A synthetic element is one of 24 chemical elements that do not occur on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor or particle accelerator, or explosion of an atomic bomb. The synthetic elements are those with atomic numbers 95–118, as shown in purple on the accompanying periodic table: these 24 elements were first created between 1944 and 2010; the mechanism for the creation of a synthetic element is to force additional protons onto the nucleus of an element with an atomic number lower than 95. All synthetic elements are unstable, but they decay at a varying rate: their half-lives range from 15.6 million years to a few hundred microseconds. Seven other elements that were created artificially—and thus considered to be synthetic—were discovered to exist in nature in trace quantities; the first, was created in 1937. Plutonium, atomic number 94, first synthesized in 1940, is another such element, it is the element with the largest number of protons to occur in nature, but it does so in such tiny quantities that it is far more practical to synthesize it.
Plutonium is well-known due to its use in atomic bombs and nuclear reactors. No elements with an atomic number greater than 99 have any uses outside of scientific research, since they have short half-lives, thus have never been produced in large quantities. Any elements with atomic number greater than 94 present at the formation of the earth about 4.6 billion years ago have decayed sufficiently into lighter elements relative to the age of Earth that any atoms of these elements that may have existed when the Earth formed have long since decayed. Atoms of synthetic elements now present on Earth are the product of atomic bombs or experiments that involve nuclear reactors or particle accelerators, via nuclear fusion or neutron absorption. Atomic mass for natural elements is based on weighted average abundance of natural isotopes that occur in Earth's crust and atmosphere. For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning.
Therefore, for synthetic elements the total nucleon count of the most stable isotope, i.e. the isotope with the longest half-life—is listed in brackets as the atomic mass. The first element discovered through synthesis was technetium—its discovery being confirmed in 1937; this discovery filled a gap in the periodic table, the fact that no stable isotopes of technetium exist explains its natural absence on Earth. With the longest-lived isotope of technetium, 97Tc, having a 4.21-million-year half-life, no technetium remains from the formation of the Earth. Only minute traces of technetium occur in the Earth's crust—as a spontaneous fission product of uranium-238 or by neutron capture in molybdenum ores—but technetium is present in red giant stars; the first discovered purely synthetic element was curium, synthesized in 1944 by Glenn T. Seaborg, Ralph A. James, Albert Ghiorso by bombarding plutonium with alpha particles; the discoveries of americium and californium followed soon. Einsteinium and fermium were discovered by a team of scientists led by Albert Ghiorso in 1952 while studying the radioactive debris from the detonation of the first hydrogen bomb.
The isotopes discovered were einsteinium-253, with a half-life of 20.5 days, fermium-255, with a half-life of about 20 hours. The discoveries of mendelevium and lawrencium followed. During the height of the Cold War, teams from the Soviet Union and the United States independently discovered rutherfordium and dubnium; the naming and credit for discovery of these elements remained unresolved for many years, but shared credit was recognized by IUPAC/IUPAP in 1992. In 1997, IUPAC decided to give dubnium its current name honoring the city of Dubna where the Russian team made their discoveries since American-chosen names had been used for many existing synthetic elements, while the name rutherfordium was accepted for element 104. Meanwhile, the American team had discovered seaborgium, the next six elements had been discovered by a German team: bohrium, meitnerium, darmstadtium and copernicium. Element 113, was discovered by a Japanese team; the following elements do not occur on Earth. All have atomic numbers of 95 and higher.
