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Water-shedding surfaces can be made to last
On typical hydrophobic coatings, droplets forming from high-temperature steam soon spread out to coat the surface, quickly degrading their performance. The new coating, seen here, maintains its ability to foster droplet formation over long periods. New approach to hydrophobic material could benefit power plants, cooling systems.
David L. Chandler, MIT News Office
September 20, 2013
Steam condensation is key to the worldwide production of electricity and clean water: It is part of the power cycle that drives 85 percent of all electricity-generating plants and about half of all desalination plants globally, according to the United Nations and International Energy Agency. So anything that improves the efficiency of this process could have enormous impact on global energy use.
Now, a team of researchers at MIT says they have found a way to do just that.
It has been known for years that making steam-condenser surfaces hydrophobic — that is, getting them to repel water — could improve the efficiency of condensation by causing the water to quickly form droplets. But most hydrophobic materials have limited durability, especially in steamy industrial settings. The new approach to coating condenser surfaces should overcome that problem, the MIT researchers say.
The findings are reported this week in the journal Advanced Materials by MIT professors Karen Gleason and Kripa Varanasi, graduate student Adam Paxson and postdoc Jose Yagüe.
“Over the last several decades,” says Varanasi, the Doherty Associate Professor of Mechanical Engineering, “people have always searched for a durable surface treatment” to make condensers hydrophobic. With the discovery of a way to make highly durable polymer coatings on metal surfaces, “the potential impact this can have has now become real.”
The covalent-bonding process the team developed is significantly more stable than previous coatings, he says, even under harsh conditions.
Tests of metal surfaces coated using the team’s process show “a stark difference,” Paxson says. In the tests, the material stood up well even when exposed to steam at 100 degrees Celsius in an accelerated endurance test. Typically, the steam in power-plant condensers would only be about 40 degrees Celsius, Varanasi says.
When materials currently used to make surfaces hydrophobic are exposed to 100 degrees Celsius steam, “after one minute, you start to see them degrade,” Paxson says: The condensing water becomes “a film that covers the surface. It kills the hydrophobic surface, and degrades heat transfer by a factor of seven.” By contrast, the new material shows no change in performance after prolonged endurance tests.
“There was really negligible degradation,” says Gleason, the Alexander and I. Michael Kasser Professor of Chemical Engineering at MIT. According to degradation models, the material might be durable for much longer than these initial tests: “We’re thinking tens of years,” Gleason says.
Varanasi and Paxson were part of a team that published research earlier this year on a different kind of durable hydrophobic material, a rare-earth ceramic. Varanasi says that the two approaches will likely both find useful applications, but in different situations: The ceramic material can withstand even higher temperatures, while the new coating should be less expensive and appropriate for use in existing power plants, he says. “Before, we had nothing, and we have two possible systems now,” he says.
The new coating can easily be applied to conventional condenser materials — typically titanium, steel, copper or aluminum — in existing facilities, using a process called initiated chemical vapor deposition (iCVD).
Another advantage of the new coating is that it can be extremely thin — just one-thousandth of the thickness of conventional hydrophobic coatings. That means other properties of the underlying surface, such as its electrical or thermal conductivity, are hardly affected. “You can create ultrathin films, with no effect on thermal conductivity,” Varanasi says, “so you’re getting the best of all worlds here.”
Sumanta Acharya, the program director for the National Science Foundation's Thermal Transport Processes Program, who was not involved in this research, says, "In my opinion this work represents a major breakthrough in condenser technology. It offers the potential for significantly higher heat-transfer coefficients, high vapor-condensation rates and rapid removal of the condensate." He adds that condensers are widely used in the power industry and in residential heating and cooling, and says this work "can potentially provide radical improvements."
The work was supported by the U.S. Army Research Office though MIT’s Institute for Soldier Nanotechnologies, and by the National Science Foundation.
http://web.mit.edu/newsoffice/2013/water-shedding-surfaces-can-be-made-to-last-0920.html
Understanding a new kind of magnetism
Researchers use low-frequency laser pulses to probe the properties of a kind of fluctuating magnetism known as a spin-liquid state.
David L. Chandler, MIT News Office
September 23, 2013
Using low-frequency laser pulses, a team of researchers has carried out the first measurements that reveal the detailed characteristics of a unique kind of magnetism found in a mineral called herbertsmithite.
