Creating your next generation cathodes.
Creating your next generation cathodes.
Applied Physics Technologies, Inc. specializes in thermionic and field emission cathodes. APTech is a specialty producer and supplier of CeBix® cathodes (cerium hexaboride), LaB6 cathodes (lanthanum hexaboride), HfC cathodes (hafnium carbide), CFE and ESE sources. Their cathodes have been used in many different applications: microscopy, microanalysis, additive manufacturing, and other industries that use electron sources in their products and work.
Custom design and fabrication capabilities augment their standard catalogue items to meet their customers’ needs. Partnering is available for your more challenging product requirements, particularly for key component realization. Applied Physics Technologies maintains an active role in fundamental electron emission research and publication, striving to bring the newest ideas from the laboratory to the marketplace.
Kore are proud to be an agent of APTech since 2004. As their agent, we will be happy to help you find the cathodes you need for replacement, OEM, and custom applications. The scope of APTech’s work is outlined below, but please see the Resources tab for their full brochures and guides. They welcome custom cathode applications, so please do not hesitate to get in touch with us.
The unique properties of cerium hexaboride crystals provide stable electron emitting media with work functions less than 2.6 eV. The low work function yields higher currents at lower cathode temperatures, which means greater brightness, or current in the beam, and longer CeBix® cathode life.
CeBix® cathodes have modest vacuum requirements and long shelf life. They only need only be brought up to operating temperature to provide emission, eliminating the activation procedure required of dispenser cathodes. Cebix® cathodes can provide long-term, stable operation at current densities up to 20 A/cm², and may be fabricated in a variety of shapes, with many different heating and mounting configurations as well. CeBix® cathodes are the materials of choice for high current cathodes in a variety of advanced and custom applications. APTech is the only producer of CeBix® cathodes worldwide.
The rare earth hexaboride compounds exhibit an excellent combination of low work function and low volatility. In fact, lanthanum hexaboride (LaB6) is in wide-spread everyday use in a variety of fine-focused electron beam instruments. LaB6 cathodes are ideal for many small spot size applications such as SEM, TEM, sterilization, surface analysis and metrology, and for high current applications such as lithography, x-ray sources, small space-craft thrusters, and even electron-beam welders.
The unique properties of lanthanum hexaboride crystals provide stable electron emitting media with work functions near 2.70 eV. The low work function yields higher currents at lower cathode temperatures than tungsten, which means greater brightness, or current at the beam focus, and longer LaB6 cathode life. Typically, LaB6 cathodes exhibit 10 times the brightness and 50 times the service life of tungsten cathodes. In electron microscope applications, these characteristics translate to more beam current in a smaller spot at the sample, improved resolution, and less frequent cathode replacement. APTech provide the purest and lowest work function LaB6 available anywhere in the world.
APTech grow and fabricate their own high quality, single-crystal materials using a well-defined process called “Inert Gas Arc Float Zone Refining.” An electric arc melts a pressed-powder stick of CeBix® or LaB6 in a controlled atmosphere of inert gas, allowing the liquid-phase zone to freeze onto a selected-orientation seed crystal as the arc is moved along the stick. The finished crystal assumes the desired orientation of the seed with less than 30 parts per million by weight metal impurities. Correct melt zone temperature and process speed minimize excessive boron evaporation to achieve the optimum ratio of metal to boron atoms in the grown crystal.
Crystal orientation can be selected to match the cathode design or application. For electron microscopy, the <100> orientation is most desirable due to its brightness and crystal plane symmetry about the optical axis. As the cathode ages, the plane symmetry ensures an even evaporation rate relative to the axis, maintaining a centred, flat emitting surface. Also, the emission patterns from the symmetric crystal planes will remain consistent as they become more exposed by evaporation, contributing to a brighter beam spot.
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.
Applied Physics Technologies offers both single crystal tungsten wire and single crystal tungsten rod. Diameters range from 0.12 mm (etched wire) to 2.5 mm (as-refined rods). Orientations offered include (100), (111), (210) and (310).
