Ernst Kessler

Deep space instrument design for thermal infrared imaging with MERTIS

MERTIS is a miniaturized thermal infrared imaging spectrometer onboard of ESAs cornerstone mission BepiColombo to Mercury. It shall provide measurements in the spectral range from 7-14 μm with a spatial resolution of maximal 300 m and 80 spectral channels in combination with radiometric measurements in the spectral range from 7-40 μm. The instrument concept therefore integrates two detector systems sharing a common optical path consisting of mirror entrance optics and reflective Offner spectrometer. Uncooled micro-bolometer and thermopile radiometer technology are implemented for lowest power consumption. Subsequent viewing of different targets including on-board calibration sources will provide the desired performance. Special attention is spent on the fully passive thermal design in the harsh environment around Mercury. The article will provide an overview of the 3 kg - instrument design and highlight the concept of the subsystems and technologies used. The status of the development process will be reported.

New high detectivity linear array for analytical measurement in the room temperature range

A newly designed highly detective intra-slit thermoelectric room-temperature linear array is presented. The thermoelectric sensor array ZS-64-2 with 64 individually readable channels was designed and developed for IR spectroscopy. It is suitable for analytical measurement technology, for example, in determining the age of technical oils in real time. For this, the selected absorption bands of the oil are analyzed and evaluated. This allows conclusions be drawn about the condition of lubricants and coolants, so that they can be replaced when it is needed. This method helps to save valuable resources and it helps to avoid costly damages. In order to achieve the high detectivity of D* = 1.8 x 109 Jones the sensor was designed and optimized to be operated under vacuum conditions. For minimizing the thermal cross talk between the individual pixels, they are separated from each other by a 50 micron slit in the self-supporting silicon nitride membrane, which has a thickness of nearly 1 micron.

Responsivity and detectivity modelling of thermal radiation sensors based on a biased thermocouple

Thermal radiation sensors are based on two signal transduction stages: radiation to thermal and thermal to electrical. The most common of these sensors are the radiation thermocouple using the Seebeck effect and the bolometer applying the thermoresistive effect. While the bolometer requires a bias current for signal generation the thermocouple is generally operated unbiased. The paper theoretically investigates a biased thermocouple instead, which can be thought of as a combination of both thermal radiation sensor types. Its responsivity and detectivity are calculated based on previous theories of the performance of bolometers and radiation thermocouples, respectively, thereby including the Peltier effect. The electrical resistance and thermal conductance of the thermocouple as input parameters for these calculations are modelled using a simple strip geometry to facilitate one-dimensional analytical electrothermal modelling.

A Planar Thin-Film Peltier Cooler for the Thermal Management of a Dew-Point Sensor System

The development of new application fields of microelectromechanical systems (MEMS) devices often implies a high degree of complexity and the integration of several functionalities within these devices. In this paper, thermoelectric planar thin-film microcoolers become more and more important due to their cooling and temperature stabilization ability, respectively, in MEMS devices. This paper reports on the investigation of the design, manufacturing, and characterization of a membrane-integrated thin-film thermoelectric cooling arrangement for active temperature control and precise local cooling of the sensitive region in a thin-film dew-point sensor. The sensor concept is based on the combination of a thermal sensor heater and a planar thin-film Peltier cooler, which are all arranged on a freestanding and thermally insulated membrane. To obtain a high performance concerning the maximum temperature decrease of the active-cooled membrane, a highly efficient thermoelectric materials combination of Sb and Bi0.87Sb0.13 was used for the fabrication of the in-plane Peltier configuration. For the first sensor setup, a temperature decrease of 10.6 K was achieved under atmospheric conditions at 293 K. In combination with an externally assembled two-stage Peltier cooler dew-point, temperature measurements down to 213 K (-60 °C) were performed in a climatic exposure test cabinet.

Chemical and Electrochemical Synthesis of Platinum Black

We present electrochemical and chemical synthesis of platinum black at room temperature in aqueous and non-aqueous media. X-ray analysis established the purity and crystalline nature. The electron micrographs indicate that the nanostructures consist of platinum crystals that interconnect to form porous assemblies. Additionally, the electron micrographs of the platinum black thin layer, which was electrochemically deposited on different metallic and semiconductive substrates (aluminium, platinum, silver, gold, tin-cooper alloy, indium-tin-oxide, stainless steel, and copper), indicate that the substrate influences its porous features but not its absorbance characteristics. The platinum black exhibited a broad absorbance and low reflectance in the ultraviolet, visible, and infrared regions. These characteristics make this material suitable for use as a high-temperature resistant absorber layer for the fabrication of microelectronics.

