Physical Bases of Dental Material Science

Physical Bases of Dental Material Science

e-book

Editors:

Ferenc Tölgyesi, István Derka, Károly Módos

Semmelweis University, Biophysics and Radiation Biology Institute • Budapest, 2012

© Ferenc Tölgyesi, István Derka, Károly Módos, 2012

FAFA

Manuscript completed: 2012. május 31.

Semmelweis University, Biophysics and Radiation Biology Institute

This publication is produced under the next tender: TÁMOP-4.1.2/A/2-10/1-2010-0008 , „Képzés és tartalomfejlesztés a Semmelweis Egyetemen” .

Foreword

Almost every day many new materials are introduced into the dental practice. This fact requires deeper knowledge of material science. Faculty of Dentistry of Semmelweis University followed the international tendency when introduced „Physical Bases of Dental Material Science” subject. The task of this subject is to give general education in material science. It is also necessary to understand well the most important relationships that helps students to learn more easily other subjects later. Anyway else material science is not only important but a very interesting “game”.

There is a serious problem: students, who start to learn this subject, has no enough deep knowledge in science mainly in physics. Editors try to give a more easily understandable text. It is impossible to learn anything without any base knowledge, so if somebody has no any previous study we propose to look for any textbook that contains the base physical or mathematical knowledge.

We will be very grateful for every notices or critical comments:

ferenc.tolgyesi@eok.sote.hu.

There are different colored frames in the text. Meaning of the colors is:

Good learning!

Budapest, 31. 05. 2012.. Editors

Other texts, examples which belong to the topics on the exam. Here are examples on values of the quantities.

Curiosities which are not necessary to know on the exam.

Most important concepts of the chapter.

There is a collection of problems at the end of the textbook. Proposed problems to the given chapters are in this frame. The collection contains the results and there are a few complete solutions, too.

Contents:

Interactions between atoms and molecules, bonds

Consistencies

Gaseous state

Liquid state

Solid state

Phase, phase diagram, phase transition

Surface phenomena

Methods of structural analysis

Material families

Structure of metals, metal alloys

Structure of ceramics

Structure of polymers

Structure of the composites

Mechanical properties – introduction

Mechanical properties – elastic behavior

Mechanical properties — plastic behavior

Mechanical properties – fracture

Mechanical properties – viscoelasticity

Thermal and electric properties

Optical properties

Comparative summary of material families characteristics

Problems

Chapter 1. Interactions between atoms and molecules, bonds

The properties of the atoms and molecules and the interactions between them determine the characteristics of a matter. This chapter deals with the forces between atoms and molecules and the „so-called” primary and secondary bonds.

What determines the type of the primary bond?

Ionic, covalent and metallic bond are the primary bonds. The electronegativity of the atoms plays an important role in forming a bond. The definition of the electronegativity is:

AIEN += ,

where I is the ionization energy (the smallest energy that is necessary to remove an electron from the atom), A is the electron affinity (the energy that is released capturing an electron – this energy is negative, according to the agreement, so to calculate the affinity we must take the absolute value). The usual unit is the eV (electronvolt), or we can use aJ (attojoule), or for one mol kJ/mol.

Fig. 1.1 Electronegativity of the elements (according to Pauling). (www.ptable.com site is very

informative, where not only the electronegativity but other characteristics of the elements may be found)

There are different electronegativity scales in the literature. One of them is the most widely used Pauling’s scale. In Fig. 1.1 height of the columns shows the electronegativity of the elements. The electronegativity increases from left to right according to the more complete electron shell that is closer to the noble gas configuration in each period. In the columns the electrons are far from

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Chapter 1. Interactions between atoms and molecules, bonds

the nucleus because the radius increases and the electronegativity decreases from up to down. We can summarize these in a sketchy, but understandable statement that from the left bottom corner (Fr) to the right top one (F) along any way the electronegativity increases. (The electronegativity of the noble gases nowadays is defined but previously not. We neglect it due to the less practical importance.)

What is the role of the electronegativity in determination of the type of the bond? If two atoms have very different electronegativity the atom that has higher value is able to accept electron from the other. The previous one gets negative the latter positive charge and the electrostatic attraction results ionic bond (Fig. 1.2).

