K. Sigfrid Yngvesson

Microwave Synthesis of Zeolites: Effect of Power Delivery

The effect of microwave power magnitude and pulsing frequency on the synthesis enhancement of zeolites, silicoaluminophosphate SAPO-11, silicalite, and NaY, was studied. Pulsing the microwave power compared to continuous delivery at the same averaged fed microwave power showed no effect on the nucleation and crystallization rates of SAPO-11, silicalite, or NaY. However, SAPO-11 synthesized with continuous microwave power delivery produced larger particles compared to pulsed microwave power with the same reaction time (3.77 microm for continuous versus 2.49 microm for pulsed 1 s on; 3 s off). Further, pulsed microwave power delivery used lower steady state power to maintain the same reaction temperature compared to continuous power delivery (55 W compared to 65 W, respectively). The microwave power used to heat the reaction precursors of SAPO-11 and silicalite was varied by applying cooling gas at various rates while maintaining the reaction temperatures. Significant enhancement of the crystallization rate for SAPO-11 was observed with increasing the fed microwave power (0.014 min(-1) at 65 W, 0.030 min(-1) at 130 W, and 0.066 min(-1) at 210 W), with little effect on the nucleation time. The crystallization rate to microwave power relation was found to obey a power curve (y = 0.4259x(2) - 0.2776x + 0.8517). Lower microwave power produced larger crystals but required longer reaction time to complete crystallization (3.77 microm at 65 W compared to 2.04 microm at 210 W). Conversely, silicalite synthesis at 150 degrees C was found to be independent of the magnitude of the applied microwave power.

Imaging Front-End Systems For Millimeter Waves And Submillimeter Waves

Imaging (multi-beam) front-end systems utilize a reflector or a lens, as well as a focal-plane array of receptor elements (beams). To be practical, such systems must utilize focal plane arrays which can be fabricated using hybrid integrated (and eventually monolithic) technology. This paper reviews alternative approaches to focussing elements, auxiliary devices required for local oscillator injection and similar functions, and focal plane arrays. It also discusses general limitations and advantages of these approaches, as well as typical applications to millimeter and submillimeter astronomical instrumentation.

Wideband IF-Integrated Terahertz HEB Mixers: Modeling and Characterization

This paper presents a detailed study of the IF properties of phonon-cooled NbN hot electron bolometer (HEB) terahertz mixers and receivers, performed over the widest intermediate frequency range reported so far. First, a broadband technique is used to obtain the IF small-signal output impedance of HEB mixers using network analyzer measurements. The impedance is measured over an IF range of 300 kHz to 8.5 GHz under the presence of dc bias and local oscillator (LO) illumination. The LO frequencies (f LO) used in this investigation range from 694 GHz to nearly 2 THz, covering frequencies below and above the superconducting bandgap frequency of the NbN film for the first time. The impedance (real and imaginary parts) can be fitted to the Standard model for HEB devices over almost the entire range of frequencies measured. The impedance data are then used in conjunction with a computer-aided design model for the monolithic micorwave integrated circuit IF amplifier to design and analyze the performance of integrated receivers (mixer/IF amplifier combinations). The measured and predicted variations of the receiver noise, mixer gain, and mixer output noise with IF are in good agreement. A record IF noise bandwidth of 8 GHz (obtained for f LO= 694 GHz) is demonstrated for a receiver designed using the technique described in this paper.

Microwave effects in exhaust catalysis

Many attempts have been made to employ electromagnetic energy as a selective coreagent in catalytic reactions. Photocatalysis employs visible light for a variety of oxidation reactions on a very limited series of catalysts, those employing TiO2. In contrast, attempts to excite specific atom-atom bonds of adsorbed species in the infrared by vibrational resonance have not proved to break the specific bonds selectively. The differences are significant. Many recent studies have suggested that microwave energy can be employed in catalysis and that the results differ from “conventional” heating of the systems studied. Although “photocatalysis” is well accepted, “microwave-catalysis” has not here-to-fore been accepted as an approach to change the selectivity or efficiency of catalysis in the presence of microwave energy. We have studied the influence of microwave energy on sorption and catalysis, particularly on automotive exhaust catalysis. The development of a new generation of automotive exhaust catalysts faces several significant challenges, which might be overcome by the use of microwave energy. The first challenge is to shorten the time required for catalyst “light-off,” since a disproportionate fraction of the pollutants are produced as the catalyst is heated up to operating temperature. The second challenge is the influences of sulfur contaminants that impede the catalytic activity, particularly during light-off. Based on preliminary experiments, we propose that indeed microwave energy can induce catalyst light-off more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst.

Corrections to “Wideband IF-Integrated Terahertz HEB Mixers: Modeling and Characterization” [May 10 1140-1150]

In the above titled paper (ibid., vol. 58, no. 5, pp. 1140-1150, May 10), some errors have been detected. The corrections are presented here.

Ultrafast terahertz detector and mixer using a hot electron 2DEG device

Hot electron bolometric (HEB) detectors and mixers for the THz frequency range, which use thin-film superconductors, have been developed recently. They have short response times due to efficient cooling of ht hot electrons by either (i) phonon transmission from the film to the substrate, or (ii) diffusion of the electrons into the contacts. We have previously demonstrated a 2DEG detector which uses the heated 2D electron gas medium, as well as phonon-cooling. Here we propose and analyze a new version of this detector which employs diffusion cooling. A response time of 1 ps and responsivity of 3,000 V/W are calculated for a device which is 0.8 (mu) l long. This response time is considerably shorter than for any of the superconducting HEB detectors. The predicted double sideband receiver noise temperature for the mixer version is in the range 1,000 K to 2,000 K at 1 THz, with a 100 GHz intermediate frequency bandwidth. The new detector could be operated at 77K and the local oscillator power is estimated to be about 1 (mu) W.