Ba and BaO on W and on Sc2O3 coated W

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 Sc2O3- 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 8Cb (1272 K), a typical operating temperature, or at 1125 8Cb (1477 K), a typical activation temperature. Our investigation found that a partial layer of BaO formed onW, 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 Sc2O3-coated W substrate, heating to the activation temperature is necessary for the inter-diffusion between the Sc2O3 andWto occur. BaO layers form a stronger bond to the inter-diffused layer than to pureW. However, the most important finding is that a stable BaO-containing compound forms and continues to accumulate under the impinging flux on the Sc2O3 and W covered substrate at 940 8Cb. 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, Sc2O3 and W. However, further investigation is necessary to determine if the Raju and Maloney type layer is indeed present on top of scandate cathodes.