HP3-RAD: A compact radiometer design with on-site calibration for in-situ exploration

Many processes on planetary bodies are driven by their respective surface energy balance, and while planetary climate is influenced by the dynamics of the atmospheric boundary layer, surface radiation drives the Yarkovksy and YORB effects on small airless bodies. In addition, insolation governs cometary activity and drives the dust cycle on Mars. The radiative flux received and emitted at the surface of solar system bodies is thus a fundamental quantity, which is driven by the reception of solar radiation in the visible wavelength band, while re-radiation primarily occurs in the thermal infrared. Knowledge of the relevant radiative fluxes enables studies of thermo-physical surface properties, and radiometers to measure surface brightness temperatures have been payloads on many missions. The HP3-RAD is part of the Heat Flow and Physical Properties Package (HP3) on the InSight mission to Mars. It is a light-weight thermal infrared radiometer with compact design. HP3-RAD measures radiative flux in 3 spectral bands using thermopile detectors. The 120 g device includes integrated front-end electronics as well as a deployable cover that protects the sensors from dust contamination during landing. In addition, the cover is simultaneously used as a calibration target. The instrument concept as well as its implementation will be described, and special emphasis will be put on technological challenges encountered during instrument development. Potential future improvements of the design will be discussed.

Membrane based thermoelectric sensor array for space debris detection

As manmade space debris in the low earth orbit becomes an increasing risk to space missions, which could even result in total mission loss, it has become even more critical to have detailed knowledge of the properties of these particles like the mass, the velocity and the trajectory. In this paper, we present a newly designed, highly sensitive impact detector array with 16 pixels for space debris analysis. The thermopile sensor array, which was developed in the project, consists of 16 miniaturized multi-junction thermopile sensors made by modern thin-film technology on Si wafers. Each thermopile sensor consists of 100 radially arranged junction pairs formed from evaporated antimony and bismuth thin films. The centrally located active (hot) junctions comprise the active area of 1 mm². The output e.m.f. of the sensor is proportional to the temperature difference between the active and the reference junctions. The thermopile requires no cooling and no bias voltage or current for operation. It generates no 1/f noise but only the thermal resistance (Nyquist) noise. The sensor can be used for DC and low frequency AC measurements. The impact energy of micro sized particles is measured by a calorimetric principle. This means that the kinetic energy of the particle is converted into heat by hitting the absorbing foil, which is glued on the surface of the membrane area. This setup in combination with a preceded velocity detector allows the measurement of the most interesting particle quantities mass, velocity and trajectory.

Thermoelectric radiation sensors for the space mission BepiColombo to Mercury

We present a newly designed thermoelectric detector chip of high detectivity for the space mission BepiColombo to Mercury. The sensor is part of the MERTIS radiometer, which enables radiometric measurements in the spectral range from 7-40 micron to study the thermo-physical properties of the planets surface material. In collaboration with the DLR Institute of Planetary Research, the Institute of Photonic Technology has developed a sensor array with a specific detectivity D* of 1.3 x 109 Jones in vacuum environment and 2 x 15 individual readable channels. In addition, it has an optical slit in the middle, which serves as the entrance slit of a spectrometer downstream. The sensor area is coated with an absorbing layer, in this case silver black having an absorption coefficient of nearly 100 percent in a wavelength range from 0.4 up to 20 micron. To minimize the thermal cross talk between the individual pixels, each pixel is separated by a 50 micron slit in the self-supporting silicon nitride membrane. For good mechanical stability of the pixels, the pixel membrane is tensioned by 10 micron bridges like braces. The sensor is electrically contacted with a star-flex PCB by direct wire bonding and both are mounted on milled aluminum housing. At the Institute of Photonic Technology (IPHT), high detectivity radiation sensors are developed and based on the thermoelectric principle. The thermoelectric materials used are the highly effective combination of n-bismuth(87%)- antimony(13%) / p-antimony. The sensors are designed, in the main, as miniaturized multi-junction thermocouples and made by state of the art thin film technologies allowing for achievable and reproducible detectivities D* in the range of 108 up to 2 x 109 Jones.