玻璃结构理论GLASS_THEORY

The Nature of Glass Remains Anything but Clear The New York Times 2008 Science It is well known that panes of stained glass in old European churches are thicker at the bottom because glass is a slow moving liquid that flows downward over centuries Well known but wrong Medieval stained glass makers were simply unable to make perfectly flat panes and the windows were just as unevenly thick when new The tale contains a grain of truth about glass resembling a liquid however The arrangement of atoms and molecules in glass is indistinguishable from that of a liquid But how can a liquid be as strikingly hard as glass They re the thickest and gooiest of liquids and the most disordered and structureless of rigid solids said Peter Harrowell a professor of chemistry at the University of Sydney in Australia speaking of glasses which can be formed from different raw materials They sit right at this really profound sort of puzzle Philip W Anderson a Nobel Prize winning physicist at Princeton wrote in 1995 The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition He added This could be the next breakthrough in the coming decade Thirteen years later scientists still disagree with some vehemence about the nature of glass Peter G Wolynes a professor of chemistry at the University of California San Diego thinks he essentially solved the glass problem two decades ago based on ideas of what glass would look like if cooled infinitely slowly I think we have a very good constructive theory of that these days Dr Wolynes said Many people tell me this is very contentious I disagree violently with them Others like Juan P Garrahan professor of physics at the University of Nottingham in England and David Chandler professor of chemistry at the University of California Berkeley have taken a different approach and are as certain that they are on the right track It surprises most people that we still don t understand this said David R Reichman a professor of chemistry at Columbia who takes yet another approach to the glass problem We don t understand why glass should be a solid and how it forms Dr Reichman said of Dr Wolynes s theory I think a lot of the elements in it are correct but he said it was not a complete picture Theorists are drawn to the problem Dr Reichman said because we think it s not solved yet except for Peter maybe Scientists are slowly accumulating more clues A few years ago experiments and computer simulations revealed something unexpected as molten glass cools the molecules do not slow down uniformly Some areas jam rigid first while in other regions the molecules continue to skitter around in a liquid like fashion More strangely the fast moving regions look no different from the slow moving ones Meanwhile computer simulations have become sophisticated and large enough to provide additional insights and yet more theories have been proffered to explain glasses David A Weitz a physics professor at Harvard joked There are more theories of the glass transition than there are theorists who propose them Dr Weitz performs experiments using tiny particles suspended in liquids to mimic the behavior of glass and he ducks out of the theoretical battles It just can get so controversial and so many loud arguments and I don t want to get involved with that myself For scientists glass is not just the glass of windows and jars made of silica sodium carbonate and calcium oxide Rather a glass is any solid in which the molecules are jumbled randomly Many plastics like polycarbonate are glasses as are many ceramics Understanding glass would not just solve a longstanding fundamental and arguably Nobel worthy problem and perhaps lead to better glasses That knowledge might benefit drug makers for instance Certain drugs if they could be made in a stable glass structure instead of a crystalline form would dissolve more quickly allowing them to be taken orally instead of being injected The tools and techniques applied to glass might also provide headway on other problems in material science biology and other fields that look at general properties that arise out of many disordered interactions A glass is an example probably the simplest example of the truly complex Dr Harrowell the University of Sydney professor said In liquids molecules jiggle around along random jumbled paths When cooled a liquid either freezes as water does into ice or it does not freeze and forms a glass instead In freezing to a conventional solid a liquid undergoes a so called phase transition the molecules line up next to and on top of one another in a simple neat crystal pattern When a liquid solidifies into a glass this organized stacking is nowhere to be found Instead the molecules just move slower and slower and slower until they are effectively not moving at all trapped in a strange state between liquid and solid The glass transition differs from a usual phase transition in several other key ways Energy what is called latent heat is released when water molecules line up into ice There is no latent heat in the formation of glass The glass transition does not occur at a single well defined temperature the slower the cooling the lower the transition temperature Even the definition of glass is arbitrary basically a rate of flow so slow that it is too boring and time consuming to watch The final structure of the glass also depends on how slowly it has been cooled By contrast water cooled quickly or cooled slowly consistently crystallizes to the same ice structure at 32 degrees Fahrenheit To develop his theory Dr Wolynes zeroed in on an observation made decades ago that the viscosity of a glass was related to the amount of entropy a measure of disorder in the glass Further if a glass could be formed by cooling at an infinitely slow rate the entropy would vanish at a temperature well above absolute zero violating the third