Structure of Matter, General Classes of Material and Principles of Adhesion

Practice flashcards covering the structure of matter, atomic arrangement, classes of materials (metals, ceramics, polymers, composites), and principles of adhesion in dentistry.

What is the definition of adhesion?

An attraction between two contacting surfaces promoted by the interfacial force of attraction between the molecules or atoms of two different species.

In what forms may adhesion occur?

As chemical adhesion, mechanical adhesion (structural interlocking), or a combination of both.

What is an adhesive?

A substance that promotes adhesion of one substance or material to another.

What is an adherend?

A material substrate bonded to another material by means of an adhesive.

What is a metallic alloy?

A crystalline solid with metallic properties composed of two or more elements, at least one of which is a metal, and all of which are mutually soluble in the molten state.

How is an alloy system defined?

All possible alloyed combinations of two or more elements, at least one of which is a metal.

What does a binary gold-silver system include?

All possible alloys of gold and silver, varying from $100\%$ gold and $0\%$ silver to $100\%$ silver and $0\%$ gold.

What is bonding?

The action of joining objects or particles together by means of an adhesive or force of attraction.

What defines a ceramic?

Solid-phase compounds of metallic and nonmetallic elements.

What is cohesion?

Bonding between molecules or atoms of the same species.

What is a composite material?

A material made of two or more constituent components with significantly different physical or chemical properties that produce characteristics different from individual components.

How is the contact angle measured?

The angle of intersection between a liquid and a solid surface, measured from the solid surface through the liquid to the liquid/vapor tangent line.

What does a contact angle of $180^\circ$ indicate?

No wetting occurs.

What does a contact angle of $0^\circ$ indicate?

Complete wetting occurs.

What is a copolymer?

A polymer made of two or more types of monomers.

What is a random copolymer?

A copolymer where there is no sequential order of monomer types along the polymer chain.

What is a block copolymer?

A copolymer where groups of each type of monomer appear in the same polymer chain.

What is a graft or branched copolymer?

A copolymer where a sequence of one type of mer unit is grafted onto the backbone of a second type of monomer.

What is the function of a crosslink in polymerization?

A difunctional or multifunctional monomer forms a link between two growing polymer chains, resulting in a three-dimensional interconnected network.

What occurs during the curing process?

Low-molecular-weight monomers are converted into higher-molecular-weight materials through polymerization.

What is elastic recovery?

The process where an object recovers to its original shape after an applied force is removed.

How does the viscous nature of a material affect elastic recovery?

The greater the viscous nature, the less complete the recovery.

What is the first definition of glass-transition temperature ($T_g$)?

The temperature above which a sharp increase in the thermal expansion coefficient occurs, indicating increased molecular mobility.

What is the second definition of glass-transition temperature ($T_g$)?

The temperature at which macromolecule molecular motion begins to force polymer chains apart, causing softening.

What is a grain in a metal's microstructure?

A single crystal in the microstructure of a metal.

What is a grain boundary?

The interface between adjacent grains in a solid metal or alloy.

What is a metal?

An element or alloy whose atoms readily lose electrons to form positively charged ions.

What are the physical characteristics of a metallic material?

Opaque, ductile, relatively malleable, good electrical and thermal conductor, and usually lustrous.

What constitutes the microstructure of a material?

Structural features including grains, grain boundaries, phases, and defects like porosity.

How is the microstructure of a polished specimen revealed?

By microscopic imaging of a chemically or electrolytically etched surface.

What is micromechanical bonding?

Retention associated with an adhesive penetrating a roughened adherend surface.

What is a monomer?

A chemical compound capable of reacting to form a polymer; each monomer becomes a mer.

What is a phase diagram (constitution diagram)?

A graph of equilibrium phases and solubility limits for an alloy system as a function of composition and temperature.

What is a polymer?

Large molecules (macromolecules) formed by the union of many simple repeating units called mers.

What are the three types of primary bonds?

Ionic, covalent, and metallic bonds.

What is a synthetic resin?

A blend of monomers and/or macromolecules with other components forming a material with useful properties.

What is a metallic solid solution?

A solid crystalline phase containing two or more elements whose atoms share the same crystal lattice.

What is strain?

The magnitude of deformation (stretching, compression, or shear) occurring in response to an applied force.

What is stress?

The perpendicularly directed force, exerted as pressure or tension, measured in units of force per unit area ($mN/m^2$).

What causes surface energy?

Excess energy of attraction at the surface compared with the bulk because surface atoms are not surrounded by fellows.

What is the unit of surface energy?

$mJ/m^2$ or $mN/m^2$.

What is surface tension?

The tendency of fluid surfaces to contract to the smallest surface area by an inward force caused by imbalanced mutual attraction.

What is the unit for surface tension?

$mN/m$.

What is a thermoplastic polymer?

A macromolecule material made of linear or branched chains that softens when heated above $T_g$ and can be remolded.

What is a thermoset polymer?

A polymeric material that retains a crosslinked structure and does not soften enough to be molded upon heating.

What are van der Waals forces?

Short-range forces of physical attraction (secondary bonds) between molecules of liquids or molecular crystals.

How is wetting defined?

The ability of a liquid to maintain contact with a solid surface reflecting intermolecular interactions.

What is a wetting agent?

