**3. Solid phase processes of self-organization of substance at nanolevel**

In the chemical reduction of palladium from a solution at an early stage of nucleation, metal ions are reduced to a state with zero valency and, when approaching, are combined into atomic clusters to form irreversible nuclei (0.1–1 nm in size) [1]. These nuclei can increase in size due to the addition of other atoms or coagulation among them, forming floccules. To reduce the effect of coagulation and stabilization of nanoparticles, special agents are introduced into solutions that provide a monodisperse state and the required forms of nanoparticles [5, 6].

Thus, metal particles (dispersed phase) are released from the solution (dispersion medium), which are suspended in a liquid. The interactions between them lead to clustering (the first stage of substance integration). Clusters of various shapes gradually increase in size and form loose flocs. After liquid removal, the system also remains biphasic: solid particles in the form of floccules with a developed surface and gas (air) filling the spaces between them. The system is energy saturated, and it is in a non-equilibrium state. Metallic materials are dissipative systems capable of energy dissipation. This can be manifested in the activation of morphostructural solid phase transformations.

An analysis of the Debye powder diagrams obtained from the starting powders confirms their ultrafine state. The X-ray diffraction pattern of palladium powder is characterized by broadening of symmetric diffraction peaks for all crystallographic directions, with an increase in the degree of blurriness and intensity with an increase in the diffraction angle of reflection (**Figure 1**). Doublet (422) is not split. The diffraction broadening of reflections caused by a decrease in coherent dissipation blocks begins at crystallite sizes less than 100 nm. In the area of small angles, a wide halo is observed, which is characteristic of X-ray amorphous phases, due to the presence of a significant number of particles with sizes less than 10 nm. To study the degree of broadening of diffraction reflections, we used a method based on extracting from a graphical representation of the dependence of the intensity on the wavelength of a linear optical spectrum obtained by scanning X-ray diffraction patterns and then storing it in the form of a full-profile bi-dimensional description. It was found that the diffraction peaks have a symmetrical shape and the broadening is proportional to the tangent of the diffraction reflection angle, which indicates the dispersion of the powder under study. The lines with Miller indices (*hkl*) *h* = *k* are approximated by Gaussian curves, which also indicates a high dispersion. An analysis of the degree of broadening of the diffraction reflections of the X-ray diffraction pattern of the initial palladium powder indicates that according to the size criterion of the composing particles, the powder can be divided into two dominant fractions of 30–60 nm and less than 10 nm.

Electron microscopic studies show that palladium particles are a quasiamorphous substance in which isolated particles of two scale levels are identified: 5–20 nm and 40–150 nm (**Figure 2**). Among the first group, rounded particles with

#### **Figure 1.**

*X-ray diffraction patterns of palladium powder (T = 293 K)—initial state (a, b1) and state after 6 (b2), 12 (b3), 18 (b4) and 24 (b5) months. Changes in the preferred orientations of crystallites in palladium grains at different stages of conversion (state after 6 (c1), 12 (c2), 24 (c3) months).*

fuzzy boundaries of 5.5–7 nm in size predominate; those larger than 15–20 nm are observed occasionally. The second group is mainly represented by particles of various configurations formed as a result of aggregation of particles of the first group or having a quasi-amorphous structure without isolation of individual elements with a predominant size of ~60 nm (**Figure 2**). At this stage, differentiation is already observed in the total mass of particles with the individualization of elements of a denser structure.

**107**

**Figure 2.**

*of multilayer plates.*

becomes indistinguishable.

*Solid Phase Evolution of Nanodispersed Palladium Powders*

The system is an ultrafine mixture of metal particles and gas (air) filling the spaces between them. The morphostructural organization of metal nanoparticles formed during synthesis is represented by clusters of various configurations (up to 2 nm) and floccules (5–20 nm). The floccule structure is locally heterogeneous with a pronounced gradient of density decrease from the centre to the periphery. In the solid phase, the integration process continues. Floccules have a developed contact surface; smaller clusters, forming contact with them, significantly increase floccules in size, due to the expansion of a structurally loose boundary. A small amount of substance covers a large amount of space. Floccules with many interpenetrating and point contacts coagulate and form aggregates. The system begins the processes of self-organization and structuring. Floccule nuclei become denser, their flocculent boundary outgrowths and smaller free clusters grow together to form a quasi-amorphous cement (**Figure 3**). Later, the floccules are acquire a spherical shape. The substructure of the cementitious substance

*The structural organization of palladium powders (T = 293 K), which transforms over time: separation into grains, their gradual compaction and the formation of flakes, their imposition on each other, and the formation* 

The process of interaction of nanoparticles with an active unfolded surface, which provides multiple contacts, contributes to the development of their collective effects aimed at self-organization and structuring of the system. As a result,

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 2.**

*Synthesis Methods and Crystallization*

**106**

**Figure 1.**

denser structure.

fuzzy boundaries of 5.5–7 nm in size predominate; those larger than 15–20 nm are observed occasionally. The second group is mainly represented by particles of various configurations formed as a result of aggregation of particles of the first group or having a quasi-amorphous structure without isolation of individual elements with a predominant size of ~60 nm (**Figure 2**). At this stage, differentiation is already observed in the total mass of particles with the individualization of elements of a

*X-ray diffraction patterns of palladium powder (T = 293 K)—initial state (a, b1) and state after 6 (b2), 12 (b3), 18 (b4) and 24 (b5) months. Changes in the preferred orientations of crystallites in palladium grains* 

*at different stages of conversion (state after 6 (c1), 12 (c2), 24 (c3) months).*

*The structural organization of palladium powders (T = 293 K), which transforms over time: separation into grains, their gradual compaction and the formation of flakes, their imposition on each other, and the formation of multilayer plates.*

The system is an ultrafine mixture of metal particles and gas (air) filling the spaces between them. The morphostructural organization of metal nanoparticles formed during synthesis is represented by clusters of various configurations (up to 2 nm) and floccules (5–20 nm). The floccule structure is locally heterogeneous with a pronounced gradient of density decrease from the centre to the periphery. In the solid phase, the integration process continues. Floccules have a developed contact surface; smaller clusters, forming contact with them, significantly increase floccules in size, due to the expansion of a structurally loose boundary. A small amount of substance covers a large amount of space. Floccules with many interpenetrating and point contacts coagulate and form aggregates. The system begins the processes of self-organization and structuring. Floccule nuclei become denser, their flocculent boundary outgrowths and smaller free clusters grow together to form a quasi-amorphous cement (**Figure 3**). Later, the floccules are acquire a spherical shape. The substructure of the cementitious substance becomes indistinguishable.