In this material, the magnetic elements constantly fluctuate, leading to an exotic state of fluid magnetism called a “quantum spin liquid.” This is in contrast to conventional magnetism, found in materials called ferromagnets — where all of the magnetic forces align in the same direction, reinforcing each other — or antiferromagnets, where adjacent magnetic elements align in opposite directions, leading to complete cancellation of the material’s overall magnetic field.
Although a spin-liquid state has previously been observed in herbertsmithite, there has never been a detailed analysis of how the material’s electrons respond to light — a key to determining which of several competing theories about the material is correct.
Now a team at MIT, Boston College and Harvard University has successfully carried out these measurements. The new analysis is reported in a paper in Physical Review Letters, co-authored by NuhGedik, the Biedenharn Career Development Associate Professor of Physics at MIT, graduate student Daniel Pilon, postdoc Chun Hung Lui and four others.
Their measurements, using laser pulses lasting just a trillionth of a second, reveal a signature in the optical conductivity of the spin-liquid state that reflects the influence of magnetism on the motion of electrons. This observation supports a set of theoretical predictions that have not previously been demonstrated experimentally. “We think this is good evidence,” Gedik says, “and it can help to settle what has been a pretty big debate in spin-liquid research.”
“Theorists have provided a number of theories on how a spin-liquid state could be formed in herbertsmithite,” Pilon explains. “But to date there has been no experiment that directly distinguishes among them. We believe that our experiment has provided the first direct evidence for the realization of one of these theoretical models in herbertsmithite.”
The concept of quantum spin liquids was first proposed in 1973, but the first direct evidence for such a material was only found within the last few years. The new measurements help to clarify the fundamental characteristics of this exotic system, which is thought to be closely related to the origins of high-temperature superconductivity.
Gedik says, “Although it is hard to predict any potential applications at this stage, basic research on this unusual phase of matter could help us to solve some very complicated problems in physics, particularly high-temperature superconductivity, which might eventually lead to important applications.” In addition, Pilon says, “This work might also be useful for the development of quantum computing.”
Leon Balents, a professor of physics at the University of California at Santa Barbara who was not involved in this work, says, “If the observed optical conductivity in these measurements is truly intrinsic, it is an important and exciting result, which will be very important in understanding the nature of the spin-liquid state”.
Balents adds that further work is needed to confirm this result, but says “this is clearly an exciting and important measurement, which I hope will be pursued further by extending the frequency and magnetic field range in the future.”
The work was supported by the U.S. Department of Energy, and also included Young Lee and Tian-Heng Han of MIT, David Shrekenhamer and Willie J. Padilla of Boston College, and graduate student Alex J. Frenzel of MIT and Harvard.
http://web.mit.edu/newsoffice/2013/understanding-a-new-kind-of-magnetism-0923.html
An answer to why our galaxy’s black hole is a finicky eater
Researchers find material ejects itself before black hole can devour it.
Jennifer Chu, MIT News Office
August 29, 2013
For years, scientists have observed that the black hole at the center of our galaxy has a surprisingly small appetite.
While the black hole, named Sagittarius A* (pronounced Sagittarius A-star), is 4 million times as massive as the sun, it is unusually inactive for its size, devouring very little of the surrounding gas and other galactic material.
Now a team including researchers from MIT and the University of Massachusetts at Amherst has analyzed what little activity exists around the black hole. By studying 3 million seconds of observations taken by the Chandra X-Ray Observatory, the team found that much of the nearby radiation comes from material that is ejected before reaching the black hole, with very little energy arising from the black hole itself.
“The black hole doesn’t have a chance to do its meat-grinder thing and turn that matter into energy,” says Joey Neilsen, who contributed to the research as a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “All of that stuff basically escapes before the black hole can destroy it.”
The researchers publish their results in this week’s issue of Science.
In a related paper, published in The Astrophysical Journal, many of the same researchers took a closer look at the small amount of energy originating from the black hole, finding that what little activity there is comes from low-level, continuous flares close to the event horizon — the very outermost edge of the black hole.
“It’s this sort of constant burn — this little sizzle of flares that is always happening,” Neilsen explains. “It’s doing all this flaring and popping, and all sorts of little activity on a fairly faint scale.”
The new results provide the most detailed look yet at the activity surrounding the center of the Milky Way
A finicky eater
Over the years, the most powerful telescopes have only been able to detect a faint trace of activity, in the form of X-ray emissions, from the center of the galaxy. Theories abound as to why a supermassive black hole such as Sagittarius A* would be so lackluster: Some scientists have suggested that material escapes the region before the black hole has a chance to devour it, while others say the black hole is simply ineffective at producing radiation. Still others theorize that the observed X-ray emissions arise not from the black hole, but from a cluster of nearby stars.
