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Topic Review
Green Monopropellants
Green propellants are usually defined as low-hazard, low-toxicity, environmentally friendly propellants that are considered safe during various phases of spacecraft development, launch, and operations. Such propellants provide safe handling and storability when compared to conventional toxic propellants such as hydrazine and its derivatives that require special handling protocols and adhering to strict safety measurements that, in addition to others, include using Self-Contained Atmospheric Protective Ensemble (SCAPE) suits. Due to their favorable characteristics, green propellants demonstrate higher commercial value by being able to cut costs related to transportation, storage, handling, and further reduces ground operations time. Recently, a more specified definition has been noted by Mayer et al. (2018) [3], based on the Acute Toxicity Classification (ATC) by the Global Harmonized System of classification and labeling of chemicals (GHS) [6], which denotes that propellants possessing ATC levels of three and safer are considered as green propellants. ATC levels are typically categorized on a 1:5 scale where level one denotes the most toxic class and level five is considered the least toxic class. Moreover, a controversial topic arises when referring to some modern green propellants, whether to address them by the term “monopropellants” or by more specific terms including (premixed propellants, fuel blends, or mixtures). Monopropellants are fundamentally defined as propellants consisting of chemical compounds (for example N2H4), which release energy through exothermic chemical decomposition. Since the evolution of liquid gun propellants based on HAN compound and other nitrate salts aqueous solutions, the term “monopropellants” was used to describe such premixed formulations. As widely used in literature and industry, some modern green propellants, for instance the Energetic Ionic Liquids (EILs), are undoubtedly classified and described as “monopropellants.” Basically, it can be interpreted from the previous that a "green monopropellant" may be defined as: "A low-hazard, low-toxicity, and safe-to handle propellant that is stored in a single tank and is able to decompose from its storage state by the help of a catalyst or other ignition method, such as thermal or electric ignition, can be considered a “green monopropellant” as long as it does not require another separately stored propellant for decomposition."
  • 5.2K
  • 28 Sep 2021
Topic Review
Supercritical Water Gasification (SCWG)
Gasification with supercritical water (SCWG) is a thermochemical process which, exploiting the properties of supercritical water (374.1 °C and 22.1 MPa ), allows to obtain a syngas rich in hydrogen. Both biomass and waste plastic can be used as feedstock.
  • 5.0K
  • 18 Sep 2020
Topic Review
Compact Cassette
The Compact Audio Cassette (CAC) or Musicassette (MC), also commonly called the cassette tape or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. It was developed by Philips in Hasselt, Belgium, and released in 1962. Compact cassettes come in two forms, either already containing content as a prerecorded cassette, or as a fully recordable "blank" cassette. Both forms are reversible by the user. The compact cassette technology was originally designed for dictation machines, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel tape recording in most non-professional applications. Its uses ranged from portable audio to home recording to data storage for early microcomputers. The first cassette player (although mono) designed for use in car dashboards was introduced in 1968. Between the early 1970s and the early 2000s, the cassette was one of the two most common formats for prerecorded music, first alongside the LP record and later the compact disc (CD). Compact Cassettes contain two miniature spools, between which the magnetically coated, polyester-type plastic film (magnetic tape) is passed and wound. These spools and their attendant parts are held inside a protective plastic shell which is 4 by 2.5 by 0.5 inches (10 cm × 6.3 cm × 1.3 cm) at its largest dimensions. The tape itself was commonly referred to as "eighth-inch" tape, supposedly ​1⁄8 inches wide, but it was slightly larger: 0.15 inches (3.81 mm). Two stereo pairs of tracks (four total) or two monaural audio tracks are available on the tape; one stereo pair or one monophonic track is played or recorded when the tape is moving in one direction and the second (pair) when moving in the other direction. This reversal is achieved either by flipping the cassette, or by the reversal of tape movement ("auto-reverse") when the mechanism detects that the tape has come to an end.