The process of interaction of nanoparticles with an active unfolded surface, which provides multiple contacts, contributes to the development of their collective effects aimed at self-organization and structuring of the system. As a result,

**Figure 3.**

*Evolutionary scheme of transformation of a solid phase system at the nanolevel: floccule coagulation → compaction with structuring elements → formation of structural elements (flakes).*

new structural units form in the form of plates, flakes of various configurations from the total mass of loose flaky substance. Due to the compaction of the substance, their edges are often curled up, and the plates are separated (**Figure 2**). The substructure of the flakes in the form of stacking spheres at this stage remains clearly distinguishable. Observation of the dynamics of the system showed that at room temperature, the process of separation of particles of the second scale level is gradually activated, and larger fragments up to 200 nm are also individuated. They acquire clear boundaries as they condense. Their structure is composed of denser spheres cemented by a quasi-amorphous loose substance.

Developing over time, the processes of structural self-organization evolve to the consequent scale level, where the main structural units are no longer floccules and clusters but nanosized lamellar formations (~50–180 × 5–15 nm) which are formed from them. The process develops according to the planar contact deposition of nanoplates (flakes). This determines the morphostructural specificity of this stage of transformation. The scales continue to condense, the relief is gradually smoothed out and they become more closely adjacent to each other (**Figure 2**). As a result, a multilayer thin surface structure is formed of contact superimposed flakes, the substructure of which is still quite pronounced. Since the density of the substance at this stage of conversion is still quite low, the processes of compaction and structuring are decisive.

Thus, in the area of nanolevel scales, the solid phase evolution of palladium powders proceeds at three meso-levels of structural organization. The first meso-level is characterized by the integration of substance with the formation of clusters up to 2 nm in size (point, linear, volumetric of various configurations, the substructure is filamentous, flocculent) and floccules—segregations with a relatively dense core and indefinite flaky boundaries (5–20 nm). The second is the integration of floccules and smaller clusters, densification and structuring with the formation of nanoplates (flakes) (~50–180 × 5–15 nm). The third is the integration of flakes ("tiling" of flakes), the formation of thin multilayer submicroscopic plates with a movable easily transformable substructure. All the transformations described above took place in a thin layer of powder placed on a carbon tape substrate which is put on an observation table in an electron microscope for 6 months of observation.

A study of palladium powders in a sealed ampoule for 6 months at 293 ± 5 K showed that larger formations appeared in them in the form of branched micrograins, which are intergrowths of subgrains of rounded elongated shapes (**Figure 4**). The microstructure of subgrains is composed of nanosized polydisperse

**109**

*Solid Phase Evolution of Nanodispersed Palladium Powders*

particles of varying degrees of compaction, on the surface of the subgrains without their mutual ordering and at the boundaries of intergrowth with other subgrains with polygonization elements (**Figure 4**). The particles are identical in their morphostructural features to the above-described particles which are formed in a thin

*Structural organization of palladium micrograins formed from powder after 6 months at 293 ± 5 K and their* 

layer of powder deposited on a conductive carbon tape.

powders kept at a temperature of 293 ± 5 K (T < *ΘD* Pd).

In this case, discontinuous discontinuities are often observed.

hollow structure (**Figure 6**).

**Figure 4.**

*surfaces.*

**4. Low-temperature solid phase self-organization processes**

In finely dispersed palladium powders aged at negative temperatures, the integration processes take place at substantially higher rates than at a temperature of 293 ± 5 K. After 6 months of exposure to nanodispersed powders at a temperature of 258 ± 1 K, the occurrence of micrograins with sizes up to 250 μm is observed (**Figure 5**). The grains have a branched spongy structure, in the form of intergrowth of subgrains of rounded elongated shape with a diameter of 1–3 μm. The ratio of the volumes of voids and solids in the grains is comparable to each other. Many subgrains have end-to-end surface discontinuities, which indicate their

Studies of the structural organization of powder particles and palladium micrograins maintained at a temperature of 293 ± 5 K (T < *ΘD* Pd) showed that the main processes here also include coagulation, densification, the formation of thin flakes and their aggregation with the creation of larger forms of thin multilayer plates, but in structural organization, they advanced significantly further than

The surface of the plates (flakes) is more compacted, smoothed, with welldefined boundaries. Thin multilayer structural units gradually increase their area and, having reached micron sizes, begin to bend and twist, forming hollow, rounded, oval or tubular shapes (**Figures 6–8**). The multilayer structural organization of the plates ("tile laying" of the flakes) that form the walls of the hollow subgrains during bending provides them mobility due to the displacement of the layers relative to each other (**Figure 7**). With the increasing surface curvature, the upper plates move apart, revealing the inner layers with a quasi-amorphous surface.

