Ba and BaO on W and on Sc 2O 3 coated W

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Temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) are used to characterize the surface layers that form under an evaporating flux of a dispenser cathode (which is a Ba and BaO source) on a W substrate and Sc 2O 3-coated W substrate to simulate the surface layer of a conventional dispenser cathode and scandate cathode, respectively. The surface layers were prepared while the substrate was either at 940 °C b (1272 K), a typical operating temperature, or at 1125 °C b (1477 K), a typical activation temperature. Our investigation found that a partial layer of BaO formed on W, similar to the surface layer that forms on a dispenser cathode. Heating to the activation temperature causes the BaO to form a stronger bond with W. For the Sc 2O 3-coated W substrate, heating to the activation temperature is necessary for the inter-diffusion between the Sc 2O 3 and W to occur. BaO layers form a stronger bond to the inter-diffused layer than to pure W. However, the most important finding is that a stable BaO-containing compound forms and continues to accumulate under the impinging flux on the Sc 2O 3 and W covered substrate at 940 °C b. Surface emission models describe successfully all other dispenser cathodes, but fail to explain the emission characteristics of scandate cathodes. Raju and Maloney proposed an alternate model, which requires the presence of a thick layer of semi-conducting material. Our finding suggests that it is possible to form a thick layer from simultaneous presence of BaO, Sc 2O 3 and W. However, further investigation is necessary to determine if the Raju and Maloney type layer is indeed present on top of scandate cathodes.