Intelligent inorganic nonmetallic materials
The environment in which structural materials are located is extremely complex, and the risk of accidents caused by material damage is increasing. It is a very important and urgent task to research and develop structural materials that can self-diagnose and repair damage. In this paper, the development, concept, inorganic and nonmetallic smart materials of smart materials are summarized, and the further research of smart materials is prospected.
Keywords intelligence; Inorganic nonmetal; material
Intelligent materials refer to new materials that can perceive and respond to the environment and have the ability to discover functions. Professor Yi's research has gradually expanded to other fields such as civil engineering.
At the same time, the American R? e？ Professor Newnham put forward the concept of smart materials around materials with sensing and executive functions, which some people call smart materials. He divided intelligent materials into three categories:
Passive intelligent materials-materials that can only respond to external changes;
Active intelligent materials-materials that can not only identify external changes, but also respond to environmental changes by implementing line-induced feedback loops;
Smart materials-materials that have the functions of perception and execution, and can respond to environmental changes, thus changing the performance coefficient.
r？ e？ The similarity between Newnham's smart materials and Takagi Shun's smart materials lies in the responsiveness of materials to the environment.
Since 1989, Japan, the United States, Western Europe and the whole world have been studying smart materials. Scientists study the introduction of necessary bionic functions into materials, so that materials and systems can reach a higher level and become new materials with self-detection, self-judgment, self-conclusion, self-command and execution functions. Intelligent structures often combine high-tech sensors or sensitive elements with traditional structural materials and functional materials, endowing materials with brand-new properties, making inanimate materials have "feeling" and "perception", adapting to environmental changes, not only finding problems, but also solving problems by themselves.
Because the performance of intelligent materials and systems can change with the environment, their application prospects are very broad [7]. For example, the wing of an aircraft can change its shape in response to air pressure and flight speed after introducing intelligent systems; Smart structures entering space are equipped with shock absorption systems, which can compensate for weightlessness and prevent metal fatigue; The submarine can change its shape and eliminate turbulence, making the noise of flow difficult to be detected and easy to hide; Metal intelligent structural materials can detect damage and inhibit crack propagation by themselves, and have self-repairing function to ensure the reliability of the structure; Many intelligent systems, such as air-fuel oxygen sensor and piezoelectric raindrop sensor, are used in high-tech cars, which increases their functions. Other intelligent water purification devices can sense and remove harmful pollutants; Electrochromic smart window can respond to climate change and human activities, adjust heat flow and illumination; Intelligent toilet can analyze urine samples and make early diagnosis; Intelligent drug delivery system can respond to blood glucose concentration, release insulin and maintain blood glucose concentration at a normal level.
The trend of research and development of smart materials abroad is to develop smart materials into smart material systems and structures. This is the international frontier of the current development of engineering discipline, which will bring a revolution to the development of engineering materials and structures. In the construction of urban infrastructure abroad, how to use smart materials to build floors, bridges and buildings that can respond to environmental changes sensitively is being conceived. This is a system synthesis process, and new features and functions need to be introduced into the existing structure.
American scientists are designing various methods to make bridges, wings and other key structures have their own "nervous system", "muscles" and "brain" so that they can feel the impending failure and solve it by themselves. For example, it can warn the pilot before the plane breaks down, or automatically repair the bridge when cracks appear. One of their methods is to embed tiny optical fiber materials in high performance composites. Because the composite material is full of crisscross optical fibers, they can feel different pressures on their wings like "nerves". In extremely serious cases, optical fibers will break and light transmission will be interrupted, so they will warn of impending accidents.
1, the idea of intelligent materials [8]
A new concept is often a synthesis of different viewpoints and concepts. The thinking of intelligent material design is related to the following factors: (1) the history of material development, structural materials → functional materials → intelligent materials. (2) The influence of artificial intelligence computers, that is, the future models of biological computers, learning computers and three-dimensional recognition computers, put forward new requirements for materials. (3) Considering the manufacture of intelligent materials from the perspective of material design. (4) Software function introduction materials. (5) Expectation of materials. (6) Energy transfer. (7) Materials have the viewpoint of time axis, such as life prediction function, self-repair function, and even self-learning, self-proliferation and self-purification. Due to external stimulation, the timeline can make a positive dynamic response, that is, imitate the function of organisms. For example, intelligent artificial bone is not only compatible with biology, but also can be decomposed according to the growth and healing state of biological bones and eventually disappear.