  • 4.5K
  • 18 Oct 2022
Topic Review
Pole Weapon
A pole weapon or pole arm is a close combat weapon in which the main fighting part of the weapon is fitted to the end of a long shaft, typically of wood, thereby extending the user's effective range and striking power. Pole weapons are predominantly melee weapons, with a subclass of spear-like designs fit for both thrusting and throwing. Because many pole weapons were adapted from agricultural implements or other fairly abundant tools, and contained relatively little metal, they were cheap to make and readily available. When warfare would break out and the belligerents had a poorer class who could not pay for dedicated military weapons, leaders would often appropriate tools as cheap weapons. The cost of training was comparatively minimal, since these conscripted farmers had spent most of their lives using these "weapons" in the fields. This made polearms the favored weapon of peasant levies and peasant rebellions the world over. Pole arms can be divided into three broad categories: those designed for extended reach and thrusting tactics used in pike square or phalanx combat; those designed to increase leverage (thanks to hands moving freely on a pole) to maximize centrifugal force against cavalry; and those designed for throwing tactics used in skirmish line combat. Because their versatility, high effectiveness and cheap cost, polearms experimentation led to many variants and were the most frequently used weapons on the battlefield: bills, picks, dane axes, spears, glaives, guandaos, pudaos, poleaxes, halberds, harpoons, sovnyas, tridents, naginatas, bardiches, war scythes, and lances are all varieties of pole arms. Pole arms were common weapons on post-classical battlefields of Asia and Europe. Their range and impact force made them effective weapons against armored warriors on horseback, because they could be dismounted and/or penetrate said armor. The Renaissance saw a plethora of varieties. Pole arms in modern times are largely constrained to ceremonial military units such as the Papal Swiss Guard or Yeomen of the Guard, or traditional martial arts. Chinese martial arts in particular have preserved a wide variety of weapons and techniques.
  • 4.5K
  • 09 Nov 2022
Topic Review
Possessions of Muhammad
The possessions of the Islamic prophet Muhammad are a group of his items and possessions such as weapons, animals and others known with unique names. There is doubt about the attribution of these possessions to Muhammad, as many of them were lost during wars and tribulations. The scholar Ahmed Taymour says - after listing the remnants attributed to Muhammad in Constantinople (later Istanbul): "It is no secret that some of these remnants are likely to be true. However, we did not see any of the trustworthy people who mentioned it with proof or denial, for Allah (God), Glory be to Him, who knows about it best, and some of them cannot conceal what the idea or notion that stumbles on of suspicion and disputes it in doubts." The emir Ahmad Ibn Tulun mentioned in his book “Mufākahat al-Khullān fī Hawādith Az-Zamān”, in the incidents of the year 19 AH (640 CE) and 900 AH (1500 CE) that some claimed that they had a mug and some crutches of Muhammad and that “it was found that they are not of the possessions of the Prophet Muhammad, but rather they are the fragments of Al-Layth ibn Sa'd." The scholar Al-Suyuti said, on the authority of Muhammad (his garment): "This (the possessions) was used by the caliphs to inherit it and put it on their shoulders in processions sitting and riding, and it was on the able-bodied when he was killed and contaminated with blood, and I think it was lost in the temptation of the Tatars, for we belong to Allah (God) and to Him we shall return."
  • 4.5K
  • 11 Nov 2022
Topic Review
Intelligent Tutoring System
An intelligent tutoring system (ITS) is a computer system that aims to provide immediate and customized instruction or feedback to learners, usually without requiring intervention from a human teacher. ITSs have the common goal of enabling learning in a meaningful and effective manner by using a variety of computing technologies. There are many examples of ITSs being used in both formal education and professional settings in which they have demonstrated their capabilities and limitations. There is a close relationship between intelligent tutoring, cognitive learning theories and design; and there is ongoing research to improve the effectiveness of ITS. An ITS typically aims to replicate the demonstrated benefits of one-to-one, personalized tutoring, in contexts where students would otherwise have access to one-to-many instruction from a single teacher (e.g., classroom lectures), or no teacher at all (e.g., online homework). ITSs are often designed with the goal of providing access to high quality education to each and every student.