For each theory, scientists have developed models to simulate the radiation around the black hole, with each model consisting of a characteristic X-ray spectrum. To see which theory is most likely to explain the black hole’s inactivity, the team first looked at X-ray emissions surrounding Sagittarius A*.
The researchers analyzed 3 million seconds of X-ray data from the Chandra observatory, NASA’s orbiting X-ray telescope. The group dissected the X-ray data and focused on the observed emission lines, signifying the characteristic light given off by individual atoms. The researchers then zeroed in on the emissions from atoms of iron, a relatively abundant element in the galaxy.
Iron can be found on the surface of stars, as well as in the gas surrounding a black hole — but stars tend to be many times cooler than a black hole’s circulating gas. To determine where the majority of X-rays were coming from, the researchers calculated the temperature of the iron, given its X-ray emission lines. They observed a large amount of very hot iron atoms, suggesting much of the X-ray activity arose not from a cluster of stars, but from gas surrounding the black hole.
While the group’s observations also appear to suggest that there is plenty of hot, gaseous material available for the black hole to consume, the minimal activity of the black hole itself suggests that that is not the case. The likeliest explanation for such finicky eating, the team concluded, was that the gaseous material ejects itself in the form of hot wind, escaping before the black hole can devour it.
“We think most of the energy, or a lot of it, is going toward pushing the gas away from the black hole and not letting it fall in,” says Mike Nowak, a research scientist at MIT Kavli. “Now we have a better understanding of what parts are active, and what aren’t.”
A buzz of activity
As for what little energy is given off by the black hole itself, Neilsen, Nowak and others looked at some of the very faintest X-ray signals from the same Chandra dataset — signals that indicated flares, or small peaks in activity, from close to the black hole’s event horizon.
Within the data, the group was able to detect large and medium-sized flares occurring about once every few days. They wondered, however, whether smaller flares might be present, but too faint to see.
The researchers calculated the frequency of observed flares at various luminosities, and based on that distribution, estimated the frequency of even smaller flares that were undetectable by the satellite. The result was a near constant “buzz” of low-grade flares that exactly matched the group’s previous estimates of X-ray activity at the galactic center.
“Rarely do we have a chance to answer these questions in detail,” Neilsen says. “This is a really great chance to actually dive in and say, ‘How do we understand what normal galaxies are doing, as opposed to gigantic luminous quasars and active galaxies?’ This is the first time we could really work on answering a question like this with high-quality data.”
ReinhardGenzel, a professor at the Max Planck Institute for Extraterrestrial Physics, says the activity — or lack thereof — observed near Sagittarius A* may be typical of other nearby black holes, most of which are relatively dormant.
“However, the degree of inactivity — a factor of a million or so — is remarkable, in part because we have so much better and higher-resolution data in this case,” notes Genzel, who was not part of the research team. “The beautiful high-resolution Chandra spectroscopy presented in this paper will, for the foreseeable future, never be available in other galactic nuclei.”
http://web.mit.edu/newsoffice/2013/why-our-galaxys-black-hole-is-a-finicky-eater-0829.html
How to make ceramics that bend without breaking
When subjected to a load, the molecular structure of the ceramic material studied by the MIT-Singapore team deforms rather than cracking. When heated, it then returns to its original shape. Though they have the same chemical composition, the two molecular configurations correspond to different natural minerals, called austenite and martensite.
Photo - Graphic: Lai et al
New materials developed at MIT could lead to actuators on a chip and self-deploying medical devices.
David L. Chandler, MIT News Office
September 26, 2013
Ceramics are not known for their flexibility: they tend to crack under stress. But researchers from MIT and Singapore have just found a way around that problem — for very tiny objects, at least.
The team has developed a way of making minuscule ceramic objects that are not only flexible, but also have a “memory” for shape: When bent and then heated, they return to their original shapes. The surprising discovery is reported this week in the journal Science, in a paper by MIT graduate student Alan Lai, professor Christopher Schuh, and two collaborators in Singapore.
Shape-memory materials, which can bend and then snap back to their original configurations in response to a temperature change, have been known since the 1950s, explains Schuh, the Danae and VasilisSalapatas Professor of Metallurgy and head of MIT’s Department of Materials Science and Engineering. “It’s been known in metals, and some polymers,” he says, “but not in ceramics.”