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 4.**

*Synthesis Methods and Crystallization*

new structural units form in the form of plates, flakes of various configurations from the total mass of loose flaky substance. Due to the compaction of the substance, their edges are often curled up, and the plates are separated (**Figure 2**). The substructure of the flakes in the form of stacking spheres at this stage remains clearly distinguishable. Observation of the dynamics of the system showed that at room temperature, the process of separation of particles of the second scale level is gradually activated, and larger fragments up to 200 nm are also individuated. They acquire clear boundaries as they condense. Their structure is composed of denser

*Evolutionary scheme of transformation of a solid phase system at the nanolevel: floccule coagulation →* 

*compaction with structuring elements → formation of structural elements (flakes).*

Developing over time, the processes of structural self-organization evolve to the consequent scale level, where the main structural units are no longer floccules and clusters but nanosized lamellar formations (~50–180 × 5–15 nm) which are formed from them. The process develops according to the planar contact deposition of nanoplates (flakes). This determines the morphostructural specificity of this stage of transformation. The scales continue to condense, the relief is gradually smoothed out and they become more closely adjacent to each other (**Figure 2**). As a result, a multilayer thin surface structure is formed of contact superimposed flakes, the substructure of which is still quite pronounced. Since the density of the substance at this stage of conversion is still quite low, the processes of compaction and structur-

Thus, in the area of nanolevel scales, the solid phase evolution of palladium powders proceeds at three meso-levels of structural organization. The first meso-level is characterized by the integration of substance with the formation of clusters up to 2 nm in size (point, linear, volumetric of various configurations, the substructure is filamentous, flocculent) and floccules—segregations with a relatively dense core and indefinite flaky boundaries (5–20 nm). The second is the integration of floccules and smaller clusters, densification and structuring with the formation of nanoplates (flakes) (~50–180 × 5–15 nm). The third is the integration of flakes ("tiling" of flakes), the formation of thin multilayer submicroscopic plates with a movable easily transformable substructure. All the transformations described above took place in a thin layer of powder placed on a carbon tape substrate which is put on an observation table in an electron microscope for 6 months

A study of palladium powders in a sealed ampoule for 6 months at 293 ± 5 K showed that larger formations appeared in them in the form of branched micrograins, which are intergrowths of subgrains of rounded elongated shapes (**Figure 4**). The microstructure of subgrains is composed of nanosized polydisperse

spheres cemented by a quasi-amorphous loose substance.

**108**

ing are decisive.

**Figure 3.**

of observation.

*Structural organization of palladium micrograins formed from powder after 6 months at 293 ± 5 K and their surfaces.*

particles of varying degrees of compaction, on the surface of the subgrains without their mutual ordering and at the boundaries of intergrowth with other subgrains with polygonization elements (**Figure 4**). The particles are identical in their morphostructural features to the above-described particles which are formed in a thin layer of powder deposited on a conductive carbon tape.

#### **4. Low-temperature solid phase self-organization processes**

In finely dispersed palladium powders aged at negative temperatures, the integration processes take place at substantially higher rates than at a temperature of 293 ± 5 K. After 6 months of exposure to nanodispersed powders at a temperature of 258 ± 1 K, the occurrence of micrograins with sizes up to 250 μm is observed (**Figure 5**). The grains have a branched spongy structure, in the form of intergrowth of subgrains of rounded elongated shape with a diameter of 1–3 μm. The ratio of the volumes of voids and solids in the grains is comparable to each other. Many subgrains have end-to-end surface discontinuities, which indicate their hollow structure (**Figure 6**).

Studies of the structural organization of powder particles and palladium micrograins maintained at a temperature of 293 ± 5 K (T < *ΘD* Pd) showed that the main processes here also include coagulation, densification, the formation of thin flakes and their aggregation with the creation of larger forms of thin multilayer plates, but in structural organization, they advanced significantly further than powders kept at a temperature of 293 ± 5 K (T < *ΘD* Pd).

The surface of the plates (flakes) is more compacted, smoothed, with welldefined boundaries. Thin multilayer structural units gradually increase their area and, having reached micron sizes, begin to bend and twist, forming hollow, rounded, oval or tubular shapes (**Figures 6–8**). The multilayer structural organization of the plates ("tile laying" of the flakes) that form the walls of the hollow subgrains during bending provides them mobility due to the displacement of the layers relative to each other (**Figure 7**). With the increasing surface curvature, the upper plates move apart, revealing the inner layers with a quasi-amorphous surface. In this case, discontinuous discontinuities are often observed.

#### **Figure 5.**

*Transformations of palladium micrograins at a temperature of 258 ± 1 K: sponge aggregate formed from nanopowder (after 6 months) (a), grain with a dense structureless surface and sponge internal structure (after 2 years) (b), X-ray diffraction patterns of palladium grain (after 6 (c), 12 (d), 24 months (e) at 258 K) and changes in the preferred orientations of crystallites in palladium grains at different stages of conversion (state after 6 (f1), 12 (f2), 24 (f3) months).*

As a result of the processes of coagulation and compaction of the substance and the collective actions of nanoparticles of several scale levels, thin-walled hollow formations of rounded, oval, tubular shapes with a diameter of about 1–3 μm

**111**

**Figure 6.**

**Figure 7.**

*258 ± 1 K (after 6 months).*

of the multilayer wall is 30–60 nm.

*multilayer lamellar formations during the formation of hollow forms.*

*Solid Phase Evolution of Nanodispersed Palladium Powders*

are formed (**Figure 8**). These formations are structural units of the microscopic organization of matter. The structure formed in this way has an unfolded inner and outer surface. The diameter of the internal channels is close to 1 μm. The thickness

*Structural organization of multilayer planar intergrowth of plates (flakes). Destruction of the surface of* 

*Structural organization: sponge aggregate of palladium formed from nanopowder at a temperature of* 

unification and expansion of the thin channel internal structure.

