Smart materials are materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields.
Piezoelectric:
Piezoelectric materials are materials that produce a voltage when stress is applied. Since this effect also applies in the reverse manner, a voltage across the sample will produce stress within the sample. Suitably designed structures made from these materials can therefore be made that bend, expand or contract when a voltage is applied.
Reason for the piezoelectric called as smart material:
Smart materials are comprised of those that transform energy from one form to output energy in another form, and again do so directly and reversibly. Thus, an electro-restrictive material transforms electrical energy into elastic (mechanical) energy which in turn results in a physical shape change. Changes are again direct and reversible. Among the materials in this category are piezoelectrics, thermoelectrics, photovoltaics, pyroelectrics, photoluminescents and others.
Applications of piezoelectric ceramics:
Piezoelectric components are ideal for all kinds of electromechanical transducers. Here are just some examples of the many applications you will find our PZT materials used in.
Generators (conversion of mechanical into electrical energy)
Sonic and ultrasonic transducers (conversion of electrical into mechanical energy)
Sonic (< 20 kHz)
Ultrasonic
Measurement of distance in air
Higher-frequency ultrasonic
Sensors (conversion of mechanical force or movement into a (proportional) electric signal)
PZT as a generator
Actuators (conversion of electrical signals into a (proportional) mechanical displacement)
Limitation on using piezoelectric materials:
Each piezoelectric material has a particular operating limit for temperature, voltage, and stress. The particular chemical composition of the material determines the limits. Operating a material outside of these limitations may cause partial or total depolarization of the material, and a diminishing or loss of piezoelectric properties.
Here are the some of the limitation of piezoelectric materials:
Temperature Limitations:
As the operating temperature increases, piezoelectric performance of a material decreases, until complete and permanent depolarization occurs at the material's Curie temperature.
The Curie point is the absolute maximum exposure temperature for any piezoelectric ceramic. Each ceramic has its own Curie point. When the ceramic element is heated above the Curie point, all piezoelectric properties are lost. In practice, the operating temperature must be substantially below the Curie point.
The material's temperature limitation decreases with greater continuous operation or exposure. At elevated temperatures, the ageing process accelerates, piezoelectric performance decreases and the maximum safe stress level is reduced.
Voltage Limitations
A piezoelectric ceramic can be depolarized by a strong electric field with polarity opposite to the original poling voltage.
The limit on the field strength is dependent on the type of material, the duration of the application, and the operating temperature. The typical operating limit is between 500V/mm and 1 000V/mm for continuous application.
It should be noted that alternating fields can have the same affect during the half cycle which is opposite to the poling direction.
Mechanical Stress Limitations
High mechanical stress can depolarize a piezoelectric ceramic. The limit on the applied stress is dependent on the type of ceramic material, and duration of the applied stress.
For dynamic stress (impact ignition) the limit is less severe; materials with higher energy output (high g constant) can be used.
For impact applications, the material behaves quasi statically (non-linear) for pulse durations of a few milliseconds or more. When the pulse duration approaches a microsecond, the piezoelectric effect becomes linear, due to the short application time compared to the relaxation time of the domains.
Power Limitations
The acoustic power handling capacity of a radiating transducer is limited by the following factors.
Manufacturing Process
The manufacturing process involves a number or stages shown schematically in figures 1 to 12. The first step is to weigh, dry mix and ball mill the raw materials. The uniform mixture is when heat treated (calcined), during which the components react to form the polycrystalline phase. The calcined powder is spray dried to add binder in order to increase it's reactivity and to improve pressing properties.
After shaping by dry-pressing, the binder is burnt out by slowly heating the green ceramics to around 700oC. The parts are transferred to another furnace, where they are sintered between 1200 and 1300oC. The dimensional tolerance of fired parts (± 3%) is improved by cutting, grinding, lapping etc.
Electrodes are applied either by screen printing or chemical plating or vacuum deposition. Poling then is carried out by heating in an oil bath at 130-220oC, and applying an electrical field of 2-8 kV/mm to align the domains in the material. The oil bath is used a heat source and to prevent flash over.