In principle, the molecular structure of ceramics should make shape memory possible, he says — but the materials’ brittleness and propensity for cracking has been a hurdle. “The concept has been there, but it’s never been realized,” Schuh says. “That’s why we were so excited.”
The key to shape-memory ceramics, it turns out, was thinking small.
The team accomplished this in two key ways. First, they created tiny ceramic objects, invisible to the naked eye: “When you make things small, they are more resistant to cracking,” Schuh says. Then, the researchers concentrated on making the individual crystal grains span the entire small-scale structure, removing the crystal-grain boundaries where cracks are most likely to occur.
Those tactics resulted in tiny samples of ceramic material — samples with deformability equivalent to about 7 percent of their size. “Most things can only deform about 1 percent,” Lai says, adding that normal ceramics can’t even bend that much without cracking.
David Dunand, a professor of materials science and engineering at Northwestern University, says the MIT team “achieved something that was widely considered impossible,” finding “a clever solution, based on fundamental materials-science principles, to the Achilles’ heel of ceramics and other brittle materials.”
“Usually if you bend a ceramic by 1 percent, it will shatter,” Schuh says. But these tiny filaments, with a diameter of just 1 micrometer — one millionth of a meter — can be bent by 7 to 8 percent repeatedly without any cracking, he says.
While a micrometer is pretty tiny by most standards, it’s actually not so small in the world of nanotechnology. “It’s large compared to a lot of what nanotech people work on,” Lai says. As such, these materials could be important tools for those developing micro- and nanodevices, such as for biomedical applications. For example, shape-memory ceramics could be used as microactuators to trigger actions within such devices — such as the release of drugs from tiny implants.
Compared to the materials currently used in microactuators, Schuh says, the strength of the ceramic would allow it to exert a stronger push in a microdevice. “Microactuation is something we think this might be very good for,” he says, because the ceramic material has “the ability to push things with a lot of force — the highest on record” for its size.
The ceramics used in this research were made of zirconia, but the same techniques should apply to other ceramic materials. Zirconia is “one of the most well-studied ceramics,” Lai says, and is already widely used in engineering. It is also used in fuel cells, considered a promising means of providing power for cars, homes and even for the electric grid. While there would be no need for elasticity in such applications, the material’s flexibility could make it more resistant to damage.
The material combines some of the best attributes of metals and ceramics, the researchers say: Metals have lower strength but are very deformable, while ceramics have much greater strength, but almost no ductility — the ability to bend or stretch without breaking. The newly developed ceramics, Schuh says, have “ceramiclike strength, but metallike ductility.”
Robert Ritchie, a professor of materials science and engineering at the University of California at Berkeley, says, “The very notion of superelastic ceramics is somewhat of a surprise. … We all know that ceramics invariably are extremely brittle.”
Ritchie, who was not connected with this work, points out that shape-memory metals are already used in satellite antennae and in self-expanding dental and cardiovascular prostheses. “Applying these concepts to ceramics, however,” he says, “is somewhat startling and raises many interesting possibilities.”
http://web.mit.edu/newsoffice/2013/how-to-make-ceramics-that-bend-without-breaking-0926.html
Tips for travelling with your mobile phone
Before you depart, think how you’ll want to use your phone and how best to protect it.
Robyn FizzJuly 8, 2013
Is your mobile phone the next best thing to your BFF? Are you to taking it on your travels this summer, to the shore, or a national park, or a foreign country? There are so many photos waiting to be taken…
But before you depart, think through how you'll want to use your mobile phone and how best to protect it. Just how much will those calls in Paris cost? Can you make your phone safe for the beach? How can you thwart pickpockets?
Read on for a round-up of tips and links for traveling with your phone. Just remember, it's important to do your homework up front, so you can enjoy your travels with your mobile buddy — and not come home to a whopping bill.
Before you go (travel in the United States)
Here's some basic advice for going mobile in the U.S.:
Before you go (travel abroad)
There's a denser layer of decisions to make about your phone if you'll be traveling in other countries. International voice and data usage without the right plan is very expensive. According to IS&T consultant Matt Sullivan, the cost of data without a plan is "crazy money" (about $20/MB; opening one web page could cost you $20 or more). Many unsuspecting tourists receive bills in the thousands of dollars on their return.