All elements with atomic numbers 1 through 94 occur at least in trace quantities, but the following elements are produced through synthesis. Technetium, astatine and plutonium were discovered through synthesis before being found in nature. "einsteinium - chemical element". Britannica.com. Retrieved 23 May 2017. "mendelevium - chemical element". Britannica.com. Retrieved 23 May 2017. "synthetic elements". Encyclopedia2.thefreedictionary.com. Retrieved 23 May 2017. "It's Elemental - The Element Fermium". Education.jlab.org. Retrieved 23 May 2017. Kulkarni, Mayuri. "A Complete List of Man-made Synthetic Elements". ScienceStuck. Retrieved 15 May 2019
Steven Logan Bennett was a United States Air Force pilot who posthumously received the Medal of Honor for heroism during the Vietnam War on August 8, 1974. He is the namesake of the ship MV Capt. Steven L. Bennett and his name is engraved on the Vietnam Memorial at Panel 01W - Row 051. There have been numerous other dedications held in his honor, they range from streets being named after him to buildings, including a gymnasium and a cafeteria, a sports arena and VFW posts, many monuments. He has been mentioned in several military history books. Captain Bennett has a public park named in his honor in Texas; the President of the United States takes pride in presenting the MEDAL OF HONOR posthumously to CAPTAIN STEVEN L. BENNETT UNITED STATES AIR FORCE20th Tactical Air Support Squadron, Pacific Air Forces. Place and date of action: Quang Tri, Republic of Vietnam, June 29, 1972. For service as set forth in the following Citation: Capt. Bennett was the pilot of a light aircraft flying an artillery adjustment mission along a defended segment of route structure.
A large concentration of enemy troops was massing for an attack on a friendly unit. Capt. Bennett was advised that none was available, he requested artillery support but this too was denied due to the close proximity of friendly troops to the target. Capt. Bennett was elected to strafe the hostile positions. After 4 such passes, the enemy force began to retreat. Capt. Bennett continued the attack, but, as he completed his fifth strafing pass, his aircraft was struck by a surface-to-air missile, which damaged the left engine and the left main landing gear; as fire spread in the left engine, Capt. Bennett realized that recovery at a friendly airfield was impossible, he instructed his observer to prepare for an ejection, but was informed by the observer that his parachute had been shredded by the force of the impacting missile. Although Capt. Bennett had a good parachute, he knew that if he ejected, the observer would have no chance of survival. With complete disregard for his own life, Capt. Bennett elected to ditch the aircraft into the Gulf of Tonkin though he realized that a pilot of this type aircraft had never survived a ditching.
The ensuing impact upon the water caused the aircraft to cartwheel and damaged the front cockpit, making escape for Capt. Bennett impossible; the observer made his way out of the aircraft and was rescued. Capt. Bennett's unparalleled concern for his companion, extraordinary heroism and intrepidity above and beyond the call of duty, at the cost of his life, were in keeping with the highest traditions of the military service and reflect great credit upon himself and the U. S. Air Force. GERALD R. FORD USAF Pilot Badge Capt. Bennett studied at the University of Southwestern Louisiana from 1964-1968, prior to commissioning into the US Air Force in 1968, he attended Pilot Training at Webb Air Force Base in Big Spring, Texas. He and Linda Leveque married in September 1968; the Bennett's had one child, Angela Bennett Engele, who lives in the Dallas–Fort Worth area and is the current president of the OV-10 Association located in Fort Worth and the Volunteer Administrator for the Fort Worth Aviation Museum.
List of Medal of Honor recipients for the Vietnam War "Tropical Times article". Archived from the original on December 3, 2006. Retrieved October 5, 2010. "Medal of Honor citation". Retrieved October 5, 2010. At msc.mil "article at Air Force News". Retrieved October 5, 2010. "Handbook of Texas Online article". Retrieved October 5, 2010. Schneider, Donald K.. Air Force Heroes in Vietnam. Washington: University Press of the Pacific. ISBN 1-4102-0384-0. "article at mishalov.com". Retrieved October 5, 2010. "Steven L. Bennett". Claim to Fame: Medal of Honor recipients. Find a Grave. Retrieved 2009-01-28
Engine balance refers to those factors in the design, engine tuning and the operation of an engine that benefit from being balanced. Major considerations are: Balancing of structural and operational elements within an engine Longevity and performance Power and efficiency Performance and weight/size/cost Environmental cost and utility Noise/vibration and performanceThis article is limited to structural and operational balance within an engine in general, balancing of piston engine components in particular. Balancing of heat engines is a complicated subject that covers many areas in the design, production and operation; the engine considered to be well balanced in a particular usage may produce unacceptable level of vibration in another usage for the difference in driven mass and mounting method, slight variations in resonant frequencies of the environment and engine parts could be big factors in throwing a smooth operation off balance. In addition to the vast areas that need to be covered and the delicate nature, terminologies used to describe engine balance are incorrectly understood and/or poorly defined not only in casual discussions but in many articles in respected publications.