1. 1 bionic and intelligent materials
The performance of smart materials is a function of composition, structure, shape and environment, and it has environmental responsiveness. The biggest feature of living things is their adaptation to the environment, from plants, animals to humans. Cells are the basis of organisms and can be regarded as a fusion material with three functions: perception, processing and execution, so cells can be used as a blueprint for intelligent materials.
The research from simple materials to complex matter can be realized by establishing a model. This model can solve complex biological materials, thus creating bionic intelligent materials. For example, polymer materials are artificially designed synthetic materials. In the study, we used the macromolecular structure of silk for reference, and then synthesized nylon with higher strength. At present, according to protein (an analog information receiving function) and protein (an executive function), various levels of intelligent materials have been created from ultra-micro to macro.
1.2 intelligent material design
Intelligent materials can be obtained by combining existing materials and introducing various functions, especially software functions. With the rapid development of information science, automata can be used not only for artificial machinery such as robots and computers, but also for biological machinery with conditioned reflex.
When an automatic device inputs a signal (information), it can generate an output signal (information) according to the past input signal (information). Information entered in the past can be stored in the system as an internal state. Therefore, the automatic device consists of three parts: input, internal state and output. Comparing intelligent materials with automation equipment, their concepts are similar.
The robot M can be described by the following six parameters:
M=(θ，X，Y，f，g，θ0)
Where θ is a set of internal states; X and y represent input and output information sets respectively; F represents the state transition coefficient that the current internal state changes to the internal state at the next moment due to the input information; G is the output coefficient of the current internal state of the output information due to the input information; θ0 is a set of initial states.
In order to make the material intelligent, its internal state θ, state transition coefficient f and output coefficient g can be controlled. For example, for ceramics, the relationship between θ, F and G is the relationship between material structure, composition and functionality. These parameters should be considered when designing materials. If the function of ceramics is improved to intelligence, F and G need to be controlled.
Generally, ceramics are polycrystals composed of tiny grains, and their properties are often controlled by adding a little second component. The properties of the bulk boundary and grain boundary of the second component affect the properties of the obtained material.
In fact, when the ions of the second component are introduced into the system, their free energy (G=H-TS) changes. In order to minimize the free energy (G) of matter, it is necessary to control the enthalpy (H) and make the entropy (S) reach the most appropriate value. Entropy is related to the distribution of additives, so the function control of ceramics can be realized by optimizing entropy. Entropy is controlled by the enthalpy of matter itself. Therefore, in order to make ceramics have high functions and achieve the purpose of intelligence, materials should be in non-equilibrium state, quasi-equilibrium state and metastable state.
For smart materials, the concepts of material and information are the same. And the average information φ of a certain L symbol is related to the information logP of the probability P state, that is
This formula is similar to the entropy of thermodynamics, but the sign is opposite, so it is called negative entropy. Because entropy is a measure of disorder and negative entropy is a measure of order.
1.3 methods of creating intelligent materials
Intelligent materials have the functions of perception, processing and execution, so their creation is actually to introduce such software functions (information) into materials. This is similar to the information processing unit of human body-neuron, which can integrate various functions (Figure 1(a)) and put various software functions in different hierarchical structures with a thickness of several nanometers to several tens of nanometers (Figure 1(b)) to make materials intelligent. At this time, the properties of materials are not only related to their composition, structure and morphology, but also a function of the environment. The research and development of intelligent materials involves intelligent materials and systems in the fields of metals, ceramics, polymers and biology.
2. Intelligent inorganic nonmetallic materials
There are many kinds of intelligent inorganic nonmetallic materials. Here are some typical intelligent inorganic nonmetallic materials.
