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Hybrid Materials

Hybrid Materials
Hybrid Materials

1 1

Introduction to Hybrid Materials

Guido Kickelbick

1.1

Introduction

Recent technological breakthroughs and the desire for new functions generate an enormous demand for novel materials. Many of the well-established materials, such as metals, ceramics or plastics cannot ful?ll all technological desires for the various new applications. Scientists and engineers realized early on that mixtures

of materials can show superior properties compared with their pure counterparts. One of the most successful examples is the group of composites which are formed by the incorporation of a basic structural material into a second substance, the https://www.sodocs.net/doc/4c5493194.html,ually the systems incorporated are in the form of particles, whiskers, ?bers, lamellae, or a mesh. Most of the resulting materials show improved mechanical properties and a well-known example is inorganic ?ber-reinforced polymers. Nowadays they are regularly used for lightweight materials with advanced me-chanical properties, for example in the construction of vehicles of all types or sports equipment. The structural building blocks in these materials which are incorporated into the matrix are predominantly inorganic in nature and show a size range from the lower micrometer to the millimeter range and therefore their heterogeneous composition is quite often visible to the eye. Soon it became evident that decreasing the size of the inorganic units to the same level as the organic building blocks could lead to more homogeneous materials that allow a further ?ne tuning of materials’ properties on the molecular and nanoscale level, generating novel materials that either show characteristics in between the two orig-inal phases or even new properties. Both classes of materials reveal similarities and differences and an attempt to de?ne the two classes will follow below. However, we should ?rst realize that the origin of hybrid materials did not take place in a chemical laboratory but in nature.

1.1.1

Natural Origins

Many natural materials consist of inorganic and organic building blocks distrib-uted on the (macro)molecular or nanoscale. In most cases the inorganic part Hybrid Materials. Synthesis, Characterization, and Applications.Edited by Guido Kickelbick

Copyright ? 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31299-3

21Introduction to Hybrid Materials

provides mechanical strength and an overall structure to the natural objects while the organic part delivers bonding between the inorganic building blocks and/or the soft tissue. Typical examples of such materials are bone, or nacre.

The concepts of bonding and structure in such materials are intensively stud-ied by many scientists to understand the fundamental processes of their forma-tion and to transfer the ideas to arti?cial materials in a so-called biomimetic approach. The special circumstances under which biological hybrid inorganic–organic materials are formed, such as ambient temperatures, an aqueous envi-ronment, a neutral pH and the fascinating plethora of complex geometries pro-duced under these conditions make the mimicking of such structures an ultimate goal for scientists. In particular the study of biomineralization and its shape control is an important target of many scienti?c studies. This primarily interface-controlled process still reveals many questions, in particular how such a remarkable level of morphological diversity with a multiplicity of functions can be produced by so few building blocks. In addition to questions concerning the composition of the materials, their unique structures motivate enquiry to get a deeper insight in their formation, often not only because of their beauty but also because of the various functions the structures perform. A complex hierarchical order of construction from the nanometer to the millimeter level is regularly found in nature, where every size level of the speci?c material has its function which bene?ts the whole performance of the material. Furthermore these differ-ent levels of complexity are reached by soft chemical self-assembly mechanisms over a large dimension, which is one of the major challenges of modern mate-rials chemistry.

Chapter 7 describes the fundamental principles of biomineralization and hybrid inorganic–organic biomaterials and many applications to medical problems are shown in Chapter 8.

1.1.2

The Development of Hybrid Materials

Although we do not know the original birth of hybrid materials exactly it is clear that the mixing of organic and inorganic components was carried out in ancient world. At that time the production of bright and colorful paints was the driving force to consistently try novel mixtures of dyes or inorganic pigments and other inorganic and organic components to form paints that were used thousands of years ago. Therefore, hybrid materials or even nanotechnology is not an invention of the last decade but was developed a long time ago. However, it was only at the end of the 20th and the beginning of the 21st century that it was realized by scientists, in particular because of the availability of novel physico–chemical char-acterization methods, the ?eld of nanoscience opened many perspectives for approaches to new materials. The combination of different analytical techniques gives rise to novel insights into hybrid materials and makes it clear that bottom-up strategies from the molecular level towards materials’ design will lead to novel properties in this class of materials.

