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Marjan Aslani       

University: The George Washington University
Major: Electrical Engineering
Gradation Date: May 09
Hometown: Vienna, Va
Project:  Multijunction Thermal Converters for Precision AC Voltage and Current Metrology


Multijunction thermal converters (MJTCs) are the most accurate devices used for the measurement of AC voltage and current and they are the primary standards in AC metrology. In this method, the heating effect of an unknown AC waveform is compared to that of a known DC signal using thermal voltage and current converters to find the rms value for the AC signal.
Today, the design and fabrication of MJTCs are based on thin-film technology in which MJTCs are fabricated on a silicon (for high currents) or quartz (for high frequencies) substrate with a dielectric membrane over it. They consist of a heater resistor and an array of thermocouples on either sides of the heater to measure the heater temperature along its length.

The ability of a thermal converter to relate an rms AC voltage or current to a DC value is characterized by a quantity called `ac-dc transfer difference' that is ideally zero.  Currently the average ac-dc difference of thin-film MJTCs is less than 1 µV/V in the audio frequency range and up to a few µV/V at 1 MHz. At frequencies greater than 1 MHz however, the ac-dc uncertainties are too large, in the range of a few hundreds µV/V. 

 

The AC-DC laboratory at NIST offers calibration services to national and commercial laboratories, and is also engaged in research to advance the thermal, electrical, and mechanical properties of these devices in order to reduce their ac-dc differences. 

To analyze the frequency behavior of the MJTCs and to calculate their ac-dc transfer differences, an electrical model has been developed. The heater is modeled as a lossy transmission line by a series of resistors and inductors, and the thermocouples are modeled by blocks of parallel capacitors and resistors. The values for the inductors, capacitors and resistors are carefully measured by a precision LCR meter in the lab at different frequencies from 1KHz to 1MHz. These numbers are then plugged into the model for simulation and the current through out the heater and different branches are measured.

The work in progress for this project includes fabrication of new MJTCs with improved design and alternative materials that would eventually lead to MJTCs with low ac-dc differences in frequencies above 1MHz.


About me:   I grew up Tehran, Iran in a family that emphasized greatly on math and science. I came to the US for my undergraduate studies and I graduated from the George Washington University in DC in May09. In Fall 09, I will be attending Stanford for my graduate studies in VLSI and IC design.


During my undergraduate years, I attended 2 UREs. My firs internship was at UC Berkeley and my second internship was here at NIST. One main reason that I applied to SURF was because my brother who a few years ago attended NIST in Boulder strongly encouraged me to do so. These two internships have truly been enlightening and made me realize that I would like to pursue a carrier in research.


I am really grateful that I had the opportunity of working with two really amazing mentors, Tomas Lipe and Josef Kinard. They did not hesitate to offer me their knowledge and support and I am very thankful for every thing they taught me.

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Marjan Aslani