Follow all the tips for traveling in the U.S. and then add these to the mix:
While you're there
Ah, you've arrived! Your phone is ready to be your guide, to take photos, to let you place calls and send text messages to friends and family. Here are a last few tips to keep in mind.
http://web.mit.edu/newsoffice/2013/tips-for-traveling-with-your-mobile-phone.html
What is email spoofing all about?
In this modern form of forgery, email information is masked in an attempt to trick recipients into believing the message came from someone else.
Monique Yeaton
May 6, 2013
You've been receiving emails from a friend recently telling you about some "amazing" information with a link that seemingly leads nowhere. When you ask her about the emails, she tells you she didn't send them. Why is this happening? It's probably a case of email spoofing. Email spoofing, used in a large portion of spam, is a modern form of forgery where certain email information is masked in an attempt to trick the recipient into believing the message came from someone else.
Spoofed emails are designed to elicit a certain behavior from you, the email recipient. The goal could be for you to click on a link leading to a website containing malware, to open a virus-laden attachment, or to reply with information that is personal or confidential.
Between Friends
A common way spammers trick you is by using the name of a friend or someone you know in the "From:" field and as the signature. Fraudulent messages often contain urgent requests such as: "Your email account has been suspended," "Help, I'm stuck abroad and need money," or "Please open this invoice." The tactics are nearly endless, but the goal is always the same: to try, through social engineering, to get you to complete an action. After all, you trust your friends. Right?
Not so fast. If you look closely, the "From:" email address is not legitimate, even though the name that appears before it may be. Look also at the email's full headers. You can use these headers to verify the original source of the message. In a legitimate email, the return path (the email address the message was really sent from) will usually match the address that appears in the email's "From:" field. A fraudulent email will show a different address as the return path. In most spoofed email, the "Reply-To:" address in the email will also be different.
Spammers get names and addresses through compromised email accounts, which give them access to contact lists. If a friend has his or her email account compromised, then you may become a target for spoofed email. Information about relationships can also be obtained from social network profiles that are public or have weak privacy settings.
What Can Be Done?
Because there's no effective way to stop spammers from spoofing, there's generally nothing you can do about these messages except to delete them. Luckily, spammers tend to abandon address books quickly, moving on to other lists and new targets.
IS&T recommends that MIT emails users leverage Spam Quarantine to reduce the number of spam messages that reach your inbox. Filters can catch most — although not all — unwanted email. Awareness and a keen eye are also crucial to catching these messages.
The bottom line: never open attachments, or click links in the body of any email message that seems suspicious. If you do receive a suspicious email from a friend or colleague, you can always pick up the phone to verify its authenticity… they are your friends and colleagues, after all.
http://web.mit.edu/newsoffice/2013/email-spoofing-whats-it-all-about.html
Multiple choices: How do you handle eWaste?
MIT is committed to responsibly recycling its eWaste. Find out what you or your DLC can do to help.
Robyn FizzApril 4, 2013
At MIT, we're surrounded by electronic and digital devices. Over time we outgrow these devices or they become obsolete or stop working. Collectively, that amounts to a heap of electronic waste (eWaste) — and that's not the worst of it. According to MIT Environment, Health & Safety (EHS), eWaste contains as many as 38 different toxic or hazardous substances, including lead, mercury, cadmium, PCBs and arsenic. When eWaste is not properly handled and recycled, these substances get into our groundwater, air and soil.
The Institute is committed to responsibly recycling its eWaste — from larger equipment such as monitors, TVs, computers, copiers, fax machines and printers to smaller daily-use devices such as mobile phones, pagers, CDs and inkjet cartridges.
If devices are still operable, MIT provides several options for selling, donating or reusing them. This article focuses on recycling eWaste, but you can find out about these other options by visiting the Disposition of Equipment page on the VPF website or by contacting Michael McCarthy in the MIT Property Office.
First, deactivate
Before you take steps to recycle equipment, be sure to deactivate its Property Office tag. Note that excess property originally purchased through a government contract or grant should be screened to determine whether it meets the needs of other contracts. The Property Office will do this screening as part of the deactivation process.
To have equipment tags deactivated, send an email to John Erkkila that includes the item type, its seven-digit MIT tag number, the reason for deactivation and contact information. Once equipment has been deactivated, the Property Office will send the assigned contact person a red tag to affix to the device.
Next, erase data and recycle
To safeguard against identity theft and other cybercrimes, it's important to erase personal information and sensitive data from devices before they get recycled. You can do this yourself or, depending on the situation, have it handled by IS&T or an outside vendor.