By approaching each other, hollow micrograins grow together according either contact or bridge mechanism (**Figures 9** and **10**). The first mechanism is due to direct contact of curved surfaces (**Figure 9**). When the curved surfaces of two thin layers come into mutual contact, the layer thickness in the contact zone doubles. The scales composing the walls of the hollow grains come into motion due to the collective effects of the interaction. Mass transfer is directed away from the contact area (**Figure 9**). The outflow of particles causes opening of the cavity, the internal channels are combined and the wall thickness is levelled. The intergrowth process is not a sintering process with an increase in thickness in the contact area, but the

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 6.**

*Synthesis Methods and Crystallization*

**110**

**Figure 5.**

*after 6 (f1), 12 (f2), 24 (f3) months).*

As a result of the processes of coagulation and compaction of the substance and the collective actions of nanoparticles of several scale levels, thin-walled hollow formations of rounded, oval, tubular shapes with a diameter of about 1–3 μm

*Transformations of palladium micrograins at a temperature of 258 ± 1 K: sponge aggregate formed from nanopowder (after 6 months) (a), grain with a dense structureless surface and sponge internal structure (after 2 years) (b), X-ray diffraction patterns of palladium grain (after 6 (c), 12 (d), 24 months (e) at 258 K) and changes in the preferred orientations of crystallites in palladium grains at different stages of conversion (state* 

*Structural organization: sponge aggregate of palladium formed from nanopowder at a temperature of 258 ± 1 K (after 6 months).*

#### **Figure 7.**

*Structural organization of multilayer planar intergrowth of plates (flakes). Destruction of the surface of multilayer lamellar formations during the formation of hollow forms.*

are formed (**Figure 8**). These formations are structural units of the microscopic organization of matter. The structure formed in this way has an unfolded inner and outer surface. The diameter of the internal channels is close to 1 μm. The thickness of the multilayer wall is 30–60 nm.

By approaching each other, hollow micrograins grow together according either contact or bridge mechanism (**Figures 9** and **10**). The first mechanism is due to direct contact of curved surfaces (**Figure 9**). When the curved surfaces of two thin layers come into mutual contact, the layer thickness in the contact zone doubles. The scales composing the walls of the hollow grains come into motion due to the collective effects of the interaction. Mass transfer is directed away from the contact area (**Figure 9**). The outflow of particles causes opening of the cavity, the internal channels are combined and the wall thickness is levelled. The intergrowth process is not a sintering process with an increase in thickness in the contact area, but the unification and expansion of the thin channel internal structure.

**Figure 8.** *Spherical and oval palladium micrograins formed as a result of aggregation of nanoparticles.*

#### **Figure 9.**

*Models of the contact mechanism of intergrowth of hollow masses. Contact interactions. Contact expansion and fusion of hollow structures.*

The second aggregation mechanism, aimed at expanding the thin channel system, ensures the intergrowth of hollow thin-walled structures that do not contact each other. This mechanism is due to the effects of long-range bonds. When the curved surfaces of thin-walled (~30–60 nm) hollow formations are located at a distance comparable to the diameter of the internal channels (~1 μm) between the dispersed particles from which they are composed, collective effects aimed at combining the surface also appear. Collective actions of particles are manifested in mass transfer to the area of maximum approximation of surfaces (**Figure 10**). In this zone, a thin "bridge" begins to form, either in the form of a whisker-like outgrowth or a twisting effect forming an outgrowth cone that extends to contact with each other. After the formation of the bridge, mass transfer is activated. The contact bridge expands, and the transverse size becomes comparable with the size of the channels. The rupture structures and partings between the plates and scales are

**113**

further transformation.

*Solid Phase Evolution of Nanodispersed Palladium Powders*

smoothed, forming concentric bands similar to growth zones. A single expanded

*The "bridge" mechanism of intergrowth of palladium micrograins, the model of intergrowth by the "bridge"* 

Thus, the palladium grains formed at this stage are a two-phase mixture consisting of a solid phase (Pd) and two types of voids (air). The first type is a void encapsulated in the internal cavities of micrograins and complex branched channels (~1 μm) formed when they merge. The second type is a spongy volumetric structure formed during the intergrowth of hollow microparticles. A void is almost

The formed system has a developed inner and outer surface; it is energetically saturated and metastable. The structural configuration of the dissipative system involves its further transformation. The activation of interactions between hollow subgrains is characterized by two processes occurring simultaneously: the formation of new contacts and the destruction of previously formed ones. The constant movement of matter associated with the formation of new contacts often results in rupture of the surface and opening of channel structures. Viscous deformation discontinuities, shears and disruptions without visible deformations of the accretion boundaries are observed (**Figure 11**). Microcracks appear along the boundaries of intergrowth, new contacts form and open-channel cavities are closed. The system is

The processes of structural self-organization of particles in thin-walled bodies quickly form geometrically regular shapes (negative crystals) on the inner side of the channel wall, the outer surface is a multilayer packing of flakes and sometimes ordering effects are also observed in their arrangement, with the formation of face shapes (**Figure 12**). Particles whose structural organization is constructed in this way (tubes, spheres whose walls consist of nanostructured particles in a quasiamorphous phase) are energetically saturated and have a high degree of freedom for

Opening of internal channels causes collective effects in the system aimed at closing them. The process is due to transformations of the substance in the area of the open hole and depends on the mechanism of rupture. If a rupture occurred due to tensile forces, leading to the formation of a tension zone and a thin isthmus at the time of the rupture, the boundaries of the rupture have thin ragged edges, with a significant number of nanosized separation protrusions. Such discontinuities are

transformed in a mobile way, reducing the volume of second type voids.

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

channel system is being formed.

**Figure 10.**

evenly distributed over the grain volume.