Fig. 1.2 Formation of the strong (primary) bonds

If the electronegativities are the same or the difference is small the outer electrons become common and the common electron orbitals –that produces a certain attraction between atoms – results the covalent or the metallic bond. We can say that atoms have a common electron cloud. Type of the bond depends on the average electronegativity of the partners. If it is big spreading of the „cloud” is not too large and we can speak about covalent bond. In opposite case the spreading may be very large involving billions of atoms. If electron cloud entirely delocalized, so spreads entirely in the crystal the metallic bond is formed. (We can say that this crystal is a ”huge” molecule.)

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Chapter 1. Interactions between atoms and molecules, bonds

Fig. 1.3 Examples of the primary bonds (At the vertex of the triangle in the pure form at the other examples in mixed form.)

On the base of the previous discussion we can place the different bonds into a coordinate system (Fig. 1.3) where the average electronegativity is on the horizontal axis and the difference of them is on the vertical one. The different chemical bonds may be found in a triangle area. The peak at the top and the area around it represent the ionic bond (large difference, e.g. CsF). Left peak and its environment represent the metallic bond (small difference and average, e.g. Cs). The last one on the right corresponds to the covalent bond (e.g. F2). Most chemicals are formed by the mixture of the different bonds according to the examples in the figure.

Why is a bond formed?

Of course there is no always bond between two atoms. The energy of the two atoms enough close to each other forming a system must be smaller than the sum of their own energy far and separated from each other. Figure 1.4 shows the typical total energy of the two atoms as the function of the distance between them. The energy decreases if they are closer to each other. After a certain minimum energy if the distance decreases more the energy increases rapidly. The optimal distance is where the energy of the system has minimum. Increasing the distance they attract each other trying to return to the minimum. Moving closer them they repulse each other. The energy curve on the figure is the sum of the two dashed ones which represent the attractive and repulsing force.

The origin of the attraction may be electrostatic interaction between two ions with different charge or common electron orbitals (bonding orbitals). The repulsion may derive from the electrostatic force between nuclei being too close to each other or from the non-bonding orbitals produced by the electrons due to the Pauli’s principle. The equilibrium distance (r0) is the bond distance in the range of 0.1 nm. The depth of the energy minimum (E0) is the bond energy typically 100-1000 kJ/mol. In this range an average covalent bond has higher and the ionic or metallic bond has lower value.

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Chapter 1. Interactions between atoms and molecules, bonds

Fig. 1.4 Formation the bonds between atoms (molecules)

An interesting and important property of the energy curve that in the neighborhood of the minimum the shape of the curve approximately parabolic as in the case of the spring as we learnt in the previous study. The parabolic energy function of the spring is the consequence of the linear force law. We expect similar linear force function in the case of atomic interaction too at least close to the equilibrium state. It becomes important later in the discussion of the deformation of the objects. In the case of the deformation like elongation the distance between atoms a little bit increases and how much forces arise inside the substance the question that should be answered.

In brief we must mention one question yet about the directionality of the bonds. The ionic and covalent bond results strong interaction to specific directions. There is no preferred direction in the case of metallic bond due to the delocalized electron cloud.

Secondary bonds

There will be attractive force among atoms and molecules if there are no common electron orbitals and no net electric charge but they have permanent or temporary electric dipole moment. The electric interaction between the properly positioned dipoles is the base of the secondary bonds like the van der Waals or H-bond. This force is much weaker than the previously discussed bonds so the energy range of them is about 0.2-50 kJ/mol. The H-bond is stronger. (Sufficiently large number of these bonds is able to produce very large cohesive strength among the particle of a body or among different bodies.)

The types of the van der Waals interaction: orientation induction and dispersion. Orientation one is produced between two permanent dipoles with proper direction. In the case of the induction type a permanent dipole induces dipole moment in a neutral molecule due to the sharing charges having proper direction. The dispersion interaction is the weakest one. In this case there is no atom or molecule having permanent dipole but due to the fluctuations of the electron clouds results temporary dipoles. Temporary dipole produced on this way may induce a properly orientated dipole in the neighboring atom or molecule (Fig. 1.5)

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Chapter 1. Interactions between atoms and molecules, bonds