If you plan to erase the data yourself, be aware that it isn't enough to reformat a hard drive or delete files using an "erase" or "empty trash" command. To wipe a hard drive, use software designed to overwrite each sector. For recommendations, see the IS&T Knowledge Base article, Removing Sensitive Data.
You can ask Facilities to pick up eWaste at no charge as long as each item weighs less than 50 pounds. To do this, fill out the request form through the Recycling and Bulk Trash option under the Building Services tab in SAPweb. (Pickup can take three to five days because of high demand for this service.) Facilities arranges for a third-party vendor, M&K Recovery, to destroy the drives of computers they recycle.
If your department, lab or center (DLC) has a contract with IS&T through the Distributed IT Resource (DITR) group, your consultant will arrange for your systems to be wiped on campus before being picked up by Intechra, an IT assets disposition firm and MIT partner vendor.
If your DLC plans to recycle large amounts of equipment, it can contract with Intechra to wipe the drives before they recycle the equipment. The company retires IT assets using a secure, centrally managed process. To make arrangements for this fee-based service, email Intechra representative Melanie Jaques or call her at 401.225.6429.
TechnoCycle
Facilities also provides the TechnoCycle service for recycling smaller devices. TechnoCycle bins are available at Distributed Mail Centers throughout campus. If you are recycling old mobile phones, follow the advice for Removing Data from Mobile Devices on the Removing Sensitive Data page.
Reuse or recycle, but don't abandon
Ruth Davis of Facilities has a final word on eWaste: "Please don't leave equipment in MIT hallways." The reuse@mit.edu email list on campus provides a green way to find new homes on campus for old equipment and other goods. Sometimes, however, the items don't get picked up. If this happens with your eWaste, contact Facilities to remove the equipment.
Mechanical engineering technology at Perdue
The careers of mechanical engineering technology graduates take them to a variety of employers (e.g. Rockwell Automation, Fender Guitars, Lockheed Martin, Caterpillar). Yet they have many skills in common: problem-solving, leadership and teamwork. The program focuses on the methods, materials, machinery and manpower necessary to effectively operate in a manufacturing environment. You'll learn how to manage people, machines, and production resources to ensure maximum efficiency and safety.
Areas of emphasis include nano/micro-manufacturing, sustainable energy, green manufacturing and robotics.
The College of Technology has a reputation for its strength in instruction and applied research — applying technology to real-world needs. Making things better is the driving force behind our research efforts.
Current signature areas of applied research within the college include:
We are using computational and technological methods to study biological phenomena at the molecular level. Current research includes computational genomics, biotechnology, computational life sciences, and information systems in support of the pharmaceutical, biotechnology and life sciences industry. We also have faculty focused on scanning probe microscopy techniques for applications in biology and medicine.
Security is defined as the detection of, and protection from, activities that would cause harm to processes, systems, or groups. Forensics is the application of scientific methods for the recovery, authentication, and analysis of data and events. We are working to improve cyberforensics, computer network security and biometrics.
We are finding ways to apply technology to improve the production and distribution of manufactured goods. This includes such areas as materials, methods, manufacturing processes and distributions, flexible manufacturing systems, integration and automation of manufacturing operations and product lifecycle management.
We are constantly striving to improve the integration, coordination, and deployment of information technology and people to pursue novel scientific theories and knowledge. This area involves computationally intensive science that is carried out in highly distributed network environments or science that uses immense data sets that require grid computing.
Our faculty are taking a systematic approach to addressing the sustainable delivery, production, and consumption of energy, using new and current technologies and considering their environmental impacts.
https://tech.purdue.edu/degrees/mechanical-engineering-technology
STEM Education (Science, Technology, Engineering, and Math)
STEM Education in the College of Technology at Purdue University promotes pre-school through grade 12 student understanding of technology and engineering in formal and informal settings. Through a comprehensive research program, the initiative seeks to develop and disseminate practices proven successful in:
Through these foci, the initiative seeks to play a leadership role on the national and international level to inform policy and develop and participate in sustainable communities.
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. The combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, and data can be moved from one place to another. Circuits can be constructed of discrete components connected by individual pieces of wire, but today it is much more common to create interconnections by photolithographic techniques on a laminated substrate (a printed circuit boardor PCB) and solder the components to these interconnections to create a finished circuit. In an integrated circuit or IC, the components and interconnections are formed on the same substrate, typically a semiconductor such as silicon or (less commonly) gallium arsenide.
