The atmosphere of Venus is always a difficult subject for science. The surface of Venus is the most hostile operating environment of any of the planets in our solar system. Exploring planet Venus has been always an interesting subject for mankind. Improving understanding of Venus and its greenhouse effect atmosphere and geology has relevance to a better understanding of the Earth and solar system formation. The big problem is the high surface temperature of Venus that is approximately 452 ?C (850 F).
Venus is very similar to Earth in size, initial composition, and solar radioactive influences. However, its present planetary conditions contrast drastically from that of Earth. The surface of Venus has been explored by a number of missions from Earth, In pervious last years, some probes has been sent to Venus to examine its surface due to the high temperature and high pressure on the surface of Venus they did not survive more than two hours. However, the research on Venus planet surface is limited due to the lack of electronics components sensor systems which has to operate on the harsh environment of Venus.
The missions to the Venus planet have significant value but most of sensors and electronic components cannot operate at the 450 ?C temperature of surface of Venus. The new generation of wide band gap semiconductors devices that are silicon carbide, diamond, and gallium nitride have tendency to operate on at temperatures from 500°C to 650°C. Till now, the wide band gap devices has perform a few minutes to a few hours of durability when these wide band gap devices are electronically operate at high temperature. For very high temperature of Venus surface, wide band gap devices must best and stable for long time operation under electrical devices at 450°C temperature without any prominent drift in electrical operating parameters.
In this report, i am going to discuss about high temperature electronic devices and materials that operates on high temperature environment of Venus Planet.
2- Venus Planet
Venus is the second planet from the Sun and the sixth largest planet of solar system. Venus is very only planet that is same in structure and size to Earth. Venus is the hottest planet in our solar system with surface temperature that is enough to melt lead. Venus has a harsh atmosphere and its atmosphere consist of carbon dioxide with clouds of sulfuric acid. Recently, scientists have detected small amounts of water traces in the atmosphere. The surface of Venus is very dry. During its evolution, ultraviolet rays from the sun evaporated its water very quickly.
The orbital period of Venus is 225 days.
The atmosphere of Venus is 90 times denser than earth.
With a radius of 6,052 kilometers, Venus is roughly the same size as Earth.
Atmosphere composition is of 96.5% of CO2 and a 3% of nitrogen.
Venus is only a 30% closer to the Sun than Earth.
Surface pressure 92 bars.
Venus has an iron core that is approximately 3,200 kilometers in radius.
Venus is the second planet from the sun and our closest planetary neighbor.
Distance from the sun is approximately 67 million miles.
Venus spins slowly in the opposite direction of most planets.
3- What is Venus Probe and history?
Venus is planet that has been reached first by a space probe. In 1962, Mariner 2 was collected the information from surface of Venus and thses information was about its temperature, atmosphere and rotational periods. The Venera 7 was the first probe to land on the surface of Venus. But unfortunately, it was out of operation within an hour after reaching the surface of Venus due to high temperature and pressure. In 1982, Venera 13 has transmitted the first color pictures from the surface of Venus.
However, till now, Venera 13 has survived the longest on Venus’ surface for approximately 2 hours. Another way is to actually send a probe to the atmosphere of Venus instead of planet surface. The Vega 1 balloon in 1984 survived for 47 hours at a height of 53 Km from the planet’s surface. But for my report, I will just be talking about the probe landing on the surface and look into selecting and implementing novel design ideas for high temperature electronics for such a mission.
The Magellan spacecraft, launched in 1989, arrived at Venus on August 10, 1990. Before its demise in October 1994, Magellan was able to collect radar images of 98% of Venus’ surface. The below table shows the probe that has been landed on the atmosphere and surfaces of Venus an there survival times.
Venus Express is the ESA’s (European Space Agency) first mission to the planet Venus. It is launched in November 2005 and it reached at Venus planet in April 2006 and sending back information to the earth from Venus.