Final inspection is performed 24 hours later, and includes testing of electrode-ceramic bonding as well as measurement of dimensional tolerances, dielectric and piezoelectric properties.
Magnetic shape-memory alloys (MSM):
Magnetic shape-memory alloys are ferromagnetic materials exhibiting large changes in shape and size in an applied magnetic field.
Magnetic shape memory (MSM) alloys have the capability to produce large magnetic field-induced strain of several percent. The large strain can either be caused by a magnetic field-induced structural reorientation (usually by twin boundary motion) or by a magnetic field-induced phase transformation (usually a martensitic phase transformation). The former is mostly referred to as MSM-effect, magnetoplasticity or more precisely as magnetically induced reorientation (MIR). The magnetic field-induced phase transformation is correctly referred to as MSM effect or as magnetically induced martensite/austenite (MIM/MIA). During MIR, twin boundaries move in order to allow those twin variants having a smaller angle between easy magnetization axis and applied field direction to grow, at the expense of unfavourably oriented twin variants.
Reason for the Magnetic shape-memory alloys called as smart material:
Magnetic Shape Memory (MSM) alloys take a unique position within the class of smart materials due to their outstanding strain up to 10% obtained in a moderate magnetic field. MSM devices promise innovative applications working at frequencies not achievable by conventional shape memory alloys. In single crystals of special magnetic shape memory materials fundamentally new actuation mechanism had been discovered in the martensite phase. It was observed, that already comparably low magnetic fields (< 1 Tesla) can be sufficient to move twin boundaries contained in the material. Since the twin boundaries are separating areas of different crystallographic orientations, their displacement leads to a reorientation of the crystal. This allows controlling the microstructure and shape of the sample by applying magnetic fields. The observed change in length of up to 10 % is, compared to magnetostrictive or piezoceramic materials, by more than two orders of magnitude higher.
Examples for the Magnetic shape memory alloys:
Ni-Mn-Ga, Ni-Mn-In-Co alloys,Fe-Pt, Fe-Pd alloys, CoNiGa and CoNiAl alloys
Applications of the Magnetic shape memory alloys:
Magnetic Shape Memory Alloys (MSMAs) are promising high-frequency active materials for actuation, sensing, shape control, vibration suppression and energy harvesting applications.
Automotive, turbine industries, microscopes, tiny mirrors used in optical communication, and robots used in medicine. Because the foam is light, it could lead to aerospace applications, such as airplane wings that morph to become more aerodynamic.
Limitations of magnetic shape memory alloys:
The main disadvantages of MSMA based actuators are the brittleness of the single-crystal material, the difficulty to apply the strong magnetic field required to obtain sufficient strain and the nonlinear behaviour.
Manufacturing of magnetic shape memory alloys
Thin-film fabrication techniques and discrete MSM stripes, a hybrid actuator system was designed. The actuator system consists of four thin-film stators and two discrete MSM stripes mounted in a row. For creating an actuator motion, one of the MSM stripes has to be excited by a pair of stators to cause variant switching. This results in an elongation plus a compression of the second stripe and vice versa. The technologies required for fabricating the thin-film stator are sputter deposition, PECVD, electroplating, etching, and photolithography ordered samples of a variety of compositions of the off-stoichiometric magnetic shape memory alloy Ni2MnGa have been prepared by mechanical alloying from elemental precursors. As-milled powders are highly disordered and show very weak ferromagnetic order. Annealing produces a well-ordered L21Heusler phase with high saturation magnetization. Annealing results in a consistent loss of Ga of about 1-4at.% (of total sample composition). Structural and magnetic properties of a range of compositions have been measured and are reported in the present work. A magnetically oriented metal-polymer composite has been prepared by mixing the powdered sample in epoxy and curing under an externally applied magnetic field. The magnetic anisotropy energy of the composite sample has been measured and found to be about 20% of the value expected for a single crystal of similar composition. Possibilities for increasing the magnetic anisotropy of metal-polymer composites are discussed. Results are discussed in terms of the effects of structural and chemical order on the resulting magnetic properties in the context of a model based on indirect exchange interactions.
pH sensitive polymers:
pH sensitive polymers are materials which will respond to the changes in the pH of the surrounding medium by varying their dimensions. Such materials swell or collapse depending on the pH of their environment. This behaviour is exhibited due to the presence of certain functional groups in the polymer chain.
