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NATURE OF METALS AND ALLOYS

• Metals represent one of the four major groups of dental materials.

• The Metals Handbook (1992) defines a Metal as

"an opaque lustrous chemical substance that is a good conductor of heat and electricity and, when polished, is a good reflector of light."

• An Alloy is a substance with metallic are good thermal and electrical conductors.

• Compared with ceramics, polymers, and composites, metals have high fracture toughness (KIc), i.e. the ability to absorb energy and inhibit crack propagation under increasing tensile stress. This property is a measure of the resistance of a material to clack propagation

• Noble metals (gold, iridium, osmium, palladium, platinum, rhodium, and ruthenium) are highly resistant to corrosion and oxidation and do not require alloying elements for this purpose

TERMINOLOGY

• Liquidus: the temperature below which a metal is completely solid (i.e., the temperature at which it begins to melt).

• Solidus: the temperature above which a metal melting temperature. Since it remains solid while the rest of the metal is molten, its small particles act as seeds around which grains of the solidifying metal form. This enhances the physical properties of the solid metal.

• Sag: deformation of a metal or alloy; for our discussion, this occurs at high temperatures such as during porcelain firing.

• Generally, alloys are used in dentistry because alloying strengthens a metal.

• Pure metals such as gold are not used because their relatively poor physical properties make them a poor choice for intraoral use. For example, pure cast gold is only 1/5th as strong and 1/6th as hard as a typical gold-based casting alloy.

• During solidification (i.e., when a metal or alloy goes from liquid to solid), the following process occurs: random atoms to embryos (temporary nuclei) to nuclei to dendrites to grains

• As the grains grow, they contact each other.

The border between them is called a grain boundary.

• Generally, a metal to be used under intraoral conditions should have a fine grained structure because it tends to be more resistant to permanent deformation.

BONDING OF METALS

• Apart from ionic and covalent bonding a solid can be held together by metallic bonds.

• Metallic bonds are formed between the free moving valence electrons and the positively charged ionic cores.

• Outer shell valence electrons can be removed easily from atoms in metals, thus the nuclei contains the balance of the bound electrons forming a positively charged ionic core.

• The electrons from these metal atoms flow freely to form a ‘sea’ of electrons or “cloud” or

“gas”

• Often called delocalized electrons

• Since the electrons are negatively charged and ionic cores are positively charged, electrostatic attraction force results giving rise to metallic bonds which holds the particles in the atom together

ATOMIC STRUCTURE

• Atomic level, pure metals exist as crystalline arrays.

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• In these arrays, the nuclei and core electrons occupy the atomic centers, whereas the free electrons float among the atomic positions

• There are six crystal systems that occur and these can further be divided into 14 crystalline arrays.

• Within each array, the smallest repeating unit that captures all of the relationships among atomic centres is called a unit cell.

• The unit cells for the most common arrays in dental metals are body centred cubic, face centred cubic and hexagonal.

• Body centred cubic – metallic atoms are located at the corners of the unit cell, and one atom is at the centre of the unit cell.

• Face centred – has 90 degree angles and atomic centres that are equidistant horizontally and vertically but atoms are located at the centres of the faces but none in the centre of the unit cell

© BRIHASPATHI ACADEMY ׀ SUBSCRIBER’S COPY ׀ NOT FOR SALE In these arrays, the nuclei and core electrons

occupy the atomic centers, whereas the free electrons float among the atomic positions

here are six crystal systems that occur and these can further be divided into 14 crystalline

Within each array, the smallest repeating unit that captures all of the relationships among

the most common arrays in dental metals are body centred cubic, face

metallic atoms are located at the corners of the unit cell, and one atom is

angles and atomic centres that are equidistant horizontally and vertically but atoms are located at the centres of the faces but none in the centre of the unit

• Hexagonal - atoms are equidistant from each other in the horizontal plane but not in the vertical direction

PROPERTIES OF METALS

• All properties are a result from its structure and metallic bonds.

• Higher density – due to efficient packing of atomic centres in the crystal lattice.

• Good electrical and thermal conductivity mobility of valence electrons which transfers energy by moving readily from areas of higher energy to those of lower energy under the influence or a thermal gradient or an electrical field.

• Opacity and reflective nature of metals free electrons to absorb and re

• Melting Point – occur as metallic bonds are overcome by the applied heat. The more valence electrons to an atomic centre, the higher the melting point.

• Corrosion – depend on how the valence electrons are held. The weaker the metallic bond, the higher the corrosion.

• Good ductility and malleability

ability of the atomic centres to slide against each other into new positions.

• Crystals have dislocation

atomic centres to slide past each other one plane at a time. Force required is less to move atoms are equidistant from each other in the horizontal plane but not in the

All properties are a result from its structure

due to efficient packing of atomic centres in the crystal lattice.