Breadboards, perfboards or stripboards are common for testing new designs. They allow the designer to make quick changes to the circuit during development.
An electronic circuit can usually be categorized as an analog circuit, a digital circuit or a mixed-signal circuit (a combination of analog circuits and digital circuits).
Analogcircuits
A circuit diagram representing an analog circuit, in this case a simple amplifier.
Analog electronic circuits are those in which current or voltage may vary continuously with time to correspond to the information being represented. Analog circuitry is constructed from two fundamental building blocks: series and parallel circuits. In a series circuit, the same current passes through a series of components. A string of Christmas lights is a good example of a series circuit: if one goes out, they all do. In a parallel circuit, all the components are connected to the same voltage, and the current divides between the various components according to their resistance.
A simple schematic showing wires, a resistor, and a battery.
The basic components of analog circuits are wires, resistors, capacitors, inductors, diodes, and transistors. (Recently, memristorshave been added to the list of available components.) Analog circuits are very commonly represented in schematic diagrams, in which wires are shown as lines, and each component has a unique symbol. Analog circuit analysis employs Kirchhoff's circuit laws: all the currents at a node (a place where wires meet) must add to 0, and the voltage around a closed loop of wires is 0. Wires are usually treated as ideal zero-voltage interconnections; any resistance or reactance is captured by explicitly adding a parasitic element, such as a discrete resistor or inductor. Active components such as transistors are often treated as controlled current or voltage sources: for example, a field-effect transistor can be modeled as a current source from the source to the drain, with the current controlled by the gate-source voltage.
When the circuit size is comparable to a wavelength of the relevant signal frequency, a more sophisticated approach must be used. Wires are treated as transmission lines, with (hopefully) constant characteristic impedance, and the impedances at the start and end determine transmitted and reflected waves on the line. Such considerations typically become important for circuit boards at frequencies above a GHz; integrated circuits are smaller and can be treated as lumped elements for frequencies less than 10 GHz or so.
An alternative model is to take independent power sources and induction as basic electronic units; this allows modeling frequency dependent negative resistors, gyrators, negative impedance converters, and dependent sources as secondary electronic components
Digital circuits
In digital electronic circuits, electric signals take on discrete values, to represent logical and numeric values. These values represent the information that is being processed. In the vast majority of cases, binary encoding is used: one voltage (typically the more positive value) represents a binary '1' and another voltage (usually a value near the ground potential, 0 V) represents a binary '0'. Digital circuits make extensive use of transistors, interconnected to create logic gates that provide the functions of Boolean logic: AND, NAND, OR, NOR, XOR and all possible combinations thereof. Transistors interconnected so as to provide positive feedback are used as latches and flip flops, circuits that have two or more metastable states, and remain in one of these states until changed by an external input. Digital circuits therefore can provide both logic and memory, enabling them to perform arbitrary computational functions. (Memory based on flip-flops is known as static random-access memory (SRAM). Memory based on the storage of charge in a capacitor, dynamic random-access memory (DRAM) is also widely used.)
The design process for digital circuits is fundamentally different than the process for analog circuits. Each logic gate regenerates the binary signal, so the designer need not account for distortion, gain control, offset voltages, and other concerns faced in an analog design. As a consequence, extremely complex digital circuits, with billions of logic elements integrated on a single silicon chip, can be fabricated at low cost. Such digital integrated circuits are ubiquitous in modern electronic devices, such as calculators, mobile phone handsets, and computers. As digital circuits become more complex, issues of time delay, logic races, power dissipation, non-ideal switching, on-chip and inter-chip loading, and leakage currents, become limitations to the density, speed and performance.
Digital circuitry is used to create general purpose computing chips, such as microprocessors, and custom-designed logic circuits, known as application-specific integrated circuit (ASICs). Field-programmable gate arrays (FPGAs), chips with logic circuitry whose configuration can be modified after fabrication, are also widely used in prototyping and development.
Mixed-signal circuits
Mixed-signal or hybrid circuits contain elements of both analog and digital circuits. Examples include comparators, timers, phase-locked loops, analog-to-digital converters, and digital-to-analog converters. Most modern radio and communications circuitry uses mixed signal circuits. For example, in a receiver, analog circuitry is used to amplify and frequency-convert signals so that they reach a suitable state to be converted into digital values, after which further signal processing can be performed in the digital domain.