There are two kinds of ph sensitive materials
The response is triggered due to the presence of ionisable functional groups (like -COOH, -NH2) which get ionized and acquire a charge (+/-) in a certain pH. The polymer chains now have many similarly charged groups which causes repulsion and hence the material expands in dimensions. The opposite happens when pH changes and the functional groups lose their charge hence the repulsion is gone and the material collapses back. These materials are being extensively used in controlled drug delivery systems and biomimetics.
Why pH sensitive materials are called smart materials
They are intelligent polymers they can sense the pH changes in the environment and response to the environmental changes. (they can able to communicate environmental changes.)
pH-sensitive materials - The most interesting of these are indicators that change colors as a function of pH, and show promise in paints that change color when the metal beneath begins to corrode.
Applications:
pH sensitive polymers are used to sense the pH changes in a solution. These are responsive polymers used in Diagnostics, Separations, Bioprocess, and Drug delivery. Protons are generated and/or consumed in many biological reactions involving enzymes, pH sensors have found use as transducers in biosensor devices.
The pH change resulting from the metabolic activity of cells, with application to examining cellular response to toxins and infectious agents. The measurement of intracellular or sub cellular pH using pH-sensitive fluorescent indicators can provide insights into the physiology of the cell.
Application for pH-sensitive polymers are used to make Biodegradable Composites-widely used in the bio medical field. These are Water soluble polymers, they can easily enter into a biological cells and sense the pH changes inside the human body and helps in bio medical field, so these polymers widely used Manufacture of drugs.
Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a pre designed manner. This is used in treatment of diabetes , an optimal delivery system would be one that could deliver insulin upon detection of gucose in the blood stream.
Examples
Stimuli-sensitive polymers are suitable candidates for oral peptide drug delivery vehicles since they will prevent gastric degradation in the stomach while providing a controlled release of a peptide drug such as calcitonin later. The purpose of this study was to fabricate polymeric beads from pH/temperature sensitive linear terpolymers (poly(N-isopropylacrylamide-co-butylmethacrylate-co-acrylic acid) and load them with a peptide drug, human calcitonin, which was dissolved in aqueous phase. METHODS: The polymeric beads were formed by solubilizing a cold, aqueous solution of temperature sensitive polymer with human calcitonin. This solution was added dropwise into an oil bath kept at a temperature above the LCST of a polymer, precipitating polymer and entrapping the peptide. The quantity and the physical state of the peptide were analyzed by reverse-phase HPLC, CD and FTIR and its biological activity after loading was determined in vivo. RESULTS: The loading efficiency and stability of human calcitonin into the polymeric beads was studied as a function of pH and ionic strength of the loading buffer and temperature of the oil bath. Final optimal loading conditions were 20 mM glycine/HCl buffer, pH 3.0 containing 0.15 M NaCl as a dissolution medium and 23 degrees C as the oil bath temperature. Loading and release of human calcitonin were also studied as a function of acrylic acid content in the terpolymers. As the acrylic acid content increased from 0 to 10 mol %, the loading efficiency and stability of calcitonin improved significantly. The same trend was observed for the quantity of released calcitonin. In vivo biological activity of the released hormone was preserved. CONCLUSIONS: The results showed that the beads made of the polymers with high content of acrylic acid (most hydrophilic) provided better loading, stability and release of human calcitonin. The designed beads represent a new potential system for oral delivery of calcitonin and other peptides.
pH-Sensitive systems examples the pH range of fluids in various segments of the GIT may provide environmental stimuli for responsive drug release. Studies by servaral research group s. have been performed on polymers containing weakly acidic or basic groups in the polymetric backbone . the charge density of the polymers depends on pH and ionic composition of the outer solution (the solution into which the polymer is exposed).altering the pH of the solution will cause swelling or desselling of the polymer . Polyacidic polymers will be unswollen at low pH , since the acidic groups will be protonated and hence unionized. With increasing pH, polyidic polymers will swell. The opposite holds for plybasic polymers, since the ionization of the basic groups will increase with decreasing pH thus, drug release from devices made from these polymers will display pH -dependent release rates.
