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Phase change materials (PCM) applied from conditioned buildings, memory and neuromorphic processors to military textiles and thermal management systems

Phase change materials are substances that absorb and release thermal energy (heat) during the process of melting and freezing  at defined temperatures . They are called “phase change” materials because they  transition from one of the two fundamental states of matter – solid and liquid – to the other  during the thermal cycling process. By melting and solidifying at the phase change temperature (PCT), a PCM is capable of storing and releasing large amounts of energy  to provide useful heat/cooling. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state.


There are two kinds of heat energy: sensible and latent. Sensible heat capacity is the ability of a material to absorb heat energy as it increases in temperature (warms up). An example of this is a glass of water that heats up in the sun. Latent heat capacity is the ability of a material to absorb or release heat energy as it melts or freezes without increasing in temperature. If a glass of ice cubes is placed in sunlight, the ice will also start to heat up sensibly but then they will start to melt.


If you measure the temperature of the ice as it melts, you will find that until all the ice has melted, it will remain at 0ºC. This is because when ice changes phase, the temperature will remain constant until all the ice has melted. This is known as latent heat. The energy released/absorbed by phase transition from solid to liquid, or vice versa, the heat of fusion is generally much higher than the sensible heat.Most traditional heating systems use sensible heat to alter the temperature of a substance. But PCMs use both sensible and latent heat for thermal storage (rather than just temperature change).

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PCMs are used in many different commercial applications where energy storage and/or stable temperatures are required, including, among others, heating pads, cooling for telephone switching boxes, mattresses, clothing, pillows, and electronics. PCMs provide many advantages when incorporated into products such as energy savings, a better night’s sleep, cooling and heating relief in remote locations without access to electricity, and better performing electronics.


Phase change materials (PCMs) have been applied to the textiles in a variety of processes to improve comfort of wearer. When the temperature of the body raised due to the higher ambient temperature more than the melting temperature of the PCM, the core material (Phase change material) reacts accordingly and absorbs heat. By absorbing heat chemical bonds are broken and phase change material
is started converting from solid to liquid state. During the melting process PCMs absorbs heat energy from the surrounding and stores extra energy.


When the temperature of the body decreased due to lower ambient temperature less than the crystallization temperature of the PCM, the core material (phase change material) reacts accordingly and releases the previous stored heat. By releasing heat the chemical bond are formed and the core phase change material started converting from liquid to the solid phase. During the crystallization process releases heat to the surrounding and wearer feels thermal comfort.


By far the biggest potential market is for building heating and cooling. PCMs are currently attracting a lot of attention for this application due to the progressive reduction in the cost of renewable electricity, coupled with limited hours of availability, resulting in a misfit between peak demand and availability of supply. In North America, China, Japan, Australia, Southern Europe and other developed countries with hot summers peak supply is at midday while peak demand is from around 17:00 to 20:00. This creates a lot of demand for storage media.


They are also used in US Army  conditioned Buildings where works in conjunction with traditional insulation to decrease heat gain (or loss) by storing and releasing heat to the conditioned space at different times of the day. PCM is a substance used to increase the thermal mass of a building due to its ability to melt and solidify at certain temperatures, providing the capability to store and release large amounts of thermal energy.


They are also useful in military applications. Thermal management systems (TMS) of armored ground vehicle designs are often incapable of sustained heat rejection during high tractive effort conditions and ambient conditions. Latent heat energy storage systems that utilize Phase Change Materials (PCMs) present an effective way of storing thermal energy and offer key advantages such as high-energy storage density, high heat of fusion values, and greater stability in temperature control. Military vehicles frequently undergo high-transient thermal loads and often do not provide adequate cooling for powertrain subsystems.


PCMs fall into three main categories depending on their base material: water-based, salt hydrates, and organic material based. Organic (carbon-containing) materials derived either from petroleum, from plants or from animals; and salt hydrates, which generally either use natural salts from the sea or from mineral deposits or are by-products of other processes. A third class is solid to solid phase change. The different materials provide different advantages and usability.


Soaking, liming, Deliming-cum-bating, pickling, Tanning, Retanning, Neutralisation, Coating, lamination, finishing, melt spinning, manufacturing are some of the convenient processes for PCMs. Encapsulation is the method use for coating materials with capsules.


PCMs are generally available in three forms: unencapsulated raw PCM, microencapsulated PCM and macroencapsulated PCM. The difference between the two encapsulated options is the size of the particle. Inorganic and water-based PCMs cannot be encapsulated. Encapsulation of a PCM adds an outer shell to the PCM core to prevent leakage, degradation and contamination. Solid-liquid phase change materials are usually encapsulated for installation in the end application, to contain in the liquid state.


In some applications, especially when incorporation to textiles is required, phase change materials are micro-encapsulated. Microcapsule is the process in which small portion of materials (capsules) used to coat a desire material for specific purpose and it can be achieved by many techniques. Micro-encapsulation allows the material to remain solid, in the form of small bubbles, when the PCM core has melted.


 PCM (Phase Change Materials) Memory

Phase-change materials such as  Te-containing alloys, typically lying along the GeTe-Sb2Te3 quasibinary tie line. Their ability to switch, reversibly and extremely quickly, between the crystalline and amorphous phases, combined with the high stability of both phases, makes them ideally suitable for memory applications. They have been long used in optical data storage in the form of DVD and Blu-Ray disks and recently have also emerged as a leading candidate for electronic nonvolatile memory devices.

