page_title: Hot cathode
section_title: Boride cathodes
hierarchical_section_title: Hot cathode / Types / Boride cathodes
caption_attribution_description: English: Lanathanum Hexaboride Cathode
context_page_description: In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. The heating element is usually an electrical filament heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.
  From the 1920s to the 1960s, a wide variety of electronic devices used hot-cathode vacuum tubes. Today, hot cathodes are used as the source of electrons in fluorescent lamps, vacuum tubes, and the electron guns used in cathode ray tubes and laboratory equipment such as electron microscopes.
context_section_description: Lanthanum hexaboride (LaB₆) and cerium hexaboride (CeB₆) are used as the coating of some high-current cathodes. Hexaborides show low work function, around 2.5 eV. They are also resistant to poisoning. Cerium boride cathodes show lower evaporation rate at 1700 K than lanthanum boride, but it becomes equal at 1850 K and higher. Cerium boride cathodes have one and a half times the lifetime of lanthanum boride, due to its higher resistance to carbon contamination. Boride cathodes are about ten times as "bright" as the tungsten ones and have 10-15 times longer lifetime. They are used e.g. in electron microscopes, microwave tubes, electron lithography, electron beam welding, X-Ray tubes, and free electron lasers. However these materials tend to be expensive.
  Other hexaborides can be employed as well; examples are calcium hexaboride, strontium hexaboride, barium hexaboride, yttrium hexaboride, gadolinium hexaboride, samarium hexaboride, and thorium hexaboride.
caption: Lanthanum hexaboride hot cathodes
exif: {"Image ImageWidth": "1226", "Image ImageLength": "1523", "Image BitsPerSample": "[8, 8, 8]", "Image Compression": "LZW", "Image PhotometricInterpretation": "2", "Image Make": "Applied Graphics Technologies", "Image Model": "Digital Link", "Image Orientation": "Horizontal (normal)", "Image SamplesPerPixel": "3", "Image XResolution": "300", "Image YResolution": "300", "Image PlanarConfiguration": "1", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop Elements 4.0 Windows", "Image DateTime": "2009:01:23 12:02:54", "Image ExifOffset": "328", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "466", "Thumbnail JPEGInterchangeFormatLength": "6937", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "1226", "EXIF ExifImageLength": "1523"}