*mechanism. Formation of a contact "bridge". Expansion of the isthmus.*

*Solid Phase Evolution of Nanodispersed Palladium Powders DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 10.**

*Synthesis Methods and Crystallization*

**112**

**Figure 9.**

**Figure 8.**

*and fusion of hollow structures.*

The second aggregation mechanism, aimed at expanding the thin channel system, ensures the intergrowth of hollow thin-walled structures that do not contact each other. This mechanism is due to the effects of long-range bonds. When the curved surfaces of thin-walled (~30–60 nm) hollow formations are located at a distance comparable to the diameter of the internal channels (~1 μm) between the dispersed particles from which they are composed, collective effects aimed at combining the surface also appear. Collective actions of particles are manifested in mass transfer to the area of maximum approximation of surfaces (**Figure 10**). In this zone, a thin "bridge" begins to form, either in the form of a whisker-like outgrowth or a twisting effect forming an outgrowth cone that extends to contact with each other. After the formation of the bridge, mass transfer is activated. The contact bridge expands, and the transverse size becomes comparable with the size of the channels. The rupture structures and partings between the plates and scales are

*Models of the contact mechanism of intergrowth of hollow masses. Contact interactions. Contact expansion* 

*Spherical and oval palladium micrograins formed as a result of aggregation of nanoparticles.*

*The "bridge" mechanism of intergrowth of palladium micrograins, the model of intergrowth by the "bridge" mechanism. Formation of a contact "bridge". Expansion of the isthmus.*

smoothed, forming concentric bands similar to growth zones. A single expanded channel system is being formed.

Thus, the palladium grains formed at this stage are a two-phase mixture consisting of a solid phase (Pd) and two types of voids (air). The first type is a void encapsulated in the internal cavities of micrograins and complex branched channels (~1 μm) formed when they merge. The second type is a spongy volumetric structure formed during the intergrowth of hollow microparticles. A void is almost evenly distributed over the grain volume.

The formed system has a developed inner and outer surface; it is energetically saturated and metastable. The structural configuration of the dissipative system involves its further transformation. The activation of interactions between hollow subgrains is characterized by two processes occurring simultaneously: the formation of new contacts and the destruction of previously formed ones. The constant movement of matter associated with the formation of new contacts often results in rupture of the surface and opening of channel structures. Viscous deformation discontinuities, shears and disruptions without visible deformations of the accretion boundaries are observed (**Figure 11**). Microcracks appear along the boundaries of intergrowth, new contacts form and open-channel cavities are closed. The system is transformed in a mobile way, reducing the volume of second type voids.

The processes of structural self-organization of particles in thin-walled bodies quickly form geometrically regular shapes (negative crystals) on the inner side of the channel wall, the outer surface is a multilayer packing of flakes and sometimes ordering effects are also observed in their arrangement, with the formation of face shapes (**Figure 12**). Particles whose structural organization is constructed in this way (tubes, spheres whose walls consist of nanostructured particles in a quasiamorphous phase) are energetically saturated and have a high degree of freedom for further transformation.

Opening of internal channels causes collective effects in the system aimed at closing them. The process is due to transformations of the substance in the area of the open hole and depends on the mechanism of rupture. If a rupture occurred due to tensile forces, leading to the formation of a tension zone and a thin isthmus at the time of the rupture, the boundaries of the rupture have thin ragged edges, with a significant number of nanosized separation protrusions. Such discontinuities are

**Figure 11.** *Various types of strain breaks in palladium grains: after 6 and 12 months at 258 ± 1 K.*

closed as a result of the convergence of the walls and smoothing of the displacement formed during the rupture (**Figure 13**). When the size of the holes formed, for example, as a result of shear forces, corresponds to the diameter of the channel, their closure occurs mainly due to the effects of quasi-viscous flow (**Figure 13**). Incrusted structures grow, gradually closing the hole. This type of closure is most common in the studied system. The convergence of the walls in the rupture

**115**

*Solid Phase Evolution of Nanodispersed Palladium Powders*

boundary area is also observed; the outer diameter of the hole decreases until contact interactions between the particles of the surface layer are established, causing the processes of restoring the integrity of the outer layer and the formation of a

*Recrystallization of the inner wall of hollow channel structures in palladium grains and elements of ordering* 

The development of the compaction process over time leads to the convergence of the boundaries of hollow subgrains. When there are several surfaces of hollow formations at a critical distance at a time, a unified surface is formed forming structural units of the next scale level in the form of intergrowth of rounded bodies with sizes of 3–30 μm, which gradually form curved surfaces preserving the boundaries (**Figure 14**). Their surface also has a scaly lamellar structure with elements of polygonization, and there are residual pores of the second type, which gradually

The main mobile structural units of the surface layer of subgrains are scaly lamellar formations. They do not lose their structural identity for a long time (**Figure 15**). After the merging of the hollow subgrains and the formation of rather extensive zones with denser intergrowth, the process of surface transformation is activated. Initially, the borders and shapes of the scales are structurally separated, and then spicule-like elements appear in the structure of the scales with the ordering effect in their location; the relief contours are softened and smoothed. Then, scales are flattened and boundaries are merged. Consequently, zones with structural corrugated bands are formed (**Figure 16**) which are gradually smoothed out levelling the surface. On the surface, the elements of polygonization are manifested,

In the processes of surface transformation, the key role is played by the size effect. The collective actions of ultrafine particles determine the order of transformations that develop over time. In the initial stages, in spite of the volumetric filling of space with particles, ordering has a planar distribution. Consequently, the transformation process goes to the sub-grain level, and then it is activated in the contact zones. With the coalescence of micrograins, the intergrain zones are also involved in the collective processes of surface transformation, the boundaries gradually disappear, and the structural organization of the surface becomes unified (**Figure 16**). At later stages, the surface topography is smoothed and acquires soft forms, and then banded and other forms with a relatively smooth relief appear. A structure identical

with the development of the process at the neighbouring subgrains.