Phase-change memory (PCM) devices have in recent years emerged as a game-changing alternative to computer random-access memory. Using heat to transform the states of material from amorphous to crystalline, PCM chips are fast, use much less power and have the potential to scale down to smaller chips – allowing the trajectory for smaller, more powerful computing to continue. However, manufacturing PCM devices on a large scale with consistent quality and long endurance has been a challenge.


Researchers from the universities of Oxford, Exeter and Münster have demonstrated a new technique that can store more optical data in a smaller space than was previously possible on-chip. This technique improves upon the phase-change optical memory cell, which uses light to write and read data, and could offer a faster, more power-efficient form of memory for computers. The scientists describe their new technique for all-optical data storage in the journal Optica.


Rather than using electrical signals to store data in one of two binary states as with conventional electronics-based computers, the optical memory cell uses light to store information. The researchers demonstrated optical memory with more than 32 states; the equivalent of 5 bits. They say that this development is an important step toward an all-optical computer. Research team leader Harish Bhaskaran from Oxford University’s Department of Materials commented, “By bringing the speed-of-light-based data transmission to the circuit board, our all-optical memory could enable a hybrid computer chip that interacts with data both optically and electrically.”


Ealier, Scientists at IBM Research  demonstrated a new level of efficiency for data storage with optical memory, using a storage format called phase-change memory (PCM) to store 3 bits of data. “Phase-change memory is the first instantiation of a universal memory with properties of both DRAM and flash, thus answering one of the grand challenges of our industry,” said lead researcher Haris Pozidis from IBM Research in Zurich, Switzerland. “Reaching 3 bits per cell is a significant milestone because at this density the cost of PCM will be significantly less than DRAM and closer to flash. PCM is upto 70 times faster than flash however they are about five to 10 times slower than DRAM. As IBM points out, another advantage of PCM is that the memory can survive at least 10 million write cycles, meaning hypothetically you could be getting a life-time warranty with the storage. Flash memory, on the other hand, only lasts around 3,000 write cycles before degrading.


PCM is nonvolatile random – access memory that stores information in the structural phase of the active materials. They are based on reversible phase conversion between the amorphous and the crystalline state of a chalcogenide glass, which is accomplished by heating and cooling of the glass. It utilizes the unique behavior of chalcogenide (a material that has been used to manufacture CDs), whereby the heat produced by the passage of an electric current switches this material between two states. The different states have different electrical resistance which can be used to store data.


Robert E. Simpson, and his colleagues have engineered strain into the layered material to tune its switching properties. A strained layered phase-change memory material switches phases five times faster, using about half the voltage, than traditional phase-change memory materials.


Neuromorphic chips based on Phase change materials (PCMs)

The research team has made the pioneering breakthrough of the development of photonic computer chips that imitate the way the brain’s synapses operate. The work, conducted by researchers from Oxford, Münster and Exeter universities, combined phase-change materials – commonly found in household items such re-writable optical discs – with specially designed integrated photonic circuits to deliver a biological-like synaptic response. Crucially, their photonic synapses can operate at speeds a thousand times faster than those of the human brain.


The PCM’s ability to absorb light changes when heated, which can be used to control the amount of light that passes through the waveguide. In previous research, the group had shown that optical pulses could be used to switch between various states of absorption to store information—effectively creating a photonic memory device. The team believes that the research could pave the way for a new age of computing, where machines work and think in a similar way to the human brain, while at the same time exploiting the speed and power efficiency of photonic systems.


Professor C David Wright, co-author from the University of Exeter, said: ‘Electronic computers are relatively slow, and the faster we make them the more power they consume. Conventional computers are also pretty “dumb”, with none of the in-built learning and parallel processing capabilities of the human brain. We tackle both of these issues here – by developing not only new brain-like computer architectures, but also by working in the optical domain to leverage the huge speed and power advantages of the upcoming silicon photonics revolution.’ Professor Wolfram Pernice, a co-author of the paper from the University of Münster, added: ‘Since synapses outnumber neurons in the brain by around 10,000 to one, any brain-like computer needs to be able to replicate some form of synaptic mimic. That is what we have done here.’


Light-Powered Matrix Multiplications Fuel Processing Capabilities

Through an international collaboration, researchers from the University of Münster have developed photonic processors that combine processing and data storage on a single chip.


Led by Wolfram Pernice, a professor at the Institute of Physics and the Center for Soft Nanoscience at the University of Münster, the researchers developed a hardware accelerator for so-called matrix vector multiplications, which serve as the backbone of neural networks. The team’s process featured the combination of photonic structures with PCMs (phase-change materials) as energy-efficient storage elements.


“The basic principle for the multiplication is a simple transmission measurement of a straight waveguide with a phase-change material on top,” Johannes Feldmann, lead author on the paper, told Photonics Media. PCMs are commonly used with DVDs or Blu-ray Discs as optical data storage elements. “PCMs as nonvolatile memory elements have the additional advantage that no energy is needed to keep their phase-state, meaning that in the case of the trained neural network with fixed weights,” Feldmann said, “no energy is required to preserve the matrix once it is programmed.”


Because different wavelengths of light do not interact with one another, frequency combs were an attractive light source for achieving parallel calculations. The researchers used a chip-based frequency comb developed at École Polytechnique Fédérale de Lausanne (EPFL) as a light source to carry out matrix multiplications on multiple data sets in parallel.


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