardness, and relatively low work functions. Hafnium carbide has the highest melting point of any binary substance at 3890°C and has work functions in the 3.3 to 3.6 eV range. In general, the transition metal carbides have unique properties that can make them well suited for use as thermionic emitters in areas where conventional cathodes would quickly fail. These uses could entail operation in residual oxygen, CO, CO2 atmospheres.

 

APTech has developed zone refining techniques to obtain purified single crystal stock of specific orientations. This material has been used in plane-dependent work function and

surface chemistry studies. We have performance studies of single crystal thermionic emitters and single crystal etched field emitters made from these carbides, which show their utility and robustness for electron sources. Stoichiometry is controlled through the addition of carbon to the starting sintered stock and in controlling the zone refining process. APTech is able to create single crystal HfC and ZrC transition metal carbide material and others upon request.

Transition Metal Carbide Material Data

Molecular Weight

Stoichiometry

Metal Impurities

Lattice Parameter

Melting Temperature

Density

Hardness

Coefficient of Thermal Expansion @ 300-3000 K

Thermal Conductivity

Electrical Resistivity

Effective Work Function (100) at 1800 K

Absolute Work Function (100) at 300 K

Spectral Emissivity  (@ 0.65 microns & 2000 K)

Parameter

Units

HfC

N/A

ppm by wt.

g/cm³

α x 10^6

µΩ-cm

eV

eV

N/A

degrees

g/cm²/s

~1

7.3 ± 0.2

ZrC

NbC

TaC

TiC

VC

Evaporation rate at 1800 K (UHV)

Orientation limit for specific orientation

g/mol

Angstroms

K

Kg/mm²

cal-cm/(s-cm²-C)

~1

~1

~1

~1

~1

7.6 ± 0.2

9.5 ± 0.3

<2

<2

<2

<2

<2

<2

<10^-9

<10^-9

0.645

0.480

0.359

0.365

0.487

0.433

3.50

4.12

3.58

3.53

4.0

4.2

3.8

109

~70

74

170-200

180-250

0.049

0.034

0.053

0.049

--

--

--

2600

2470

1800

3200

12.20

6.59

7.79

14.50

~4160

~3300

~3770

~4150

~3430

4.640

4.700

4.468

4.454

4.328

4.160

190.51

<30

<30

<30

<30

<30

<30

103.23

104.92

192.96

59.91

CeBix

LaB6

Transition Metal

 Carbides

Refractory Metals

Standard

Top Hat

Guard Ring

Etched Tip

Cathode Guide

Application Guide

Rebuilding Services

Refractory Metals
 (Parts)

Source Types

Cathode Dimensions

Guides

Services & Parts

Cathodes

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