  • 4.5K
  • 10 Oct 2022
Topic Review
Heaviest Trains
The heaviest trains in the world are freight trains hauling bulk commodities such as coal and iron ore. The weight of trains generally does not include the weight of the operating locomotives; this is not considered dead weight, so is not included. If for example a train had two locomotives operating and was simply hauling a third off line, this third locomotive would be included in the payload weight.
  • 4.5K
  • 01 Nov 2022
Topic Review
List of Devices Using Mediatek SoCs
This page lists every major device using a MediaTek system-on-a-chip (SoC), organized by which chip the devices uses.
  • 4.3K
  • 24 Nov 2022
Topic Review
Sexual Robots
The area of human-robot interaction (HRI), particularly concerning sexual robots, has begun to attract interest in various social issues, such as emotions, ethics, philosophy, and psychology. These new relationships between sexual robots and humans have also awakened the interest of the media, the industry, and the maker world since with a 3D printer, it is already possible to create a sexual robot. Society has begun to consider the idea of having sex with robots, and there is the belief that this will be normal in the future. Although there is still no scientific evidence of its therapeutic benefits, many think it can help treat sexual dysfunctions or even help decrease women’s sexual exploitation. Like sex toys, some experts consider sexual robots (or sexbots) to be the future of sex relationships potentially. 
  • 4.3K
  • 11 Jan 2021
Topic Review
Blended Learning
Blended learning is an approach to education that combines online educational materials and opportunities for interaction online with traditional place-based classroom methods. It requires the physical presence of both teacher and student, with some elements of student control over time, place, path, or pace. While students still attend "brick-and-mortar" schools with a teacher present, face-to-face classroom practices are combined with computer-mediated activities regarding content and delivery. Blended learning is also used in professional development and training settings. Since blended learning is highly context-dependent, a universal conception of it is difficult. Some reports have claimed that a lack of consensus on a hard definition of blended learning has led to difficulties in research on its effectiveness. A well-cited 2013 study broadly defined blended learning as a mixture of online and in-person delivery where the online portion effectively replaces some of the face-to-face contact time rather than supplementing it. Additionally, a 2015 meta-analysis that historically looked back at a comprehensive review of evidence-based research studies around blended learning, found commonalities in defining that blended learning was "considered a combination of traditional f2f [face to face] modes of instruction with online modes of learning, drawing on technology-mediated instruction, where all participants in the learning process are separated by distance some of the time." This report also found that all of these evidence-based studies concluded that student achievement was higher in blended learning experiences when compared to either fully online or fully face-to-face learning experiences. "Blended learning" is sometimes used in the same breath as "personalized learning" and differentiated instruction.
  • 4.3K
  • 17 Oct 2022
Topic Review
Czochralski Process
The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platinum, silver, gold), salts and synthetic gemstones. The process is named after Polish scientist Jan Czochralski, who invented the method in 1915 while investigating the crystallization rates of metals. He made this discovery by accident: instead of dipping his pen into his inkwell, he dipped it in molten tin, and drew a tin filament, which later proved to be a single crystal. The most important application may be the growth of large cylindrical ingots, or boules, of single crystal silicon used in the electronics industry to make semiconductor devices like integrated circuits. Other semiconductors, such as gallium arsenide, can also be grown by this method, although lower defect densities in this case can be obtained using variants of the Bridgman-Stockbarger technique.
  • 4.2K
  • 24 Nov 2022
Topic Review
AK-630
The AK-630 is a Soviet and Russian fully automatic naval close-in weapon system based on a six-barreled 30 mm rotary cannon. In "630", "6" means 6 barrels and "30" means 30 mm. It is mounted in an enclosed automatic turret and directed by MR-123 radar and television detection and tracking. The system's primary purpose is defense against anti-ship missiles and other precision guided weapons. However it can also be employed against fixed or rotary wing aircraft, ships and other small craft, coastal targets, and floating mines. Once operational, this weapon system was rapidly adopted, with up to 8 units installed in every new Soviet warship (from mine-hunters to aircraft carriers), and hundreds produced in total.