Key High Temperature Components
Pressure vessel integrated with advanced thermal control
High temperature electronics
1. Active Components
2. Passive Components
Thermionic Vacuum Devices (TVD)
High temperature energy storage
Pressure vessel integrated with advanced thermal control
One of the most important parts of a Venus rover which directly connected to high temperature electronics is the pressure vessel with advanced thermal control. It protects the brain system of rover from harsh environment of Venus’s surface. The figure below shows the concept of a pressure vessel with passive thermal control.
Passive thermal control doesn’t have a cooling system and was used on each of the Venera landers but without using high temperature electronics. They kept their electronics cool with cumbersome hermetically sealed chambers, and sometimes the inner components were also pre-cooled to around -10°C before being dropped into the atmosphere by the orbiter.
Active thermal control contains a powerful cooling system to keep the temperature of electrical components as low as possible. Currently, this technology is still under development and engineers of Glenn Team in NASA are working on this project.
Typical analytical results from a well-designed thermal system
Active Devices Solid State
Silicon on Insulator Devices:
Silicone on Insulator (SOI) based electronic devices operating at 300°C, can be used within the thermally controlled Venus Lander for atmospheric probes. These type of electronic devices help to relieve thermal control load and mainly possible the survival and lifetime of Venus mission.
For operating mainly up to 300°C, leakage current has also to maintain using Silicone on Insulator (SOI) technology.
SOI technology is currently used in oil drilling/geothermal wells monitoring equipment.
Wide Band Gap Devices
For electronics that needs to operate above 300°C, wide band gap semiconductors devices have to be used. The following figure shows the temperature and intrinsic carrier Density of different semiconductors.
SiC devices and primitive circuits have been demonstrated to operate ~ 500°C
SiC technology is currently the most promising technology.
However, the maximum operating temperature for SiC is 600°C, but problems with diffusion and oxidation of metal contact layers dramatically reduce both operation temperature and lifetime. In that case, proper metal layers must be chosen to reduce diffusion and oxidation. Also other facts such as integration and metallization process must be considered.
The temperature has great impact on the aging of SiC devices.In the below figure it can be see that the influence of temperature on aging process of SIC devices.
Recently scientists at NASA Glenn team invented new type of SIC device. 4H-SiC JFET integrated circuits (up to 24 transistors, with two levels of metal interconnect) and ceramic packaging that have consistently functioned for more than 1000 hours (41.7 days) at 500°C in Earthatmosphere oven-testing. This is a one big step forward for Venus landers because this SIC device doesn’t need cooling or thermal system or even a pressure vessel. Figures below indicate steps of Venus surface test on 4H-SiC JFET.
Almost-completed assembly showing the 3 mm x 3 mm SIC ring oscillator chip attached to ceramic substrate and fiberglass-sleeved wiring bundle prior to any heated testing. Complete assembly prior to heat testing showing the mesh screen cap that permits chip immersion in simulated Venus atmosphere during the test. After 1000 hours (21.7 days) of simulated Venus surface conditions testing following removal of the mesh screen cap. During clip-lead electrical testing conducted with the chip disconnected from the short circuited feed through by removal of the nickel alloy wires.
Thermionic Vacuum Devices (TVD)
Thermionic vacuum tube is inherently high temperature device that controls electric current between electrodes in an evacuated reservoir. Vacuum tubes mostly rely on thermionic release of electrons from a hot filament or a cathode heated by the filament. They are one of the first and oldest high temperature electronic devices. Although by inventing transistors in 1950s these tubes almost faded from electronic industries, they are still useful for some cases related to microwave, high-frequency amplifiers and especially for pressure sensor pre-amplifier for Venus rovers.
Thermionic Vacuum Devices (TVDs) are high temperature devices and the reason is cathode is designed to be operating at 700 to 900°C.
Thermionic Vacuum Devices (TVDs) still need to be optimized for harsh environment of Venus because there are some challenges ahead of using them for Venus probe.
The graphs below indicate the Influence of temperature on thermal vacuum tubes and figure shows direction of electron flow and the grid is in between cathode and anode.
Capacitors are also key elements of high temperature wireless system for Venus rovers. Current capacitors are not ready for a long time surface mission on Venus. This technology is still developing. Before making a capacitor for extreme harsh environment, we should consider some parameters such as capacitance, leakage current, equivalent series resistance, voltage rating, dissipation factor, dielectric absorption, and volumetric or weight efficiency.