2. 1 smart ceramics
2. 1. 1 zirconia toughened ceramics
Zirconia crystals usually have three crystal forms:
Among them, the transformation from T-ZrO _ 2 to M-ZrO _ 2 has the characteristics of martensite transformation, which is accompanied by 3% ~ 5% volume expansion. ZrO2 _ 2 ceramics without stabilizer will crack seriously due to phase transformation during cooling at sintering temperature. The solution is to add metal oxides with ion radius smaller than Zr, such as Ca, Mg and Y.
The phase transformation of zirconia can be divided into phase transformation during firing and cooling and phase transformation during use. The former is temperature-induced and the latter is pressure-induced. The results of both phase transformations can toughen ceramics. The toughening mechanism mainly includes phase transformation toughening, microcrack toughening, surface toughening, crack bending and deflection toughening [9].
When the ZrO _ 2 grains are large and the stabilizer content is low, the T-ZrO _ 2 grains in the ceramics undergo phase transformation during the process of cooling to room temperature after firing, and the volume expansion accompanying the phase transformation generates compressive stress in the ceramics and forms microcracks in some areas. When the main crack propagates in this material, on the one hand, the crack propagation is hindered by the above compressive stress; At the same time, due to the extension of the original microcrack, the main crack is prevented from changing direction, the energy of crack propagation is absorbed, and the strength and toughness of the material are improved. This is microcrack toughening.
Because of the high phase transition temperature of ZrO _ 2, it is not feasible to design smart materials by means of temperature change. Stress-induced transformation toughening is the most important toughening mechanism in zirconia toughening ceramics, so it is necessary to study it.
After sintering and cooling to room temperature, T-ZrO _ 2 grains in the material still maintain tetragonal phase shape. When the material is subjected to external stress, it undergoes stress-induced phase transition, from T phase to M phase. The phase transformation of ZrO2 _ 2 grains absorbs energy and hinders crack propagation, thus improving the strength and toughness of materials. The composition of the phase change material is generally uneven, and the thermal conductivity and electrical conductivity change with the change of crystal structure, which is the signal that the material is subjected to external force, thus realizing the self-diagnosis of the material.
The crack caused by the fracture of zirconia material can be closed again after heat treatment at 300℃ for 50 hours, because the volume expansion generated during the transformation of T phase to M phase compensates the crack and the material can repair itself.
Through the changes of material size, sound wave propagation speed, thermal conductivity and electrical conductivity, the fatigue strength and expansion of materials in use can be observed in situ.
2. 1.2 smart ceramics
Smart ceramics is a kind of smart material, which can sense the change of environment and make corresponding response through feedback system. Several layers of lead zirconate titanate (PZT) can be used as an automatic positioning and tracking system for video heads, and Japanese pinball game machines are made of PZT piezoelectric ceramic blocks.
The principle of automatic positioning and tracking system for video head is that the PZT ceramic double-layer cantilever bending piece is divided into position sensing part and driving positioning part by laying electrodes. The position sensing part is a sensor, and the voltage obtained on the sensing electrode is applied to the positioning electrode through a feedback system, so that the lamina is bent and tracks on the video tape, as shown in Figure 2.
The pinball game machine also applies a similar principle.
Smart skin made of smart ceramics can reduce the noise of airplanes and submersibles when they are moving at high speed, prevent bumps, improve the running speed, reduce infrared radiation and achieve the purpose of invisibility.
Accord to that above principle, it is entirely possible to obtain very intelligent materials. This material can sense various changes in the environment and adjust one or more performance parameters of the material in time and space to achieve the best response. Therefore, perception, execution and feedback are the key functions of smart materials.
2. 1.3 piezoelectric bionic ceramics
Bionics of materials is one of the development directions of materials. Japanese researchers are studying the tail fins of whales and dolphins and the wings of birds, hoping to develop materials as soft, foldable and strong as tail fins and wings.