Limitations of pH sensitive polymers
Large unilamellar noisome and control liposome vesides were rendered pH-sensitive by complexation with a hydraphobically modified pH-responsive copolymer of N-iso proylacrylamid, N-glycidylacrylamid, and N-octadecyalcrylamide at a copolymer/lipid mass ration of 0.3 . The versides were characteriszed and tested for therir stability and pH-sensitivity in buffer and human serum. Their in vitro cytotoxicity was evaluated as well as their ability to mediate cytoplasmic delivery of encapsulated fluorescent probe using J774 murine macrophage-like cells. At pH7.2 versides were found to be stable over 90 days at 4
The monitoring of the pH of solutions is widely required in laboratories, clinics and industries since many chemical processes are pH dependent. These polymers works in liquid medium, degradable.
Manufacturing of pH sensitive polymers
Alginic acid derivatives containing hydrophobic alkylamino group were prepared by oxidation and alkylamination. They showed amphoteric properties. DPPC liposomes were conjugated with the alginic acid derivatives to add pH-sensitivity. Release ability of the polymer-liposome conjugates increased significantly at weakly acidic condition. pH-Sensitivity of these conjugates may be influenced by the molecular weight and the degree of amination (D.A.) of the conjugated polymer
Nippon Kagakkai Koen Yokoshu(2003)> Preparation of pH-Sensitive Liposomes Conjugated with Alginic Acid Derivatives and Its Application to Drug Delivery System
Temperature-responsive polymers
Temperature-sensitive polymers are based on polymer-water interactions especially specific hydrophobic-hydrophilic balancing effects, and the configuration of side groups. When polymers networks swell in a solvent, there is usually a negligible or small positive enthalpy of mixing or dilution. Although a positive enthalpy change opposes the process, the large gain in the entropy drives it. In aqueous polymer solutions, the appropriate is often observed this unusual behavior is associated with a phenomenon of polymer phase separation as the temperature is raised to a critical value, known as the LCST. Polymers characterized by LCST usually shrink, as the temperature below LCST results in the swelling of the polymer. Bioactive agents such as drugs, enzymes, antibodies, and genes may be immobilized on or within the temperature sensitive polymers. Responsive drug/genes release patterns regulated by environmental temperature changes.
Reason for temperature-sensitive polymer called as smart material
This materials respond to changes in temperature, this property of the materials widely used to sense the environmental changes.
Thermo responsive materials - Shape memory alloys, the dominant smart material, change shape in response to heat or cold. They are most commonly Nitinol, or nickel and titanium combined. Less popular but still possessing the shape memory effect are gold cadmium, silver cad-mium, copper-aluminium-nickel, copper tin, copper zinc, and copper zinc aluminium. They are useful in couplers, thermostats, automobile, plane and helicopter parts.
Applications:
Wide range of medical, defence and industrial applications. Smart textiles, smart paints that changes colour when the environment temperature change.
Manufacturing Process
A novel method, microwave irradication synthesis, is proposed for the preparation of thermo-sensitive poly(N-isoproylacrylamide) (PNIPAAm) hydrodels. The PNIPAAm hydrogels were separately synthesized by using microwave irradication method and water-bath heating method . Chemical groups, lower critical solution temperature(LCST) and surface morphology of these PNIPAAm hydrogels were characterized by FT-IR, DSC and SEM. Swelling ratios of the gels were measured gravimetrically in the temperature range from 10.0 to 60 °C. Results showed that the use of microwave irradication can greatly shorten the reaction time required for PNIPAAm gels, which were up to 99% after a short reaction time, SEM micrographs and textural measurements revealed that the gels synthesized using microwave irradiation had more porous structure, and their average pore sizes and specific surface areas were larger than thoses of the gels synthesized using water-bath heating method, and the PNIPAAm hydrogels synthesized using microwave irradication had much higher swelling ratios at 10.0°C below the LCST, and had lower swelling ratio at 60°C above the LCST compared to the hydrogels synthesized by water-bath method.
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