Diamond Capacitors are theoretically functional to almost over 500°C with stable, high capacitance. Still under development to achieve uniform diamond film and stable metal contacts
One of critical component of a wireless sensor system is the local oscillator that generates the signal, which will be modulated by the sensor and data will be transmitted to cooler environments.
Now these days, the Glenn team in NASA is working on high temperature oscillators to advance the state of the art of the SiC devices, as well as to improve the passive devices. However, the ability for high temperature wireless communication to operate up to Venus temperatures is achieved within the next years.The figure below shows high temperature oscillators that has been tested by Glenn NASA team in condition similar to surface of Venus.
For resistors, we can utilize two types, thin film resistors and thick film resistors. The very high temperatures lead to following problems for resistors. There are some factors for choosing the best resistor for Venus rover such as:
By looking at table, we can see that Ruthenium Silver and Ruthenium Oxide resistors have highest maximum operating temperature. Also Ruthenium Oxide has superior stability, low thermal stress and low noise. But the main problem with these two resistors is that ruthenium is very rare and hard to find. In that case the cost of these resistors would be high. So technically we should look for another type of high temperature resistors.
Packaging Materials and Technology
Due to the extreme temperatures being utilized, normal soldering will not be much effective and we will need to utilize brazing process. For harsh environments, the operation of electronics and sensors requires packaging technologies beyond those for regular electronics and sensors. The primary problem that can arise is the selection of interconnect material combinations involves inter-diffusion of the pad and wire metals.
Interconnect: The main concern with the selection of interconnects material combinations involves Inter diffusion of the pad and wire metals.
As can be seen Al-Al pads with melting temperature of Aluminum and Au-Au with melting temperature Gold are the best choices in order to use for harsh environment of Venus’s surface. But beside temperature, oxidation and stresses must be considered. In that case Au-Au seems to be better choice since it has better stability and reliability.
Die Attach: The other important thing to look at is the attachment material for the components. The function of a die attach material is to secure a die to the substrate, to ensure electrical connectivity to the backside of the die, and to ensure that the die does not fracture following power and temperature cycles. Thus this is a very critical and important part of our electronic assembly which can make or break the entire system irrelevant of whether we had chosen other components optimized for our environment and for the task at hand
I would recommend using 72 Ag/28Cu die attach material for our Venus probe. Finally, we have to select a substrate material itself. As this is the base on which all of our design be built, we need certain properties in our substrate material, namely
temperature resistance (min. 150°C permanent)
mechanical stability (strength, elasticity)
chemical resistance (moisture, solvents, corrosion)
Recently, ceramic (aluminum nitride and aluminum oxide) substrates and gold (Au) thick-film metallization based chip-level electronic packages and printed circuit boards have been designed and fabricated for testing high-temperature devices. The electrical interconnection system of this advanced packaging system, including the thick-film metallization and wire bonds, has been successfully tested at 500°C in an oxidizing environment for over 5000 hours with DC electrical bias. Electrically conductive die-attach materials with low curing temperature are being developed for packaging of SiC devices.
An 96% aluminum oxide based packaging material system was successfully used to facilitate the test, previously described above, of an in-house-fabricated SiC MESFET under electrical bias in a 500°C air ambient for more than 2000 hrs. The packaging components continued to successfully operate without observable electrical degradation for the full duration of the 500°C test that exceeded 2000 hours in duration. Further, the demonstration of a functional 500°C amplifier, discussed above, highlights the most recent progress in printed circuit board level packaging and passive devices for 500°C and is a significant step towards 500°C and Venus mission
Fig A: The test assembly of 500°C amplifiers based on SiC MESFETs and aluminum oxide packaging system. The test assembly includes four test circuits.
Fig B: Aluminum Nitride PCB designed for AlN packages.
By looking at those reports, I believe that 96% aluminum oxide (Al2O3) based packaging material system is the best candidate for a long time surface mission on Venus.