Fig. 3 is a bending stress sensor simulating the swimming bubble movement of fish. There is a small gas chamber between two metal electrodes in the sensor, and PZT piezoelectric ceramics play the role of covering swimming bubble muscles. Because the shape of the gas chamber is similar to a crescent moon, it is called "Moonie" compound. Piezoelectric underwater acoustic device adopts electrodes with special shapes, and the piezoelectric strain constant dh is increased to the maximum by changing the stress direction. When the thick metal electrode is subjected to hydrostatic pressure caused by sound waves, some longitudinal stresses are transformed into radial and tangential stresses with opposite signs, which makes the piezoelectric constant d3l change from negative to positive and overlap with d33, thus increasing dh value. Dh of this composite material? The gh value is 250 times that of pure PZT material.
The actuator element designed and developed with PZT fiber composite and "Moonie" composite can eliminate the steady flow caused by sound waves.
2.2 Intelligent cement-based materials
In modern society, cement is widely used as a basic building material, which makes cement-based materials intelligent and has good application prospects.
Intelligent cement-based materials include: stress, strain and damage self-checking cement-based materials [10 ~12]; Self-temperature measuring cement-based materials [13]; Cement-based materials that can automatically adjust the environmental humidity [14]; Bionic self-healing cement-based materials [15, 16] and bionic autogenous cement materials [17].
When short carbon fibers with a certain shape, size and dosage are added to cement-based materials, the resistance change of the materials corresponds to the internal structure change. Therefore, the material can monitor the internal conditions of the material under tension, bending, compression and static and dynamic loads. Using 0.5% (volume) carbon fiber as a sensor in cement slurry has much higher sensitivity than ordinary resistance strain gauges.
When a certain length of polyacrylonitrile-based short carbon fiber is mixed into cement slurry, the material produces thermoelectric effect. This material can monitor the temperature changes in the interior and surrounding environment of the building in real time. Based on the thermoelectric effect of this material, solar energy and indoor and outdoor temperature difference can also be used to supply power to buildings. If the material is further made to have the inverse effect of Seebeck effect-Peltier effect, it is possible to manufacture materials with refrigeration and heating.
Porous materials are added to cement slurry, and the relationship between moisture absorption and temperature is used to make the materials have moisture regulation function.
Some scientists are currently developing a kind of self-repairing concrete. Imagine that a large number of hollow fibers are buried in concrete. When cracks appear in concrete, the hollow fiber filled with "crack repair agent" will crack, releasing viscous repair agent, firmly sticking the cracks together and preventing concrete from breaking. This is a passive intelligent material, that is, there is no sensor embedded in the material department to monitor cracks, and there is no electronic chip embedded in the material to "guide" cracked cracks. Similar to this principle, the United States tried to prepare bionic cement-based materials according to the structure and formation mechanism of animal bones. If the material is damaged during use, porous organic fibers will release polymers to repair the damage.
American scientists are studying an active intelligent material, which can automatically reinforce bridges when problems occur. One way they design is that if some parts of the bridge have problems, other parts of the bridge will reinforce themselves to make up for them. This idea is technically feasible. With the development of computer technology, it is entirely possible to manufacture tiny signal sensors and microelectronic chips, and computers with these sensors and microcomputer chips embedded in bridge materials. Bridge materials can be all kinds of magical materials, such as shape memory materials. When the sensor buried in the bridge material gets the signal that there is something wrong with a certain part of the material, the computer will issue an instruction to make the tiny liquid buried in the bridge material turn into a solid and reinforce it automatically.
3. Conclusion
At present, intelligent materials are still in the research and development stage, and their development is closely related to social effects. The plane crash and the damage of important buildings and other structures inspire people to study intelligent aircraft and material structures with self-warning and self-repair functions. People's expectations of materials, systems and structures are met through the intelligent development of materials themselves, so that materials and structures can be "both rigid and flexible" to adapt to environmental changes. In the future research, we should focus on the following aspects.
(1) How to use the rapid development of information technology to introduce software functions into materials, systems and structures;
(2) Further strengthen the exploratory theory and mechanism research of smart composite materials, and accelerate the development of smart materials science;
(3) Strengthening applied basic research.