1.1Introduction3 Apart from the use of inorganic materials as ?llers for organic polymers, such as rubber, it was a long time before much scienti?c activity was devoted to mix-tures of inorganic and organic materials. One process changed this situation: the sol–gel process. This process, which will be discussed in more detail later on, was developed in the 1930s using silicon alkoxides as precursors from which silica was produced. In fact this process is similar to an organic polymerization starting from molecular precursors resulting in a bulk material. Contrary to many other proce-dures used in the production of inorganic materials this is one of the ?rst process-es where ambient conditions were applied to produce ceramics. The control over the preparation of multicomponent systems by a mild reaction method also led to industrial interest in that process. In particular the silicon based sol–gel process was one of the major driving forces what has become the broad ?eld of inor-ganic–organic hybrid materials. The reason for the special role of silicon was its good processability and the stability of the Si—C bond during the formation of a silica network which allowed the production of organic-modi?ed inorganic networks in one step.

Inorganic–organic hybrids can be applied in many branches of materials chemistry because they are simple to process and are amenable to design on the molecular scale. Currently there are four major topics in the synthesis of inor-ganic–organic materials: (a) their molecular engineering, (b) their nanometer and micrometer-sized organization, (c) the transition from functional to multifunc-tional hybrids, and (d) their combination with bioactive components.

Some similarities to sol–gel chemistry are shown by the stable metal sols and colloids, such as gold colloids, developed hundreds of years ago. In fact sols pre-pared by the sol–gel process, i.e. the state of matter before gelation, and the gold colloids have in common that their building blocks are nanosized particles sur-rounded by a (solvent) matrix. Such metal colloids have been used for optical applications in nanocomposites for centuries. Glass, for example, was already colored with such colloids centuries ago. In particular many reports of the scien-

ti?c examination of gold colloids, often prepared by reduction of gold salts, are known from the end of the 18th century. Probably the ?rst nanocomposites were produced in the middle of the 19th century when gold salts were reduced in the presence of gum arabic. Currently many of the colloidal systems already known are being reinvestigated by modern instrumental techniques to get new insights into the origin of the speci?c chemistry and physics behind these materials.

1.1.3

De?nition: Hybrid Materials and Nanocomposites

The term hybrid material is used for many different systems spanning a wide area

of different materials, such as crystalline highly ordered coordination polymers, amorphous sol–gel compounds, materials with and without interactions between the inorganic and organic units. Before the discussion of synthesis and properties

of such materials we try to delimit this broadly-used term by taking into account various concepts of composition and structure (Table 1.1). The most wide-ranging

de?nition is the following: a hybrid material is a material that includes two moieties blended on the molecular scale. Commonly one of these compounds is inorganic and the other one organic in nature. A more detailed de?nition distinguishes between the possible interactions connecting the inorganic and organic species. Class I hybrid materials are those that show weak interactions between the two phases, such as van der Waals, hydrogen bonding or weak electrostatic interactions. Class II hybrid materials are those that show strong chemical interations between the components. Because of the gradual change in the strength of chemical interactions it becomes clear that there is a steady transition between weak and strong interactions (Fig. 1.1). For example there are 41

Introduction to Hybrid Materials

Fig. 1.1Selected interactions typically applied in hybrid materials and their relative strength.

Table 1.1Different possibilities of composition and structure of hybrid materials.

Matrix:

crystalline ?amorphous organic ?inorganic Building blocks:

molecules ?macromolecules ?particles ??bers Interactions between components:strong ?weak

1.1Introduction5 Table 1.2Different chemical interactions and their respective strength.

Type of interaction Strength [kJ mol?1]Range Character

van der Waals ca. 50Short nonselective,

nondirectional

H-bonding5–65Short selective, directional Coordination bonding50–200Short directional

Ionic50–250[a]Long nonselective

Covalent350Short predominantly

irreversible

a Depending on solvent and ion solution; data are for organic media.

hydrogen bonds that are de?nitely stronger than for example weak coordinative bonds. Table 1.2 presents the energetic categorization of different chemical inter-actions depending on their binding energies.

In addition to the bonding characteristics structural properties can also be used

to distinguish between various hybrid materials. An organic moiety containing a functional group that allows the attachment to an inorganic network, e.g. a tri-alkoxysilane group, can act as a network modifying compound because in the ?nal structure the inorganic network is only modi?ed by the organic group. Phenyltrialkoxysilanes are an example for such compounds; they modify the silica network in the sol–gel process via the reaction of the trialkoxysilane group (Scheme 1.1a) without supplying additional functional groups intended to under-go further chemical reactions to the material formed. If a reactive functional group

is incorporated the system is called a network functionalizer (Scheme 1.1c). The situation is different if two or three of such anchor groups modify an organic seg-ment; this leads to materials in which the inorganic group is afterwards an inte-gral part of the hybrid network (Scheme 1.1b). The latter systems are described in more detail in Chapter 6.

