WE WILL DEVELOP SEVERAL TECHNOLOGIES TO ENABLE SIGNIFICANTLY LARGER ARRAYS OF X-RAY MICROCALORIMETER SENSORS FOR FUTURE NASA MISSIONS SUCH AS THE X-RAY SURVEYOR. IN PARTICULAR WE WILL DEVELOP SENSOR FABRICATION AND READOUT STRATEGIES COMPATIBLE WITH ARRAYS OF 10^5 OR MORE SENSORS. OUR READOUT STRATEGIES RELY IN PART ON A NOVEL SWITCHING ELEMENT RECENTLY DEVELOPED AT NIST THAT HAS NOT PREVIOUSLY BEEN USED WITH MICROCALORIMETER SENSORS. OUR WORK WILL ENABLE IN-FOCAL PLANE MULTIPLEXING IN WHICH THE SENSORS AND READOUT CIRCUITRY ARE CO-LOCATED. THE SCALABILITY OF THIS APPROACH IS HIGHLY FAVORABLE FOR LARGE SENSOR ARRAYS. WE WILL ALSO DEVELOP XRAY MICROCALORIMETERS USING NOVEL MATERIALS CONSISTING OF DOPED ALMN ALLOYS. THESE MATERIALS ALLOW PRECISE CONTROL OF THE TRANSITION TEMPERATURE OVER 150 MM WAFER AREA. THIS WAFER SCALE IS NECESSARY FOR ARRAYS OF 10^5 TO 10^6 SENSORS.