Good electrical and thermal conductivity – due to mobility of valence electrons which transfers energy by moving readily from areas of higher energy to those of lower energy under the influence or a thermal gradient or an electrical

nd reflective nature of metals – ability of electrons to absorb and re - emit light.

occur as metallic bonds are overcome by the applied heat. The more valence electrons to an atomic centre, the

depend on how the valence weaker the metallic bond, the higher the corrosion.

Good ductility and malleability – result from the ability of the atomic centres to slide against each other into new positions.

dislocation that allows the atomic centres to slide past each other one plane at a time. Force required is less to move

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move all planes simultaneously. Hence dislocations allow relatively easy deformation of metals.

• Strength of metals are increased by impeding the movement of dislocations.

• Fracture of metals occurs when atoms are unable to slide past one another. This occurs when there are impurities which block the dislocations.

• Emits a metallic sound

• Appear “white’ except for gold and copper.

ALLOYS

• Use of pure metals is limited in dentistry as they are soft and some like iron tend to corrode easily.

• It has been seen that metals maintain metallic behavior even when they are not pure and therefore can accept to a certain extent of addition of other elements when they solidify from liquid to solid state.

• To optimize properties, most metals are mixtures of two or more metallic elements or in some case one or more metals and/or nonmetals.

• Normally prepared by fusion of the elements above their melting points.

Categorizing Alloys

Based on either being noble or non noble

• Noble alloys have noble metals as the majority of the components whereas non noble alloys have a greater percentage of base metals

• American Dental Association has three categories high-noble, noble and predominantly base metals.

Based on the basis of their most common metal

• For instance if the alloy contains mainly gold, then it will be called gold based

Microstructure of Alloys

• As the molten alloy is cooled, the first solid alloy particles form as the temperature reaches the Liquidus – this process is called nucleation

• When cooling continues, nuclei grow into crystals – grains

• Grains enlarge until all of the liquid is gone and the grains meet and form boundaries – grain boundaries

Grains and Crystal Structure in Alloys

Size of grains depends on the cooling rate, alloy composition, presence of grain refiners.

Small grain size is generally desirable as it has better properties. Grain size influence’s an alloy’s strength, workability and even susceptibility to corrosion

Grain refiners are added to decrease grain size. Such as metals are iridium and ruthenium. Grain boundaries are important as they contain impurities such as oxides that are sites of corrosive attack.

Grain boundary regions are the final sites to undergo freezing for a molten metal that forms an equiaxed grain structure

Dendrites result from grains that grow along the major axes of the crystal lattice early in the freezing process. Dendritic skeleton structure persists to room temperature if the cooling rate is too fast.

Alloys that are predominantly base metal have larger grain size and cannot use grain refiners such as iridium.

Equiaxed means that the three dimensions of each grain are similar, in contrast to the elongated morphology of the dendrites

Most noble metal casting alloys solidify with an equiaxed polycrystalline microstructure

Cold-worked (wrought) microstructure

When the metal is to be used for wires, bands, bars, or other types of wrought structures, it is first cast into ingots that are then subjected to rolling, swaging, or wire-drawing operations that produce severe mechanical deformation of the metal. Such operations are described as hot or cold working of the metal,

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the operation is performed.

Many dental structures, such as orthodontic wires and bands, are formed by cold working operations. The finished product is often described as a wrought structure to denote that it has been formed by severe working or shaping operations.

The microscopic appearance of a cast metal is crystalline and sometimes has dendritic structure, when this metal is subjected to cold-working operations, such as drawing into a wire; the grains are broken down, entangled in each other, and elongated to develop a fibrous structure or appearance that is characteristic of wrought forms. This change in internal appearance is accompanied by a change in mechanical properties.

In general, mechanical properties of the wrought structure are superior to those of a casting prepared from the same melt or alloy

Recrystallization and Grain Growth

Metals or alloys that have been cold worked in the process of forming wires or bands change their internal structure and properties when heated or annealed.

The characteristic fibrous structure of the wrought mass is gradually lost, and the grain or crystalline structure reappears.

The process is known as recrystallization or grain growth. The degree of recrystallization is related not only to the alloy composition and mechanical treatment or strain hardening received during fabrication, but also to the temperature and the duration of the heating operation.

High temperatures and long heating periods produce the greatest amount of recrystallization.

Because the strength is usually reduced in recrystallized wrought structure (ductility often increases), it is necessary to guard

against excessive heating during the assembly of a wrought metal appliance.

Within practical limits of operation, this characteristic of recrystallization and grain growth is limited to wrought structures.

SOLIDIFICATION OF METALS

• If a pure metal is melted and allowed to cool to room temperature in a clean (and inert) container and if its temperature during cooling is plotted as a function of time, the following graph results

• The following observations can be made from the above graph

From point A to point B’ – steady decrease in temperature

From point B’ to point B – increase in temperature. Here time temperature remains constant until point C

From point C – steady decrease in temperature

Temperature Tf – indicted by straight or plateau portion of the curve at BC. It is the freezing point or solidification temperature of the pure metal and also the melting point or fusion temperature

All temperatures above T^f (plateau BC) are associated with molten metal

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All temperatures below Tf (plateau BC) are associated with solid metal

The initial cooling of the liquid metal from Tf to point B' is termed super cooling.