Blends are formed if no strong chemical interactions exist between the inor-ganic and organic building blocks. One example for such a material is the com-bination of inorganic clusters or particles with organic polymers lacking a strong (e.g. covalent) interaction between the components (Scheme 1.2a). In this case a material is formed that consists for example of an organic polymer with entrapped discrete inorganic moieties in which, depending on the functionalities of the components, for example weak crosslinking occurs by the entrapped inorganic units through physical interactions or the inorganic components are entrapped in

a crosslinked polymer matrix. If an inorganic and an organic network interpene-trate each other without strong chemical interactions, so called interpenetrating networks (IPNs) are formed (Scheme 1.2b), which is for example the case if a sol–gel material is formed in presence of an organic polymer or vice versa. Both materials described belong to class I hybrids. Class II hybrids are formed when the discrete inorganic building blocks, e.g. clusters, are covalently bonded to the

organic polymers (Scheme 1.2c) or inorganic and organic polymers are covalently connected with each other (Scheme 1.2d).

Nanocomposites After having discussed the above examples one question aris-es: what is the difference between inorganic–organic hybrid materials and inor-ganic–organic nanocomposites? In fact there is no clear borderline between these materials. The term nanocomposite is used if one of the structural units, either the organic or the inorganic, is in a de?ned size range of 1–100nm. Therefore there is a gradual transition between hybrid materials and nanocomposites,

61

Introduction to Hybrid Materials

Scheme 1.1Role of organically functionalized trialkoxysilanes

in the silicon-based sol–gel process.

because large molecular building blocks for hybrid materials, such as large inor-ganic clusters, can already be of the nanometer length scale. Commonly the term nanocomposites is used if discrete structural units in the respective size regime are used and the term hybrid materials is more often used if the inorganic units are formed in situ by molecular precursors, for example applying sol–gel reactions.Examples of discrete inorganic units for nanocomposites are nanoparticles,nanorods, carbon nanotubes and galleries of clay minerals (Fig. 1.2). Usually a nanocomposite is formed from these building blocks by their incorporation in organic polymers. Nanocomposites of nanoparticles are discussed in more detail in Chapter 2 and those incorporating clay minerals in Chapter 4.

1.1.4

Advantages of Combining Inorganic and Organic Species in One Material

The most obvious advantage of inorganic–organic hybrids is that they can favor-ably combine the often dissimilar properties of organic and inorganic components in one material (Table 1.3). Because of the many possible combinations of com-ponents this ?eld is very creative, since it provides the opportunity to invent an almost unlimited set of new materials with a large spectrum of known and as yet unknown properties. Another driving force in the area of hybrid materials is the possibility to create multifunctional materials. Examples are the incorporation of

1.1Introduction

7

Scheme 1.2The different types of hybrid materials.

81Introduction to Hybrid Materials

Fig. 1.2Inorganic building blocks used for embedment in an

organic matrix in the preparation of inorganic-organic

nanocomposites: a) nanoparticles, b) macromolecules,

c) nanotubes, d) layered materials.

Table 1.3Comparison of general properties of typical inorganic and organic materials.

, transition Properties Organics (polymers)Inorganics (SiO

2

metal oxides (TMO))

Nature of bonds covalent [C—C], van der Waals,ionic or iono-covalent [M—O]

H-bonding

low (?120°C to 200°C)high (>>200°C) T

g

Thermal stability low (<350°C–450°C)high (>>100°C)

Density0.9–1.2 2.0–4.0

Refractive index 1.2–1.6 1.15–2.7

Mechanical properties elasticity hardness

plasticity strength

)fragility

rubbery (depending on T

g

Hydrophobicity hydrophilic hydrophilic

Permeability hydrophobic low permeability to gases

±permeable to gases

Electronic properties insulating to conductive insulating to semiconductors

, TMO)

redox properties(SiO

2

redox properties (TMO)

magnetic properties Processability high (molding, casting, ?lm low for powders

formation, control of viscosity)high for sol–gel coatings

1.1Introduction9 inorganic clusters or nanoparticles with speci?c optical, electronic or magnetic properties in organic polymer matrices. These possibilities clearly reveal the power of hybrid materials to generate complex systems from simpler building blocks in a kind of LEGO ? approach.