Internal combustion piston engines, by definition, are converter devices to transform energy in intermittent combustion into energy in mechanical motion. A slider-crank mechanism is used in creating a chemical reaction on fuel with air, converting the energy into rotation; the intermittent energy source combined with the nature of this mechanism make the engine vibration-prone. Multi-cylinder configuration and many of the engine design elements are reflections of the effort to reduce vibrations through the act of balancing; this article is organized in six sections: Causes of imbalance: lists the balancing elements to establish the basics on the causes of imbalance. Types of vibration: lists different kinds of vibration as the effects of imbalance. Primary balance: discusses the term "Primary balance". Secondary balance: explains what Secondary balance is, how the confusing terminologies'Primary' and'Secondary' came into popular use. Inherent balance: goes into engine balance discussions on various multi-cylinder configurations.
Steam locomotives: an introduction to the balancing of 2-cylinder locomotives and includes the wheel hammer effect unique to steam locomotives. Although some components within the engine have complex motions, all motions can be separated into reciprocating and rotating components, which assists in the analysis of imbalances. Using the example of an inline engine, the main reciprocating motions are: Pistons moving upwards/downwards Connecting rods moving upwards/downwards Connecting rods moving left/right as they rotate around the crankshaft, however the lateral vibrations caused by these movements are much smaller than the up-down vibrations caused by the pistons. While the main rotating motions that may cause imbalance are: Crankshaft Camshafts Connecting rods The imbalances can be caused by either the static mass of individual components or the cylinder layout of the engine, as detailed in the following sections. If the weight— or the weight distribution— of moving parts is not uniform, their movement can cause out-of-balance forces.
Leading to vibration. For example, if the weights of pistons or connecting rods are different between cylinders, the reciprocating motion can cause vertical forces; the rotation of a crankshaft with uneven web weights or a flywheel with an uneven weight distribution can cause a rotating unbalance. With a balanced weight distribution of the static masses, some cylinder layouts cause imbalance due to the forces from each cylinder not cancelling each other out at all times. For example, an inline-four engine has a vertical vibration; these imbalances are inherent in the design and unable to be avoided, therefore the resulting vibration needs to be managed using balance shafts or other NVH-reduction techniques to minimise the vibration that enters the cabin. The types of imbalance are classified: Reciprocating phase imbalance. For example, the pistons in a V6 engine without an offset crankshaft reciprocate with unevenly spaced phases in a crank rotation. Reciprocating plane imbalance. For example, the offset distance between crank journals in a boxer-twin engine causes a couple on the crankshaft from the equal and opposite combustion forces.
Rotating phase imbalance. For example, if the flywheel has an unbalanced eccentric mass. Rotating plane imbalance. For example, the crankshaft of a boxer-twin engine without counterweights would have eccentric masses of each crank throw located 180° apart, which would cause a couple along the axis of the crankshaft. Torsional imbalance. See Torsional vibration. For example, the twisting deflection of the crankshaft increases as the distance from the clutch surface increases. Radial engines do not experience this type of imbalance. There are three major types of vibration caused by engine imbalances: Reciprocating: A single cylinder, 360°-crank parallel twin, or a 180°-crank inline-3 engine vibrates up and down because there are no counter-moving piston or there is a mismatch in the number of counter-moving pistons; this is a 3. Phase imbalance of reciprocating mass. Rocking: Boxer engines, 180°-crank parallel twin, 120°-crank inline-3, 90° V4, inline-5, 60° V6 and crossplane 90° V8 vibrate rotationally on Z or Y-axis.
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