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

common shell (**Figure 13**).

decrease in size.

**Figure 12.**

*of the outer layer.*

#### **Figure 12.**

*Synthesis Methods and Crystallization*

**114**

**Figure 11.**

closed as a result of the convergence of the walls and smoothing of the displacement formed during the rupture (**Figure 13**). When the size of the holes formed, for example, as a result of shear forces, corresponds to the diameter of the channel, their closure occurs mainly due to the effects of quasi-viscous flow (**Figure 13**). Incrusted structures grow, gradually closing the hole. This type of closure is most common in the studied system. The convergence of the walls in the rupture

*Various types of strain breaks in palladium grains: after 6 and 12 months at 258 ± 1 K.*

*Recrystallization of the inner wall of hollow channel structures in palladium grains and elements of ordering of the outer layer.*

boundary area is also observed; the outer diameter of the hole decreases until contact interactions between the particles of the surface layer are established, causing the processes of restoring the integrity of the outer layer and the formation of a common shell (**Figure 13**).

The development of the compaction process over time leads to the convergence of the boundaries of hollow subgrains. When there are several surfaces of hollow formations at a critical distance at a time, a unified surface is formed forming structural units of the next scale level in the form of intergrowth of rounded bodies with sizes of 3–30 μm, which gradually form curved surfaces preserving the boundaries (**Figure 14**). Their surface also has a scaly lamellar structure with elements of polygonization, and there are residual pores of the second type, which gradually decrease in size.

The main mobile structural units of the surface layer of subgrains are scaly lamellar formations. They do not lose their structural identity for a long time (**Figure 15**). After the merging of the hollow subgrains and the formation of rather extensive zones with denser intergrowth, the process of surface transformation is activated. Initially, the borders and shapes of the scales are structurally separated, and then spicule-like elements appear in the structure of the scales with the ordering effect in their location; the relief contours are softened and smoothed. Then, scales are flattened and boundaries are merged. Consequently, zones with structural corrugated bands are formed (**Figure 16**) which are gradually smoothed out levelling the surface. On the surface, the elements of polygonization are manifested, with the development of the process at the neighbouring subgrains.

In the processes of surface transformation, the key role is played by the size effect. The collective actions of ultrafine particles determine the order of transformations that develop over time. In the initial stages, in spite of the volumetric filling of space with particles, ordering has a planar distribution. Consequently, the transformation process goes to the sub-grain level, and then it is activated in the contact zones. With the coalescence of micrograins, the intergrain zones are also involved in the collective processes of surface transformation, the boundaries gradually disappear, and the structural organization of the surface becomes unified (**Figure 16**). At later stages, the surface topography is smoothed and acquires soft forms, and then banded and other forms with a relatively smooth relief appear. A structure identical

**Figure 13.**

*Closures of discontinuities in channel structures in palladium grains: walls convergence and displacement smoothing, quasi-viscous flow effect and recrystallization.*

**117**

**Figure 16.**

**Figure 15.**

contact zones acquire smoothed shapes.

*Solid Phase Evolution of Nanodispersed Palladium Powders*

to the structures of the growth planes is revealed (**Figure 17**). A relatively stable state is organized in which slow processes occur, aimed at ordering between the particles of a thin layer—the shell. Subgrains lose their structural identity; former

*Extension of the area of smoothing of the surface and with the ordering of structural elements.*

*Transformation of the surface shell of hollow grains, quasi-viscous flow effect and recrystallization.*

The next important stage in the structural transformation of palladium micrograins is associated with the creation of a unified grain shell as a whole.

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

**Figure 14.** *Different densities of micrograin intergrowths and stages of surface transformations.*

*Solid Phase Evolution of Nanodispersed Palladium Powders DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 15.**

*Synthesis Methods and Crystallization*

**116**

**Figure 14.**

**Figure 13.**

*Closures of discontinuities in channel structures in palladium grains: walls convergence and displacement* 

*Different densities of micrograin intergrowths and stages of surface transformations.*

*smoothing, quasi-viscous flow effect and recrystallization.*

*Transformation of the surface shell of hollow grains, quasi-viscous flow effect and recrystallization.*

**Figure 16.** *Extension of the area of smoothing of the surface and with the ordering of structural elements.*

to the structures of the growth planes is revealed (**Figure 17**). A relatively stable state is organized in which slow processes occur, aimed at ordering between the particles of a thin layer—the shell. Subgrains lose their structural identity; former contact zones acquire smoothed shapes.

The next important stage in the structural transformation of palladium micrograins is associated with the creation of a unified grain shell as a whole.

**Figure 17.** *Ordering within individual grains with the occurrence of growth planes and twins.*

After the formation of a micrograin (50–300 μm) of a spongy structure, the compaction processes contribute to its transformation as a whole, separating it from the rest of the fine powder. Since the aggregation and occurrence of grains have a progressive character with the simultaneous presence of both a finely dispersed phase and already formed grains in the system, particles of various sizes and degrees of compaction were found at various stages of the study. Transformation processes are aimed at the individualization of each individual grain, the creation of a common shell (the microencapsulation effect). When the displacement of voids of the second level is activated, resulting in compaction of the substance, the most active merging of particles occurs on the grain surface. A grain, as a whole object, tends to acquire a single, generalized surface. The individuality of the substructural components is first smoothed out due to effects similar to a viscous-flowing state, and then sod-deformed, smoothed, worn-out forms are formed, gradually closing the internal porous channel structure (**Figure 18**). The organized outer shell loses smoothness, becomes fine-grained and is worn smooth. The microencapsulation effect is aimed at organizing a common surface on which one can only detect relics of a structural organization that has undergone significant evolutionary transformations. The surface acquires a structure that is described as rounded with deformation elements (**Figure 18**). However, despite the presence of an external dense shell, the internal organization of the grain remains nanoporous.