• During melting, temperature remains constant

• During freezing or solidification, heat is released as the metal changes from the higher-energy liquid state to the lower-energy solid state.

This energy difference is the latent heat of solidification and is equal to the heat of fusion

• During the Super cooling process, crystallization begins for the pure metal. Once the crystals begin to form, release of the latent heat of fusion causes the temperature to rise to T^f, where it remains until crystallization is completed at point C.

• Super cooling of pure metals occurs only in clean and inert containers

Nucleus formation

• As the molten metal approaches its freezing temperature, its energy relationships change

• Solidification begins with the formation of embryos in the molten metal (embryo is a small cluster of atoms that has the same arrangement as the long-range atomic order found in the solidified metal)

• At temperatures above T^f, such embryos will also form spontaneously in the molten metal, but they are unstable since the liquid state has a lower free energy than the solid state

• Free energy is of two types – surface free energy and volume free energy

• Surface free energy (F^S) = 4ߨݎ^ଶ ߛ. It increases as the square of the embryo radius

• Volume of free energy (FV) the difference in the free energies of the solid and liquid states.

It is given by ^ସ_ଷߨݎ^ଷ. It varies as the third power of the spherical embryo radius

• FV becomes increasingly negative (more energetically favourable) as the temperature of the super cooled liquid is decreased below Tf

• Overall or Resultant free energy of an embryo (R) is the sum of the surface free energy (positive) and the volume free energy change (negative) contributions

• Observations from the above graph are as follows

At small values of embryo radius, FS is dominant and the overall free energy for the formation of the embryo is positive (energetically unfavorable)

At larger values of the embryo radius, FV

become dominant and the overall free energy of the embryo is negative (energetically favourable)

Critical nucleus size (ro) is the maximum point in the total free energy of the embryos as a function of radius

For an embryo of radius (ro), the overall flee energy (R) decreases with the addition of another atom and continues to decrease as the embryo grows. Hence, embryos with radii smaller than r^o, are unstable and spontaneously form and disappear in the liquid metal, whereas embryos with radii larger than ro, are stable nuclei and continue to grow during the solidification process

• Greater the amount of super cooling, the greater the rate of temperature reduction below Tf, the smaller is the critical radius ro,

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size becomes increasingly negative

• Hence, an increasing number of embryos become stable as the super cooling is increased, and these embryos have reduced surface energy because of their decreased radii.

• If the molten metal is cooled so rapidly that solidification must occur at a much lower temperature than Tf, there is a tendency for many, very small, stable nuclei or solidification centers to form. If a single crystal is desired, little super cooling is needed, and the molten metal must be cooled very slowly

Homogeneous Nucleation

It occurs when there are no special objects inside a phase which can cause nucleation

For instance, when a pure liquid metal is slowly cooled below its equilibrium freezing temperature to a sufficient degree, numerous homogeneous nuclei are created by slow-moving atoms bonding together in a crystalline form

It is a random process, having equal probability of occurring at any point in the molten metal

Heterogeneous Nucleation

Occurs when there are special objects inside a phase which can cause nucleation

The mold walls or particles of dust and other impurities in the molten metal may produce heterogeneous nucleation

Super cooling is not required for heterogeneous nucleation

Solidification modes & Effects on properties

• Characteristically, a pure metal crystallizes from nuclei in a pattern that often resembles the branches of a tree yielding elongated crystals that are called dendrites.

• After their nucleation, typically at mold walls for castings, dendrites grow during the solidification of pure metals by the mechanism of thermal super cooling.

• Extensions or elevated areas called protuberances form spontaneously on the advancing front of the solidifying metal and grow into regions of negative temperature gradient

• In these regions of negative temperature gradient, the temperature is higher in the liquid adjacent to the frozen metal because of the latent heat of fusion released during freezing.

• The protuberances rapidly grow in the adjacent super cooled regions that lie farther away in the molten metal; growth is along specific crystallographic directions.

• The latent heat released by the solidifying metal also lowers the amount of super cooling at the liquid-solid interface, hindering growth in regions adjacent to the protuberances and resulting in separated, and highly elongated crystals.

• A similar growth mechanism subsequently occurs at lateral sites along the protuberances and later at lateral sites along the secondary branches, resulting in the three-dimensional dendritic structure.

Implications of Dendrites in Cast Alloys

Dendritic microstructures are not generally desirable for cast dental alloys, since the inter dendritic regions can serve as sites for facile crack propagation

Hot tears (micro cracks) can form at elevated temperatures in thin areas of castings prepared from these alloys, where there is insufficient bulk metal to resist the stresses imposed by the stronger casting investment, and these cracks will degrade the mechanical properties of the restorations. To avoid hot tears, castings need to have adequate thickness, and an alloy should be selected that has an equiaxed grain structure rather than a dendritic structure along with a lower burnout temperature

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