  • 4.2K
  • 08 Oct 2022
Topic Review
Surface Passivation
Passivation, in physical chemistry and engineering, refers to coating a material so it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. In electrochemical treatment of water, passivation reduces the effectiveness of the treatment by increasing the circuit resistance, and active measures are typically used to overcome this effect, the most common being polarity reversal, which results in limited rejection of the fouling layer.[clarification needed] When exposed to air, many metals naturally form a hard, relatively inert surface layer, usually an oxide (termed the "native oxide layer") or a nitride, that serves as a passivation layer. In the case of silver, the dark tarnish is a passivation layer of silver sulfide formed from reaction with environmental hydrogen sulfide. (In contrast, metals such as iron oxidize readily to form a rough porous coating of rust that adheres loosely and sloughs off readily, allowing further oxidation.) The passivation layer of oxide markedly slows further oxidation and corrosion in room-temperature air for aluminium, beryllium, chromium, zinc, titanium, and silicon (a metalloid). The inert surface layer formed by reaction with air has a thickness of about 1.5 nm for silicon, 1–10 nm for beryllium, and 1 nm initially for titanium, growing to 25 nm after several years. Similarly, for aluminium, it grows to about 5 nm after several years. Surface passivation refers to a common semiconductor device fabrication process critical to modern electronics. It is the process by which a semiconductor surface such as silicon is rendered inert, and does not change semiconductor properties when it interacts with air or other materials. This is typically achieved by thermal oxidation, in which the material is heated and exposed to oxygen. In a silicon semiconductor, this process allows electricity to reliably penetrate to the conducting silicon below the surface, and to overcome the surface states that prevent electricity from reaching the semiconducting layer. Surface passivation by thermal oxidation is one of the key features of silicon technology, and is dominant in microelectronics. The surface passivation process was developed by Mohamed M. Atalla at Bell Labs in the late 1950s. It is commonly used to manufacture MOSFETs (metal-oxide-semiconductor field-effect transistors) and silicon integrated circuit chips (with the planar process), and is critical to the semiconductor industry. Surface passivation is also critical to solar cell and carbon quantum dot technologies.
  • 4.1K
  • 22 Nov 2022
Topic Review
Improvised Artillery in the Syrian Civil War
Improvised artillery in the Syrian Civil War are improvised firearms created and used by factions of the Syrian Civil War, most notably Syrian opposition forces. The weapons include the Hell Cannon and its variants, the Thunder Cannon and the Mortar Cannon. The weapons have been criticized for being inaccurate.
  • 4.1K
  • 21 Oct 2022
Topic Review
Smart City Planning
The smart city concept is established as those “cities with an intelligent economic, institutional, social and physical infrastructure, which ensure the centralization of their citizens in a sustainable environment”, this being a great challenge to achieve.
  • 4.1K
  • 06 Jun 2021
Topic Review
SpaceX Dragon 2
The SpaceX Dragon 2 is a class of reusable spacecraft developed and manufactured by American aerospace manufacturer SpaceX as the successor to the Dragon cargo spacecraft. It has two variants: Crew Dragon, a human-rated capsule capable of ferrying up to seven astronauts, and Cargo Dragon, an updated replacement for the original Dragon spacecraft. The spacecraft launches atop a Falcon 9 Block 5 rocket and returns to Earth via an ocean splashdown. Unlike its predecessor, the spacecraft can dock itself to the ISS instead of being berthed. Crew Dragon is equipped with an integrated launch escape system (LES) capable of accelerating the vehicle away from the rocket in an emergency at 11.8 m/s2, accomplished by using a set of four side-mounted thruster pods with two SuperDraco engines each. The spacecraft features redesigned solar arrays and a modified outer mold line compared to the original Dragon, and possess new flight computers and avionics. As of March 2020, four Dragon 2 spacecraft have been manufactured (not counting structural test articles that were never airborne). Crew Dragon serves as one of two spacecraft that is expected to transport crews to and from the International Space Station (ISS) under NASA's Commercial Crew Program, the other being the Boeing CST-100 Starliner. It is also expected to be used in flights by American space tourism company Space Adventures and to shuttle tourists to and from Axiom Space's planned space station. Crew Dragon's first non-piloted test flight occurred in March 2019, and its first crewed flight – with astronauts Robert Behnken and Douglas Hurley – occurred in May 2020. This test flight marked the first time a private company launched a crewed orbital spacecraft. Cargo Dragon is expected to supply cargo to the ISS under a Commercial Resupply Services-2 contract with NASA, along with Northrop Grumman Innovation Systems' Cygnus spacecraft and Sierra Nevada Corporation's Dream Chaser spacecraft. The first flight of the Cargo Dragon is planned to launch in October 2020.