law of thermodynamics which states that entropy vanishes at absolute zero Dr Wolynes and his collaborators came up with a mathematical model to describe this hypothetical impossible glass calling it an ideal glass Based on this ideal glass they said the properties of real glasses could be deduced although exact calculations were too hard to perform That was in the 1980s I thought in 1990 the problem was solved Dr Wolynes said and he moved on to other work Not everyone found the theory satisfying Dr Wolynes and his collaborators so insisted they were right that you had the impression they were trying to sell you an old car said Jean Philippe Bouchaud of the Atomic Energy Commission in France I think Peter is not the best advocate of his own ideas He tends to oversell his own theory Around that time the first hints of the dichotomy of fast moving and slow moving regions in a solidifying glass were seen in experiments and computer simulations predicted that this pattern called dynamical heterogeneity should exist Dr Weitz of Harvard had been working for a couple of decades with colloids or suspensions of plastic spheres in liquids and he thought he could use them to study the glass transition As the liquid is squeezed out the colloid particles undergo the same change as a cooling glass With the colloids Dr Weitz could photograph the movements of each particle in a colloidal glass and show that some chunks of particles moved quickly while most hardly moved You can see them Dr Weitz said You can see them so clearly The new findings did not faze Dr Wolynes Around 2000 he returned to the glass problem convinced that with techniques he had used in solving protein folding problems he could fill in some of the computational gaps in his glass theory Among the calculations he found that dynamical heterogeneity was a natural consequence of the theory Dr Bouchaud and a colleague Giulio Biroli revisited Dr Wolynes s theory translating it into terms they could more easily understand and coming up with predictions that could be compared with experiments For a long time I didn t really believe in the whole story but with time I became more and more convinced there is something very deep in the theory Dr Bouchaud said I think these people had fantastic intuition about how the whole problem should be attacked For Dr Garrahan the University of Nottingham scientist and Dr Chandler the Berkeley scientist the contrast between fast and slow moving regions was so striking compared with the other changes near the transition they focused on these dynamics They said that the fundamental process in the glass transition was a phase transition in the trajectories from flowing to jammed rather than a change in structure seen in most phase transitions You don t see anything interesting in the structure of these glass formers unless you look at space and time Dr Garrahan said They ignore the more subtle effects related to the impossible to reach ideal glass state If I can never get there these are metaphysical temperatures Dr Chandler said Dr Chandler and Dr Garrahan have devised and solved mathematical models but like Dr Wolynes they have not yet convinced everyone of how the model is related to real glasses The theory does not try to explain the presumed connection between entropy and viscosity and some scientists said they found it hard to believe that the connection was just coincidence and unrelated to the glass transition Dr Harrowell said that in the proposed theories so far the theorists have had to guess about elementary atomic properties of glass not yet observed and he wondered whether one theory could cover all glasses since glasses are defined not by a common characteristic they possess but rather a common characteristic they lack order And there could be many reasons that order is thwarted If I showed you a room without an elephant in the room the question why is there not an elephant in the room is not a well posed question Dr Harrowell said New experiments and computer simulations may offer better explanations about glass Simulations by Dr Harrowell and his co workers have been able to predict based on the pattern of vibration frequencies which areas were likely to be jammed and which were likely to continue moving The softer places which vibrate at lower frequencies moved more freely Mark D Ediger a professor of chemistry at the University of Wisconsin Madison has found a way to make thin films of glass with the more stable structure of a glass that has been aged for at least 10 000 years He hopes the films will help test Dr Wolynes s theory and point to what really happens as glass approaches its ideal state since no one expects the third law of thermodynamics to fall away Dr Weitz of Harvard continues to squeeze colloids except now the particles are made of compressible gels enabling the colloidal glasses to exhibit a wider range of glassy behavior When we can say what structure is present in glasses that will be a real bit of progress Dr Harrowell said And hopefully something that will have broader implications than just the glass field This article has been revised to reflect the following correction Correction July 31 2008 An article on Tuesday about the nature of glass described incorrectly the phase transition from water to ice When water molecules are lined up into ice energy called latent heat is released The phase transition does not require energy to line up the molecules In the phase transition for glass there is no latent heat Updated PEG January 1997 Original by Philip Gibbs October 1996 with thanks to many who contributed their knowledge and references Is glass liquid or solid It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid and so over several centuries it has flowed towards the bottom This is not true In Mediaeval times panes of glass were often made by the Crown