Probably the most intriguing property of hybrid materials that makes this material class interesting for many applications is their processing. Contrary to pure solid state inorganic materials that often require a high temperature treat-ment for their processing, hybrid materials show a more polymer-like handling, either because of their large organic content or because of the formation of crosslinked inorganic networks from small molecular precursors just like in poly-merization reactions. Hence, these materials can be shaped in any form in bulk and in ?lms. Although from an economical point of view bulk hybrid materials can currently only compete in very special areas with classical inorganic or organic materials, e.g. in the biomaterials sector, the possibility of their processing as thin

?lms can lead to property improvements of cheaper materials by a simple surface treatment, e.g. scratch resistant coatings.

Based on the molecular or nanoscale dimensions of the building blocks, light scattering in homogeneous hybrid material can be avoided and therefore the optical transparency of the resulting hybrid materials and nanocomposites is, dependent on the composition used, relatively high. This makes these materials ideal candidates for many optical applications (Chapter 9). Furthermore, the ma-terials’ building blocks can also deliver an internal structure to the material which can be regularly ordered. While in most cases phase separation is avoided, phase separation of organic and inorganic components is used for the formation of porous materials, as described in Chapter 5.

Material properties of hybrid materials are usually changed by modi?cations of the composition on the molecular scale. If, for example, more hydrophobicity of

a material is desired, the amount of hydrophobic molecular components is increased. In sol–gel materials this is usually achieved if alkyl- or aryl-substituted trialkoxysilanes are introduced in the formulation. Hydrophobic and lipophobic materials are composed if partially or fully ?uorinated molecules are included. Mechanical properties, such as toughness or scratch resistance, are tailored if hard inorganic nanoparticles are included into the polymer matrix. Because the compositional variations are carried out on the molecular scale a gradual ?ne tuning of the material properties is possible.

One important subject in materials chemistry is the formation of smart materials, such as materials that react to environmental changes or switchable systems, because they open routes to novel technologies, for example electroac-tive materials, electrochromic materials, sensors and membranes, biohybrid materials, etc. The desired function can be delivered from the organic or inorganic or from both components. One of the advantages of hybrid materials in this context is that functional organic molecules as well as biomolecules often show better stability and performance if introduced in an inorganic matrix.

1.1.5

Interface-determined Materials

The transition from the macroscopic world to microscopic, nanoscopic and mo-lecular objects leads, beside the change of physical properties of the material itself, i.e. the so called quantum size effects, to the change of the surface area of the objects. While in macroscopic materials the majority of the atoms is hidden in the bulk of the material it becomes vice versa in very small objects. This is demonstrated by a simple mind game (Fig. 1.3). If one thinks of a cube of atoms in tight packing of 16 ×16 ×16 atoms. This cube contains an overall number of 4096 atoms from which 1352 are located on the surface (~33% surface atoms); if this cube is divided into eight equal 8 ×8 ×8 cubes the overall number is the same but 2368 atoms are now located on the surface (~58% surface atoms); repeating this procedure we get 3584 surface atoms (~88% surface atoms). This example shows how important the surface becomes when objects become very small. In small nanoparticles (<10nm) nearly every atom is a surface atom that can inter-act with the environment. One predominant feature of hybrid materials or nanocomposites is their inner interface, which has a direct impact on the proper-ties of the different building blocks and therefore on the materials’ properties. As already explained in Section 1.1.3, the nature of the interface has been used to divide the materials in two classes dependent on the strength of interaction between the moieties. If the two phases have opposite properties, such as differ-ent polarity, the system would thermodynamically phase separate. The same can happen on the molecular or nanometer level, leading to microphase https://www.sodocs.net/doc/4c5493194.html,ually, such a system would thermodynamically equilibrate over time. However in many cases in hybrid materials the system is kinetically stabilized by network-forming reactions such as the sol–gel process leading to a spatial ?xation of the structure. The materials formed can be macroscopically homogeneous and opti-cally clear, because the phase segregation is of small length scale and therefore limited interaction with visible light occurs. However, the composition on the mo-lecular or nanometer length scale can be heterogeneous. If the phase segregation 101

Introduction to Hybrid Materials

Fig. 1.3Surface statistical consequences of dividing a cube with

16 ×16 ×16 atoms. N =total atoms, n =surface atoms.

1.1Introduction11 reaches the several hundred nanometer length scale or the refractive index of the formed domains is very different, materials often turn opaque. Effects like this are avoided if the reaction parameters are controlled in such a way that the speed of network formation is kept faster than the phase separation reactions.