Stabilization of the temperature regime at temperatures below the Debye palladium characteristic temperature favours the occurrence of preferred orientations of crystallites in palladium grains (**Figures 1**, **5** and **19**). If the spongy grains, whose crystallites do not exceed 1 μm, are kept at room temperature, no increase in crystallite size will be recorded. Radiographs are characterized by sharp lines (**Figure 19a**), and the intensity distribution is identical to the theoretical radiograph. The X-ray structural characteristics of palladium grains maintained at various temperature conditions are presented in **Table 1**.

The unit cell parameters for palladium grains transforming at room temperatures are approximately 3.882 Å. This value is reduced relative to the unit cell parameters of native palladium, which indicates the incompleteness of the transformations in the system of its metastability. And for samples aged under freezing temperatures, the parameter values are as close as possible to the values for native palladium. For negative temperatures close to the Debye characteristic

**119**

**Figure 19.**

**Figure 18.**

*Grain encapsulation and surface transformation.*

temperature of palladium, the ordering process is accelerated, and the crystallite sizes increase. It is likely that further transformation processes will lead to a single crystal state. However, the presence of internal channels/voids leaves the system

*Geological Institute of the Far Eastern Branch of the Russian Academy of Sciences).*

*Radiographs characterizing the changes in the preferred orientations of crystallites in palladium grains, aged at 293 ± 5 K (a), at 258 ± 5 K (b) and at 77 K (c) (shooting at the Collective Use Centre of the Far Eastern* 

*Solid Phase Evolution of Nanodispersed Palladium Powders*

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

**Figure 18.**

*Synthesis Methods and Crystallization*

**Figure 17.**

After the formation of a micrograin (50–300 μm) of a spongy structure, the

*Ordering within individual grains with the occurrence of growth planes and twins.*

shell, the internal organization of the grain remains nanoporous.

conditions are presented in **Table 1**.

Stabilization of the temperature regime at temperatures below the Debye palladium characteristic temperature favours the occurrence of preferred orientations of crystallites in palladium grains (**Figures 1**, **5** and **19**). If the spongy grains, whose crystallites do not exceed 1 μm, are kept at room temperature, no increase in crystallite size will be recorded. Radiographs are characterized by sharp lines (**Figure 19a**), and the intensity distribution is identical to the theoretical radiograph. The X-ray structural characteristics of palladium grains maintained at various temperature

The unit cell parameters for palladium grains transforming at room temperatures are approximately 3.882 Å. This value is reduced relative to the unit cell parameters of native palladium, which indicates the incompleteness of the transformations in the system of its metastability. And for samples aged under freezing temperatures, the parameter values are as close as possible to the values for native palladium. For negative temperatures close to the Debye characteristic

compaction processes contribute to its transformation as a whole, separating it from the rest of the fine powder. Since the aggregation and occurrence of grains have a progressive character with the simultaneous presence of both a finely dispersed phase and already formed grains in the system, particles of various sizes and degrees of compaction were found at various stages of the study. Transformation processes are aimed at the individualization of each individual grain, the creation of a common shell (the microencapsulation effect). When the displacement of voids of the second level is activated, resulting in compaction of the substance, the most active merging of particles occurs on the grain surface. A grain, as a whole object, tends to acquire a single, generalized surface. The individuality of the substructural components is first smoothed out due to effects similar to a viscous-flowing state, and then sod-deformed, smoothed, worn-out forms are formed, gradually closing the internal porous channel structure (**Figure 18**). The organized outer shell loses smoothness, becomes fine-grained and is worn smooth. The microencapsulation effect is aimed at organizing a common surface on which one can only detect relics of a structural organization that has undergone significant evolutionary transformations. The surface acquires a structure that is described as rounded with deformation elements (**Figure 18**). However, despite the presence of an external dense

**118**

*Grain encapsulation and surface transformation.*

#### **Figure 19.**

*Radiographs characterizing the changes in the preferred orientations of crystallites in palladium grains, aged at 293 ± 5 K (a), at 258 ± 5 K (b) and at 77 K (c) (shooting at the Collective Use Centre of the Far Eastern Geological Institute of the Far Eastern Branch of the Russian Academy of Sciences).*

temperature of palladium, the ordering process is accelerated, and the crystallite sizes increase. It is likely that further transformation processes will lead to a single crystal state. However, the presence of internal channels/voids leaves the system


*Mn\*\*, reliability criteria after de Wolf.*

#### **Table 1.**

*X-ray structural characteristics of palladium grains aged in various temperature conditions.*

open. A significant decrease in temperature (77 K) promotes rapid encapsulation (**Figure 20**), the formation of a dense shell, in which ordering occurs only at the micro level. That is, the processes of separation of each individual grain in this state are predominant over the processes of general ordering.

Thus, studies of interactions between ultrafine particles of palladium in free contact showed that their aggregation leads to the formation of thin channel structures with a channel diameter of ~1 μm and a wall thickness of ~30–100 nm. The sizes of the coherent dissipation blocks and the wall thickness of the channel structures are identical; their inner surface has flat faces in the form of negative crystals, which indicates the ordering between the particles composing them. The established effect of enlargement of coherent dissipation blocks and the occurrence of preferential orientations are probably associated with ordering in the arrangement of channel structures and their alignment in a certain order. Microencapsulation favours an orderly distribution of voids in the grain volume.