glass process A lump of molten glass was rolled blown expanded flattened and finally spun into a disc before being cut into panes The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass To answer the question Is glass liquid or solid we have to understand its thermodynamic and material properties Thermodynamics of glass There is still much about the molecular physics and thermodynamics of glass that is not well understood but we can give a general account of what is thought to be the case Many solids have a crystalline structure on microscopic scales The molecules are arranged in a regular lattice As the solid is heated the molecules vibrate about their position in the lattice until at the melting point the crystal breaks down and the molecules start to flow There is a sharp distinction between the solid and the liquid state that is separated by a first order phase transition i e a discontinuous change in the properties of the material such as density Freezing is marked by a release of heat known as the heat of fusion A liquid has viscosity a measure of its resistance to flow The viscosity of water at room temperature is about 0 01 poises A thick oil might have a viscosity of about 1 0 poise As a liquid is cooled its viscosity normally increases but viscosity also has a tendency to prevent crystallisation Usually when a liquid is cooled to below its melting point crystals form and it solidifies but sometimes it can become supercooled and remain liquid below its melting point because there are no nucleation sites to initiate the crystallisation If the viscosity rises enough as it is cooled further it may never crystallise The viscosity rises rapidly and continuously forming a thick syrup and eventually an amorphous solid The molecules then have a disordered arrangement but sufficient cohesion to maintain some rigidity In this state it is often called an amorphous solid or glass Some people claim that glass is actually a supercooled liquid because there is no first order phase transition as it cools In fact there is a second order transition between the supercooled liquid state and the glass state so a distinction can still be drawn The transition is not as dramatic as the phase change that takes you from liquid to crystalline solids There is no discontinuous change of density and no latent heat of fusion The transition can be detected as a marked change in the thermal expansivity and heat capacity of the material The temperature at which the glass transition takes place can vary according to how slowly the material cools If it cools slowly it has longer to relax the transition occurs at a lower temperature and the glass formed is more dense If it cools very slowly it will crystallise so there is a minimum limit to the glass transition temperature A liquid to crystal transition is a thermodynamic one i e the crystal is energetically more favourable than the liquid when below the melting point The glass transition is purely kinetic i e the disordered glassy state does not have enough kinetic energy to overcome the potential energy barriers required for movement of the molecules past one another The molecules of the glass take on a fixed but disordered arrangement Glasses and supercooled liquids are both metastable phases rather than true thermodynamic phases like crystalline solids In principle a glass could undergo a spontaneous transition to a crystalline solid at any time Sometimes old glass devitrifies in this way if it has impurities The situation at the level of molecular physics can be summarised by saying that there are three main types of molecular arrangement crystalline solids molecules are ordered in a regular lattice fluids molecules are disordered and are not rigidly bound glasses molecules are disordered but are rigidly bound Just to illustrate that no such classification could ever be complete recently scientists have succeeded in making quasi crystals that are quasi periodic They do not fit into the above scheme and are sometimes described as being halfway between crystals and glass It would be convenient if we could conclude that glassy materials changed from being a supercooled liquid to an amorphous solid at the glass transition but this is very difficult to justify Polymerised materials such as rubber show a clear glass transition at low temperatures but are normally considered to be solid in both the glass and rubber conditions It is sometimes said that glass is therefore neither a liquid nor a solid It has a distinctly different structure with properties of both liquids and solids Not everyone agrees with this terminology Material properties of glasses Usually when people talk about solids and liquids they are referring to macroscopic material properties rather than the arrangement of molecules After all glass as a material was known about long before its molecular physics was understood Macroscopically materials exhibit a very wide range of behaviours Solids liquids and gases are ideal behaviours characterised by properties such as compressibility viscosity elasticity strength and hardness But materials don t always behave according to such ideals For example it s possible to take water from being a liquid to a gas at high pressure without its passing through a phase transition so at some stage it must be between an ideal liquid and an ideal gas For crystalline substances the distinction between the solid and liquid states is very clear but what about glasses Indeed where do polymers gels foams liquid crystals powders and colloids fit into this picture Some people say that there is no clear distinction between a solid and a liquid in general A solid they claim should just be defined as a liquid with a very high viscosity They set an arbitrary limit of 1013 poises above which they say it s a solid and below which it s a liquid According to another po