The high surface area of nanobuilding blocks can lead to additional effects; for example if surface atoms strongly interact with molecules of the matrix by chemical bonding, reactions like surface reorganization, electron transfer, etc. can occur which can have a large in?uence on the physical properties of the nano-building blocks and thus the overall performance of the material formed. It is, for example, known that conjugated π-electron systems coordinated to the sur-face of titania nanoparticles can lead to charge transfer reactions that in?uence the color, and therefore the surface electronic properties of the particles. Nanosized objects, such as inorganic nanoparticles, in addition show a very high surface energy. Usually if the surface energy is not reduced by surface active agents (e.g. surfactants), such particles tend to agglomerate in an organic medium. Thus, the physical properties of the nanoparticles (e.g. quantum size effects) diminish, and/or the resulting materials are no longer homogeneous. Both facts have effects

on the ?nal material properties. For example the desired optical properties of nanocomposites fade away, or mechanical properties are weakened. However, sometimes a controlled aggregation can also be required, e.g. percolation of con-ducting particles in a polymer matrix increases the overall conductivity of the material (see Chapter 10).

1.1.6

The Role of the Interaction Mechanisms

In Section 1.1.3 the interaction mechanism between the organic and inorganic species was used to categorize the different types of hybrid materials, furthermore

of course the interaction also has an impact on the material properties. Weak chemical interactions between the inorganic and organic entities leave some potential for dynamic phenomena in the ?nal materials, meaning that over longer periods of time changes in the material, such as aggregation, phase separation or leaching of one of the components, can occur. These phenomena can be avoided

if strong interactions are employed such as covalent bonds, as in nanoparticle-crosslinked polymers. Depending on the desired materials’ properties the inter-actions can be gradually tuned. Weak interactions are, for example, preferred where a mobility of one component in the other is required for the target proper-ties. This is for example the case for ion conducting polymers, where the inor-ganic ion (often Li+) has to migrate through the polymer matrix.

In many examples the interactions between the inorganic and organic species are maximized by applying covalent attachment of one to the other species. But there are also cases where small changes in the composition, which on the ?rst sight should not result in large effects, can make considerable differences. It was,

for example, shown that interpenetrating networks between polystyrene and

sol–gel materials modi?ed with phenyl groups show less microphase segregation

Introduction to Hybrid Materials

121

than sol–gel materials with pure alkyl groups, which was interpreted to be an effect of π-π-interactions between the two materials.

In addition the interaction of the two components can have an in?uence on other properties, such as electronic properties if coordination complexes are formed or electron transfer processes are enabled by the interaction.

1.2

Synthetic Strategies towards Hybrid Materials

In principle two different approaches can be used for the formation of hybrid materials: Either well-de?ned preformed building blocks are applied that react with each other to form the ?nal hybrid material in which the precursors still at least partially keep their original integrity or one or both structural units are formed from the precursors that are transformed into a novel (network) structure.

Both methodologies have their advantages and disadvantages and will be described here in more detail.

Building block approach As mentioned above building blocks at least partially keep their molecular integrity throughout the material formation, which means that structural units that are present in these sources for materials formation can also be found in the ?nal material. At the same time typical properties of these building blocks usually survive the matrix formation, which is not the case if material precursors are transferred into novel materials. Representative examples of such well-de?ned building blocks are modi?ed inorganic clusters or nanopar-ticles with attached reactive organic groups (Fig. 1.4).

Cluster compounds often consist of at least one functional group that allows an interaction with an organic matrix, for example by copolymerization. Depending on the number of groups that can interact, these building blocks are able to mod-ify an organic matrix (one functional group) or form partially or fully crosslinked materials (more than one group). For instance, two reactive groups can lead to the formation of chain structures. If the building blocks contain at least three reactive groups they can be used without additional molecules for the formation of a

crosslinked material.

Fig. 1.4Typical well-de?ned molecular building blocks used in

the formation of hybrid materials.

1.2Synthetic Strategies towards Hybrid Materials13 Beside the molecular building blocks mentioned, nanosized building blocks, such as particles or nanorods, can also be used to form nanocomposites. The build-ing block approach has one large advantage compared with the in situ formation

of the inorganic or organic entities: because at least one structural unit (the build-ing block) is well-de?ned and usually does not undergo signi?cant structural changes during the matrix formation, better structure–property predictions are possible. Furthermore, the building blocks can be designed in such a way to give the best performance in the materials’ formation, for example good solubility of inorganic compounds in organic monomers by surface groups showing a similar polarity as the monomers.