Single crystal tungsten wire is zone refined and axially oriented to the specified crystal plane within 2°. The refining process purifies and reduces impurities to less than 30 parts per million by weight. Standard wire sizes are centerless ground, electrochemically etched to the specified diameter, +/- .0003″, and then cut to the specified length +/- 5%.
Many other refractory elements can be refined in their facilities. These are also purified during the refining process and oriented single crystal rods are produced. APTech have a ready supply of seed crystals of common orientation and can custom make virtually any orientation. Common metals include: Mo, Re, Ir, Ta, Hc, Pt, Ni, etc.
The design of the cathode tip is critical for maximum lifetime and optimum performance. Tip design must also match the specific application’s requirements for beam current, spot size, and brightness. For electron microscopy, a conical tip with a flat emitting surface at the apex has proven to be the optimum design, and what Applied Physics Technologies likes to refer to as its standard cathode style.
With the flat-tipped cone design, changes in both cone angle and flat diameter affect emission characteristics. In general, the small cone angle (60°) results in higher brightness, but a larger angle (90°) provides longer life and easier alignment. Small flat diameters also result in higher brightness plus a smaller source size, but larger flats provide longer lifetimes and more beam current.
These trends allow them to tailor our cathodes to the requirements of practically all thermionic cathode applications. For example, SEM and most transmission electron microscope (TEM) applications are best served by 90° cone angle and 16 µm flat tip. This combination provides high brightness, a moderate source size, and very good lifetime. High resolution TEMs require a 60° cone and a 5 µm flat tip for very high brightness and a small source size.
In applications requiring high total current in a large beam spot, a <310> oriented crystal in a top hat configuration may be preferred, providing a slightly lower work function and large emitting surface.
The cathode’s mount design has a significant impact on performance. The design must be simple, durable and precise. It must resist any movements of the crystal, despite the high operating temperatures, yet be easy to install and align. APTech feel they employ the best mount design in the industry with the Mini Vogel Mount.
In 1988, FEI of Hillsboro, Oregon introduced the Mini Vogel Mount (MVM) to provide the benefits of the original Vogel mount in a smaller, simpler, and more elegant design. Twin posts are rigidly fixed in a thick ceramic base, and bent towards the center in an inverted ‘V’. The posts are made of a molybdenum-rhenium alloy that maintains a high modulus of elasticity even at high temperatures. The posts are spread slightly during assembly to allow placement of small pyrolytic graphite (PG) blocks between the crystal and posts. The blocks act as resistive heaters, and help thermally isolate the hot crystal from the highly conductive posts. When the compressive force of the posts is released, the crystal is held with strength and precision. The clamping force of the posts will remain near 5,000 psi for the life of the cathode.
The structure of the MVM is amazingly robust, sustaining reasonable impact without deviating from geometric specifications. Because the graphite pads shield evaporation of the crystal in the direction of the clamping force, the emitter crystal can be fully utilized without degradation of the mount. Structural failure of the MVM is not a concern when the cathode is operated within the correct temperature and pressure range. Typically, the beam stability of the Mini Vogel Mount cathode exceeds the specification of the system in which it runs.
The use of rhenium shunts in conjunction with the MVM was inherited from FEI. Since many SEMs and their electronics were originally designed around a tungsten hairpin filament, the resistance change with temperature of the PG used in the MVM is inverted. The resistance of the 0.001” thick shunt enables the cathode to better match the temperature and resistance curve of a tungsten filament. In the shunted version, the current bypasses the outer PG block by taking the path of least resistance through the shunt. All of the heating is generated by the inner block. The design takes advantage of the anisotropic properties of PG by orienting the poor thermal conduction plane in the direction between the shunt and the Mo/Re post.
APTech excel at developing specialized cathodes for custom application and research purposes, so get in touch and together we can discuss your cathode options.
Kore Technology is a centre of excellence in time-of-flight mass spectrometer technology and has a very strong R&D capability in terms of its personnel, all of whom have been heavily involved in a variety of analytical instrumentation development programmes.