The glassy state is not a state of matter, but its structure.Solid materials are divided intocrystalandAmorphousThe atoms (or ions or molecules) constituting the crystal have a certain spatial structure (i.elattice）The crystal has a certain crystal shape and fixed melting point, but does not haveIsotropy。The glassy state is a kind of amorphous, which is a solid other than crystal in a solid.It has no fixed shape and fixed melting point, and is isotropic.They gradually become soft as the temperature rises, and finallymelt。It can be processed into various shapes after being softened.
Finger compositionatomNo structuralLong-range orderorTranslational Symmetry An amorphous solid ofstate。
The glassy state can be regarded as keepingGlass like propertiesThe solid state of.
Glassy transition
Announce
edit
GlassTransformation is a characteristic phenomenon different from other substances, and it is also one of the key and difficult problems in the field of condensed matter physics. One of the prominent characteristics of glass transformation is the change of thermodynamic parameters. The solidification of melt can be carried out through two paths, usually the melt nucleates and grows into crystalline substances near the solidification temperature point.In the process of changing from molten state to solid state, the first-order thermodynamic parameters of the system (such as volume and enthalpy) will change abruptly at the melting point temperature.If the cooling rate is fast enough or it is difficult to nucleate, so that the liquid can not nucleate and grow up in time, even if it reaches below the melting point, it still remains in the state of high temperature, that is, the liquid solidifies and enters the supercooled state through another way. At this time, the liquid is called supercooled liquid, and its volume and enthalpy show the characteristics of continuous changes during the process of cooling into glass.From the time scale, the supercooled liquid is a quasi equilibrium state with ergodic states in the phase space, and the glass state is a non-equilibrium state with ergodic states in the observable time range.In sharp contrast to the first order thermodynamic parameters, the second order thermodynamic parameters of the system (such as specific heat andCoefficient of thermal expansionThe existence of specific heat step is one of the most powerful evidences to confirm the glassy stateHowever, the thermodynamic glass transition is not a second-order phase transition, and the corresponding order parameter changes have not been observed so far.Unlike other substances, the transformation of glassy substances from liquid to solid is not carried out at a specific temperature point, but corresponds to a temperature range (i.e., the transition temperature range).In this temperature range, the glass changes from viscous body to viscoplastic body, viscoelastic body, and finally to elastic body.
Another remarkable feature of glass transition is the obvious dynamic change.When the supercooled liquid tends to the glass transition temperature, the viscosity changes sharply with the temperature (increasing by more than ten orders of magnitude instantaneously).
The traditional glass transition theory is mainly based on α relaxation. Recent studies have found that the slow β relaxation behavior is the more microscopic induction and basis of glass transition.
In addition, in addition to α relaxation and β relaxation, some special relaxation processes such as "excess wings" have also attracted wide attention, and various theoretical models have emerged in endlessly, such as phenomenological characteristics, free volume models, thermodynamic statistical models, solid model theory, energy barrier theory and modal coupling theory.