In situ formation of the components Contrary to the building block approach the

in situ formation of the hybrid materials is based on the chemical transformation

of the precursors used throughout materials’ preparation. Typically this is the case

if organic polymers are formed but also if the sol–gel process is applied to pro-duce the inorganic component. In these cases well-de?ned discrete molecules are transformed to multidimensional structures, which often show totally different properties from the original precursors. Generally simple, commercially available molecules are applied and the internal structure of the ?nal material is determined

by the composition of these precursors but also by the reaction conditions. There-fore control over the latter is a crucial step in this process. Changing one param-eter can often lead to two very different materials. If, for example, the inorganic species is a silica derivative formed by the sol–gel process, the change from base

to acid catalysis makes a large difference because base catalysis leads to a more particle-like microstructure while acid catalysis leads to a polymer-like microstructure. Hence, the ?nal performance of the derived materials is strongly dependent on their processing and its optimization.

1.2.1

In situ Formation of Inorganic Materials

Many of the classical inorganic solid state materials are formed using solid pre-cursors and high temperature processes, which are often not compatible with the presence of organic groups because they are decomposed at elevated temperatures. Hence, these high temperature processes are not suitable for the in situ formation

of hybrid materials. Reactions that are employed should have more the character

of classical covalent bond formation in solutions. One of the most prominent processes which ful?ll these demands is the sol–gel process. However, such rather low temperature processes often do not lead to the thermodynamically most stable structure but to kinetic products, which has some implications for the structures obtained. For example low temperature derived inorganic materials are often amorphous or crystallinity is only observed on a very small length scale,

i.e. the nanometer range. An example of the latter is the formation of metal nano-particles in organic or inorganic matrices by reduction of metal salts or organo-metallic precursors.

1.2.1.1Sol–Gel Process

This process is chemically related to an organic polycondensation reaction in which small molecules form polymeric structures by the loss of substituents. Usu-ally the reaction results in a three-dimensional (3-D) crosslinked network. The fact that small molecules are used as precursors for the formation of the crosslinked materials implies several advantages, for example a high control of the purity and composition of the ?nal materials and the use of a solvent based chemistry which offers many advantages for the processing of the materials formed.

The silicon-based sol–gel process is probably the one that has been most inves-tigated; therefore the fundamental reaction principles are discussed using this process as a model system. One important fact also makes the silicon-based sol–gel processes a predominant process in the formation of hybrid materials, which is the simple incorporation of organic groups using organically modi?ed silanes.Si—C bonds have enhanced stability against hydrolysis in the aqueous media usu-ally used, which is not the case for many metal–carbon bonds, so it is possible to easily incorporate a large variety of organic groups in the network formed. Prin-cipally R 4–n SiX n compounds (n =1–4, X =OR’, halogen) are used as molecular pre-cursors, in which the Si—X bond is labile towards hydrolysis reactions forming unstable silanols (Si—OH) that condensate leading to Si—O—Si bonds. In the ?rst steps of this reaction oligo- and polymers as well as cyclics are formed sub-sequently resulting in colloids that de?ne the sol. Solid particles in the sol after-wards undergo crosslinking reactions and form the gel (Scheme 1.3).

141

Introduction to Hybrid Materials

Scheme 1.3Fundamental reaction steps in the sol–gel process based on tetrialkoxysilanes.The process is catalyzed by acids or bases resulting in different reaction mech-anisms by the velocity of the condensation reaction (Scheme 1.4). The pH used therefore has an effect on the kinetics which is usually expressed by the gel point

of the sol–gel reaction. The reaction is slowest at the isoelectric point of silica (between 2.5 and 4.5 depending on different parameters) and the speed increases rapidly on changing the pH. Not only do the reaction conditions have a strong in?uence on the kinetics of the reaction but also the structure of the precursors.Generally, larger substituents decrease the reaction time due to steric hindrance.In addition, the substituents play a mature role in the solubility of the precursor in the solvent. Water is required for the reaction and if the organic substituents are quite large usually the precursor becomes immiscible in the solvent. By chang-ing the solvent one has to take into account that it can interfere in the hydrolysis reaction, for example alcohols can undergo trans -esteri?cation reactions leading to quite complicated equilibria in the mixture. Hence, for a well-de?ned material the reaction conditions have to be ?ne-tuned.

The pH not only plays a major role in the mechanism but also for the micro-structure of the ?nal material. Applying acid-catalyzed reactions an open network structure is formed in the ?rst steps of the reaction leading to condensation of small clusters afterwards. Contrarily, the base-catalyzed reaction leads to highly crosslinked sol particles already in the ?rst steps. This can lead to variations in the homogeneity of the ?nal hybrid materials as will be shown later. Commonly used catalysts are HCl, NaOH or NH 4OH, but ?uorides can be also used as catalysts leading to fast reaction times.