Dissipative processes in dispersed objects contribute to the development of long-range coherence in the system [13]. The determining parameter in this process, in our opinion, is the ratio of the sample holding temperature to the Debye characteristic temperature of palladium *Θ*D = 274 K. The Debye characteristic temperature is the temperature at which crystal lattice vibrations of all possible frequencies are realized. A further increase in temperature does not lead to the occurrence of new vibration modes but only leads to an increase in the amplitudes of existing ones, that is, the average vibration energy increases with the increase in temperature. At temperatures of the system T < *Θ*D, the atoms oscillate near a certain equilibrium position, the intensity and amplitude of which depend on the difference between the Debye characteristic temperature of the substance and the temperature of the system (ΔT = *Θ*<sup>D</sup> − T). According to Debye, with a decrease in temperature in the area T < *Θ*D, the energy of each individual vibrator decreases; moreover, the vibrations of more and more new vibrators gradually decrease.

Thus, under conditions when T < *Θ*D, interatomic interactions become predominant. Long-range bonds are established between the atoms of matter; the collective actions of atoms are aimed at ordering and tending of the system towards the perfection of its crystal structure. The structure of the substance becomes denser.

**121**

substance.

**Figure 20.**

**5. Conclusion**

• Lack of structure

• Prestructuring

• Structuring

○ Small-scale clustering

○ Large-scale aggregation

○ Recrystallization at the subgrain level

○ Recrystallization at grain level

*Solid Phase Evolution of Nanodispersed Palladium Powders*

Further lowering the temperature reduces the amplitude and energy of the natural vibrations of the lattice, thereby contributing to even greater structuring of the

*Cryotemperature transformations of palladium grains with structure compaction at 77 K: 6, 12 and 24 months.*

Thus, studies of interactions between finely dispersed palladium powders that are in free contact under dissipative conditions of low temperatures indicate the group behaviour of particles at various scale levels. Structural aggregation takes place in stages, with the allocation of an aggregation field area at each scale level:

○ Medium-scale aggregation and divergence of cluster types in the aggregate

In the collective effects of self-organization, periodic activation of mass transfer is observed, in which nanoparticles of various mesoscale structure organization are involved. The solid phase finely dispersed substance evolves mobile in the process

○ Aggregate formation and shell formation (microencapsulation)

*DOI: http://dx.doi.org/10.5772/intechopen.91822*

*Solid Phase Evolution of Nanodispersed Palladium Powders DOI: http://dx.doi.org/10.5772/intechopen.91822*

#### **Figure 20.**

*Synthesis Methods and Crystallization*

*Mn\*\*, reliability criteria after de Wolf.*

Pd 293 ± 5 K

Pd 258 ± 1 K

Pd 77 K

**Table 1.**

open. A significant decrease in temperature (77 K) promotes rapid encapsulation (**Figure 20**), the formation of a dense shell, in which ordering occurs only at the micro level. That is, the processes of separation of each individual grain in this state

**Sample For all available reflections In the precision area**

Pd 00-046-1043 3.89019(2) 3.89019(2)

**а ± 0.001 Ǻ V, Ǻ3 ∆d\* Mn\*\* а ± 0.0001 Ǻ ∆d\* Mn\*\***

1 3.882 58.482 0.00276 80.044 3.8831 0.00027 200.688 2 3.882 58.512 0.00351 68.080 3.8839 0.00019 292.893 3 3.881 58.467 0.00244 85.800 3.8825 0.00040 164.194

1 3.887 58.717 0.00135 85.196 3.8871 0.00087 62.679 2 3.890 58.870 0.00163 98.430 3.8911 0.00042 135.758 3 3.890 58.857 0.00112 165.352 3.8906 0.00022 274.520

1 3.890 58.870 0.00096 153.662 3.8909 0.00007 906.799 2 3.889 58.816 0.00072 183.220 3.8894 0.00024 233.847 3 3.888 58.765 0.00125 127.889 3.8885 0.00034 162.065

Thus, studies of interactions between ultrafine particles of palladium in free contact showed that their aggregation leads to the formation of thin channel structures with a channel diameter of ~1 μm and a wall thickness of ~30–100 nm. The sizes of the coherent dissipation blocks and the wall thickness of the channel structures are identical; their inner surface has flat faces in the form of negative crystals, which indicates the ordering between the particles composing them. The established effect of enlargement of coherent dissipation blocks and the occurrence of preferential orientations are probably associated with ordering in the arrangement of channel structures and their alignment in a certain order. Microencapsulation

Dissipative processes in dispersed objects contribute to the development of long-range coherence in the system [13]. The determining parameter in this process, in our opinion, is the ratio of the sample holding temperature to the Debye characteristic temperature of palladium *Θ*D = 274 K. The Debye characteristic temperature is the temperature at which crystal lattice vibrations of all possible frequencies are realized. A further increase in temperature does not lead to the occurrence of new vibration modes but only leads to an increase in the amplitudes of existing ones, that is, the average vibration energy increases with the increase in temperature. At temperatures of the system T < *Θ*D, the atoms oscillate near a certain equilibrium position, the intensity and amplitude of which depend on the difference between the Debye characteristic temperature of the substance and the temperature of the system (ΔT = *Θ*<sup>D</sup> − T). According to Debye, with a decrease in temperature in the area T < *Θ*D, the energy of each individual vibrator decreases; moreover, the vibra-

Thus, under conditions when T < *Θ*D, interatomic interactions become predominant. Long-range bonds are established between the atoms of matter; the collective actions of atoms are aimed at ordering and tending of the system towards the perfection of its crystal structure. The structure of the substance becomes denser.

are predominant over the processes of general ordering.

∆*d\*, average deviation of the calculated interplanar distances off the measured ones;*

*X-ray structural characteristics of palladium grains aged in various temperature conditions.*

favours an orderly distribution of voids in the grain volume.

tions of more and more new vibrators gradually decrease.

**120**

*Cryotemperature transformations of palladium grains with structure compaction at 77 K: 6, 12 and 24 months.*

Further lowering the temperature reduces the amplitude and energy of the natural vibrations of the lattice, thereby contributing to even greater structuring of the substance.