A surface-active substance applied to a solid substrate to reduce the surface tension of a liquid.

What are the three subatomic particles in a nitrogen atom?

A negatively charged electron, a positively charged proton, and an electrically neutral neutron.

What particles are contained in the nucleus of an atom?

Protons and neutrons (except for hydrogen which has no neutrons).

What force holds neutrons and protons together in the nucleus?

Nuclear force.

What is the estimated acting distance of nuclear force?

$≅ 1 \times 10^{-15}\,m$.

What are valence electrons?

Electrons in the shells of the cloud that dictate the property of matter made of those atoms.

How is a molecule defined?

An electrically neutral entity formed when two or more atoms bond together.

What is the basic unit of water?

A molecule made of two hydrogen atoms and one oxygen atom.

What allows molecules of a known mass to maintain constant motion?

Kinetic energy.

What causes water molecules to condense to liquid form?

A decrease in surrounding temperature leads to decreased kinetic energy, making attraction between molecules more prominent.

What is the change of state from liquid to ice?

Further cooling where kinetic energy is so low that molecules are immobilized by attraction forces.

How atoms are treated when interacting in space?

As discrete entities with definite boundaries established by electromagnetic fields of electrons.

What causes the attraction force between two atoms?

Interaction of opposite-charged particles of either atom.

What causes the expulsion force between two atoms?

Interaction of similar-charged particles of either atom.

What is interatomic spacing?

The distance between the centers of the cores of the atoms.

Which force increases faster initially when two atoms come close together?

The force of attraction.

What is the resultant force in atom interaction?

The sum of attraction and repulsion forces.

What occurs at the state of equilibrium between two atoms?

There is no net resultant force and the atoms reach a stable state.

What does the interatomic distance at equilibrium represent?

The sum of the radii of the two adjacent atoms.

How is bond energy (potential energy) related to interatomic force?

Integration of the interatomic force over the interatomic distance yields the interatomic energy.

When does bond energy reach a minimum?

When the resultant force between atoms becomes zero.

What defines the equilibrium interatomic distance?

The condition where the potential energy is at its minimum.

What determines the average amplitude of atomic vibration?

The temperature.

What is the phenomenon of thermal expansion?

The increase in mean interatomic distance as temperature and internal energy increase.

How is the linear coefficient of thermal expansion ($\alpha$) related to melting temperature?

It tends to be inversely proportional to the melting temperature for similar structures.

What is required for a solid with lower potential energy (deeper trough) to melt?

Greater amounts of kinetic energy, leading to higher melting and boiling temperatures.

How is stiffness (elastic modulus) measured on a force-vs-displacement curve?

By the slope of the net-force curve drawn from the equilibrium position.

A high melting point is usually accompanied by what mechanical property?

Greater stiffness.

When is the electron structure of an atom relatively stable?

If it has eight electrons in its outer valence shell (except helium).

What are secondary bonds based on?

Dipole forces induced by nonuniform distribution of electrons within a molecule.

Give an example of ionic bonding in dentistry.

Gypsum and phosphate-based cements.

How is a covalent bond characterized?

By the sharing of electron pairs between atoms.

Where does covalent bonding occur in dental materials?

In dental resins, linking to form the backbone structure of hydrocarbon chains.

What is the structure of a metallic bond?

Positive ions in a lattice bonded by a "cloud" or "gas" of free valence electrons.

Why are metals opaque to transmitted light?

Free electrons in the metallic bond absorb light energy.

What is plastic deformation in metals?

The ability of a material to be reshaped by force without fracture, associated with slip along crystal planes.

Describe the bonding in calcium sulfate ($CaSO_4$).

The sulfur and oxygen in the sulfate ion are covalent; the calcium and sulfate ions are held by ionic attraction.

What causes van der Waals forces in nonpolar molecules?

Random movement of electrons creating fluctuating dipoles.

What is a hydrogen bond?

A special case of dipole attraction in polar compounds, such as water molecules forming a hydrogen bridge.

What is a space lattice?

Any arrangement of atoms in space in which every atom is situated similarly to every other atom.

What is an amorphous structure?

A noncrystalline solid formation where molecules are distributed at random.

How many possible lattice types are there?

$14$.

What defines a unit cell?

The smallest repetitive volume of a crystal.

What is a body-centered cubic (BCC) cell?

A cubic lattice with an atom at each corner and one atom at the body center.

What is a face-centered cubic (FCC) cell?

A cubic lattice with an atom at each corner and an atom at the center of each face.

Which crystal structure is common in titanium and zirconium?

Hexagonal close-packed (HCP).

Why are polymer-based dental resins usually noncrystalline?

Because of the complexity and length of polymer chains.

What is a supercooled liquid?

A solid where atoms are arranged in nonrepeating units, typical of glass.

Do noncrystalline solids have a definite melting temperature?

No, they gradually soften as the temperature is raised.

Below the glass-transition temperature ($T_g$), how does a material behave?

It loses fluid characteristics and gains resistance to shear deformation.

What is the purpose of crystalline inclusions (fillers) in glassy matrices?

To provide color, opacity, increased thermal expansion, and increased radiopacity.

What is diffusion in solids?

The movement of atoms or molecules through a material to reach equilibrium.

What are vacancies in a solid lattice?

Finite number of missing atoms formed during solidification that provide pathways for diffusion.