The transition from a sol to a gel is de?ned as the gelation point, which is the point when links between the sol particles are formed to such an extent that a sol-id material is obtained containing internal pores that incorporate the released alcohol. However at this point the reaction has not ?nished, but condensation reactions can go on for a long time until a ?nal stage is reached. This process is called ageing. During this reaction the material shrinks and stiffens. This process is carried on in the drying process, where the material acquires a more compact structure and the associated crosslinking leads to an increased stiffness. During the drying process the large capillary forces of the evaporating liquids in the porous structure take place which can lead to cracking of the materials. Reaction param-eters such as drying rate, gelation time, pH, etc. can have a major in?uence on

1.2Synthetic Strategies towards Hybrid Materials

15

Scheme 1.4Differences in mechanism depending on the type

of catalyst used in the silicon-based sol–gel process.

the cracking of the gels and have therefore to be optimized. Under some circum-stances the destruction of the gel network can lead to the formation of powders instead of monoliths during materials formation.

Stress during the drying process can be avoided if the liquid in the pores is ex-changed under supercritical conditions where the distinction between liquid and vapor no longer exists. This process leads to so-called highly porous aerogels com-pared with the conventionally dried xerogels.

As already mentioned above many parameters in?uence the speed of the sol–gel reaction; if a homogeneous material is required all parameters must be optimized.This is particularly true if hybrid materials are the target, because undesired phase separations of organic and inorganic species in the materials or between the net-work and unreacted precursors weaken the materials’ properties. This can often even be observed by the naked eye if the material turns opaque.

The water to precursor ratio is also a major parameter in the sol–gel process. If tetraalkoxysilanes are used as precursors, two water molecules per starting com-pound are necessary to form completely condensed SiO 2. Applying a lower H 2O/Si ratio, would lead to an alkoxide containing ?nal material.

1.2.1.2Nonhydrolytic Sol–Gel Process

In this process the reaction between metal halides and alkoxides is used for the formation of the products (Scheme 1.5). The alkoxides can be formed during the process by various reactions. Usually this process is carried out in sealed tubes at elevated temperature but it can also be employed in unsealed systems under an inert gas atmosphere.

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Introduction to Hybrid Materials

Scheme 1.5Mechanisms involved in the nonhydrolytic sol–gel process.

1.2.1.3Sol–Gel Reactions of Non-Silicates

Metal and transition-metal alkoxides are generally more reactive towards hydroly-sis and condensation reactions compared with silicon. The metals in the alkoxides are usually in their highest oxidation state surrounded by electronegative –OR lig-ands which render them susceptible to nucleophilic attack. Transition metal alkox-ides show a lower electronegativity compared with silicon which causes them to be more electrophilic and therefore less stable towards hydrolysis in the sol–gel reactions. Furthermore, transition metals often show several stable coordination environments. While the negatively charged alkoxides balance the charge of the metal cation they generally cannot completely saturate the coordination sphere of

the metals, which leads to the formation of oligomers via alkoxide or alcohol bridges and/or the saturation of the coordination environment by additional coordination of alcohol molecules, which also has an impact on the reactivity of the metal alkoxides. More sterically demanding alkoxides, such as isopropoxides,lead to a lower degree of aggregation and smaller alkoxides, such as ethoxides or n -propoxides, to a larger degree of aggregation. In addition, the length of the alkyl group in the metal alkoxides also in?uences their solubility in organic solvents,for example ethoxides often show a much lower solubility as their longer alkyl chain containing homologs.

As already mentioned M—C bonds in metal alkoxides are in most cases not stable enough to survive the sol–gel conditions. Therefore, contrary to the silica route, other mechanisms have to be employed if it is desired that hybrid inorganic–organic metal oxide materials be formed in a one-step approach. One solution to the latter problem is the use of organically functionalized bi- and multidentate ligands that show a higher bonding stability during the sol–gel reaction and, in addition, reduce the speed of the reaction by blocking coordina-tion sites (Fig. 1.5).