  • 4.0K
  • 26 Oct 2022
Topic Review
Dhunge Dhara
A dhunge dhara (Nepali:ढुङ्गे धारा Listen (help·info)) or hiti (Newari) is a traditional stone drinking fountain found in Nepal. It is an intricately carved stone waterway through which water flows uninterrupted from underground sources. Dhunge dharas are part of a comprehensive drinking water supply system, commissioned by various rulers of Ancient and Medieval Nepal. The system is supported by numerous ponds and canals that form an elaborate network of water bodies, created as a water resource during the dry season and to help alleviate the water pressure caused by the monsoon rains. After the introduction of modern, piped water systems, starting in the late 19th century, this old system has fallen into disrepair and some parts of it are lost forever. Nevertheless, many people of Nepal still rely on the old hitis on a daily basis.
  • 4.0K
  • 21 Oct 2022
Topic Review
Anti Urination Devices in Norwich
Anti urination devices were a form of hostile architecture installed in Norwich and the surrounding area in the late 19th century to discourage public urination. The overcrowded and narrow streets of the city centre and a lack of public toilets led to men urinating against the side of buildings, but the installation of new public urinals to address the issue was delayed by disputes over where they were to be sited. Anti urination devices were built in places which suffered particular problems with public urination, and were intended to discourage men from urinating at that spot. Most were built of sloped or curved stone, flint or concrete, and were shaped such that anyone attempting to urinate against the wall would need to stand well away from the wall in public view, hopefully discouraging them from doing so. The slope of the structure meant that should anyone still attempt to urinate against it, the stream of urine would be deflected back onto their feet and legs. A few instead consisted of a spiked metal bar positioned across a corner at the height of a typical man's groin, and were intended to dissuade men from approaching the corner with their genitals exposed. Following improved public toilet provision from the 1890s onwards, the problems caused by the lack of urinals became less of an issue, and anti urination devices ceased to be installed. Although most metal examples were removed during the Second World War, and many others have been demolished in subsequent years, around 30 remain in place in central Norwich with further surviving examples in other parts of East Anglia.
  • 4.0K
  • 07 Oct 2022
Topic Review
Passive Films
The outstanding corrosion resistance of many metals and alloys results from the presence of a thin oxide-“passive”-film on the bare metal surface formed during its exposure to an oxidising environment. Once a film is formed, typically 1-3 nm thick, the reaction rate between the metallic material and the environment will be several orders of magnitude lower. The original theory of film formation goes back to Michael  Faraday, who in the 19th century studied iron surfaces and found them “altered”. A review of the early days of passive film research has been written by Uhlig (1979). A general introduction to the theory of passivity has been published by Sato (1990), whereas the electronic properties of passive films on different materials have been  reviewed by Schultze and Lohrengel (2000). The characterisation of the composition and structure of such thin films and the study of their interaction with corrosive environments requires a combination of sophisticated experimental techniques, namely electrochemical methods, XPS, ESCA, AES, SIMS, ISS, ARXPS, XANES, ICP-AES/MS, and others, which are leading to advanced progress over the past two decades.
  • 4.0K
  • 21 Dec 2020
Biography
K. Eric Drexler
Kim Eric Drexler (born April 25, 1955) is an American engineer best known for popularizing the potential of molecular nanotechnology (MNT), from the 1970s and 1980s. His 1991 doctoral thesis at Massachusetts Institute of Technology was revised and published as the book Nanosystems: Molecular Machinery Manufacturing and Computation (1992), which received the Association of American Publishers awa
  • 4.0K
  • 18 Nov 2022
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