All these theoretical models are reasonable and helpful for understanding the nature of glassy matter, but they still have some limitations.For example, the free volume model can well explain the relationship between viscosity and hot melting near the glass transition and temperature, but because the parameters are single and difficult to measure in the laboratory, it can not completely describe the characteristics of glass transition and can not explain the heterogeneity, relaxation and microscopic mechanism of supercooled liquid.The modal coupling theory accurately describes the relaxation of high temperature melts above the glass transition temperature at the beginning of its introduction, but it does not agree with the experimental results in predicting that the melt structure will be frozen at the cross zone temperature.Although many variables have been introduced to improve since then, there are still many other problems.In addition, there is also a lack of thorough understanding of the essence of the dynamic change that the viscosity of supercooled liquid increases sharply but the structure does not change much in such a short period of time, and it is still a difficulty to test the relaxation time far below the glass transition temperature in experiments. The core problem of glass transition is the understanding of the glass relaxation phenomenon,How to analyze the internal essence of these different experimental phenomena and clarify the connections and differences between them is a huge challenge to be faced in the future.[1-2]
formation
Announce
edit
Glassy state
Inorganic glassIt is the first human synthesisInorganic materialsOne, mostPolymerThey also belong to the glassy state.The oxides that generate glassy state are mainlyElectronegativityCentered elements, such asboron、silicon、germaniumAnd phosphorus, etc.These elements are related tooxygenStrong formationcovalent bondAnd form a relatively open three-dimensional network structure. Even if the temperature is above the melting point of the material, this covalent bond still exists, but there is noLong-range orderOr translational symmetry.Of these oxidesmeltThe state has very highviscosityWhen the system temperature decreases, it is easy toSupercoolingState, makingLong range disorderTo be able tosolid phaseHold in.Supercooled liquidThe temperature at which the glass transition occurs is calledGlass transition temperature。someGlassAfter annealing for a long time below the glass transition temperatureCrystalline stateTransformation, which is calledDevitrification。Add the above oxidesAlkali metal oxideIt can break the original three-dimensional network to a certain extent and reduce the glass transition temperature.People areSilicate glassA lot of research has been carried out and it is believed that in the glassy stateSilica tetrahedronBut there are different degrees of distortion and rotation between silicon oxygen tetrahedronsdisorder3D network of.
When the polymer material is subjected toExternal force actionOnly by changing theKey length、Bond angleTo adapt to external forces, sopolymerDisplayeddeformationVery weak.Shape variableIt is proportional to the external force. Once the external force is removed, the deformation will resume immediately.Because the polymer showsmechanical property AndSmall moleculeGlass is very similar, so this mechanical state of polymer is called glassy state.Chain segmentThe motion is frozen, only the key length, key angleLateral base、Small linkSuch small size movement unit can move.[3]
2. Gradual softening during heating: from brittle state to plastic state, high viscosity state, and finally to melt, the viscosity changes continuously.
3. Melting and solidification are reversible: repeatedly heated toMolten stateAnd heating and solidification according to the same system. If there is no phase separation and crystallization, the original properties will be restored.
4. The internal energy of the glassy state is larger than that of the crystal: under appropriate temperature conditions, the glass tends to crystallizeLiquidusAt the following temperatures, glass crystallization is spontaneous, without external work.
5. The properties of glass change continuously with the composition within a certain range: thus the composition can be changed to change the properties of glass, such as ordinary glassSilicate glassIt is an insulator, but the chalcogenide glass is a semiconductor. As2Te3conductivityUp to 10s/m
Physical state
Announce
edit
GlassstateThe surface looks solid, but it is not.For more than 50 years, scientists have been trying to understand the nature of glass.In 2008, scientists from Britain, Australia and Japan jointly found that the reason why glass cannot become a solid is the special atomic structure formed when glass cools.Relevant papers were published online in Nature Materials on June 22, 2008.
Key researchersuniversity of bristol Paddy Royall of“lattice”（lattice）。However, when the glass is cooled, the atoms are jammed together and arranged almost randomly, which hinders the formation of regular lattices. "
In the experiment, in order to observe the real movement of microscopic atoms, researchers use larger colloidal particles to simulate atoms, and useHigh power microscopeObserve.It was found that the gel formed by these particlesIcosahedronStructure without crystallizationUniversity of BRISTOL Charles Frank made the same prediction.This structure explains why glass is "glass" rather than liquid or solid.
This research is important for understandingMetastable stateMaterial is a major breakthrough, which will enable further developmentMetallic glassAnd other new materials become possible.In addition, if the metal can be cooled to form a glass like internal structure through operation, it will be possible to greatly reduce metal defects1.(Science Network Mei Jin/Compilation)
(Nature Materials),doi:10.1038/nmat2219，C. Patrick Royall，Hajime Tanaka）