1.2.1.4Hybrid Materials by the Sol–Gel Process

Organic molecules other than the solvent can be added to the sol and become phys-ically entrapped in the cavities of the formed network upon gelation. For this pur-pose the molecules have to endure the reaction conditions of the sol–gel process,namely the aqueous conditions and the pH of the environment. Hence, functional organic groups that can be hydrolyzed are not tolerated, but a partial tolerance for the pH can be obtained if the sol–gel reaction is carried out in a buffer solution.This is particularly necessary if biological molecules, such as enzymes, are to be entrapped in the gel. Physical entrapment has the disadvantage that sometimes the materials obtained are not stable towards phase separation or leaching because of differences in polarity. Chemical modi?cation of organic compounds with tri-alkoxysilane groups can partially avoid such problems due to co-condensation dur-ing the formation of the sol–gel network and thus development of covalent linkages to the network. Trialkoxysilane groups are typically introduced by a plat-inum catalyzed reaction between an unsaturated bond and a trialkoxysilane (Scheme 1.6).

1.2Synthetic Strategies towards Hybrid Materials

17

Fig. 1.5Typical coordination patterns between bi- and

multidentate ligands and metals that can be applied for the

incorporation of organic functionalities in metal oxides.

While the formation of homogeneous materials with a chemical link between the inorganic and organic component is in many cases the preferred route, there are cases where a controlled phase separation between the entrapped organic mol-ecules and the sol–gel material is compulsory for the formation of the material,for example in the preparation of mesoporous materials (Chapter 5).

Besides the entrapment of organic systems, precursors with hydrolytically stable Si—C bonds can also be used for co-condensation reactions with tetraalk-oxysilanes. In addition, organically functionalized trialkoxysilanes can also be used for the formation of 3-D networks alone forming so called silsesquioxanes (general formula R-SiO 1.5) materials. Generally a 3-D network can only be obtained if three or more hydrolyzable bonds are present in a molecule. Two such bonds generally result in linear products and one bond leads only to dimers or allows a modi?cation of a preformed network by the attachment to reactive groups on the surface of the inorganic network (Fig. 1.6). Depending on the reaction conditions in the sol–gel process smaller species are also formed in the organotrialkoxysilane-based sol–gel process, for example cage structures or ladder-like polymers (Fig. 1.6). Because of the stable Si—C bond the organic unit can be included with-in the silica matrix without transformation. There are only a few Si—C bonds that are not stable against hydrolysis, for example the Si—C ≡≡C bond where the Si—

C bond can be cleaved by H 2O if ?uoride ions are present. Some typical examples for trialkoxysilane compounds used in the formation of hybrid materials are shown in Scheme 1.7. Usually the organic functionalizations have a large in?u-ence on the properties of the ?nal hybrid material. First of all the degree of con-densation of a hybrid material prepared by trialkoxysilanes is generally smaller than in the case of tetraalkoxysilanes and thus the network density is also reduced.181

Introduction to Hybrid Materials

Scheme 1.6

Platinum catalyzed hydrosilation for the introduction of trialkoxysilane groups.Fig. 1.6Formation of different structures during hydrolysis in

dependence of the number of organic substituents compared to

labile substituents at the silicon atom.

More detailed discussions of the sol–gel process can be found in the cited literature.

1.2.1.5Hybrid Materials Derived by Combining the Sol–Gel Approach and Organic Polymers

Compared with other inorganic network forming reactions, the sol–gel processes show mild reaction conditions and a broad solvent compatibility. These two char-acteristics offer the possibility to carry out the inorganic network forming process in presence of a preformed organic polymer or to carry out the organic polymer-ization before, during or after the sol–gel process. The properties of the ?nal hybrid materials are not only determined by the properties of the inorganic and organic component, but also by the phase morphology and the interfacial region between the two components. The often dissimilar reaction mechanisms of the sol–gel process and typical organic polymerizations, such as free radical polymer-izations, allow the temporal separation of the two polymerization reactions which offers many advantages in the material formation.

One major parameter in the synthesis of these materials is the identi?cation of a solvent in which the organic macromolecules are soluble and which is compat-ible with either the monomers or preformed inorganic oligomers derived by the sol–gel approach. Many commonly applied organic polymers, such as polystyrene or polymethacrylates, are immiscible with alcohols that are released during the sol–gel process and which are also used as solvents, therefore phase separation is enforced in these cases. This can be avoided if the solvent is switched from the typically used alcohols to, for example, THF in which many organic polymers are soluble and which is compatible with many sol–gel reactions. Phase separation can also be avoided if the polymers contain functional groups that are more com-patible with the reaction conditions of the sol–gel process or even undergo an interaction with the inorganic material formed. This can be achieved, for example

1.2Synthetic Strategies towards Hybrid Materials

19

Scheme 1.7Trialkoxysilane precursors often used in the sol–gel process.

In addition, the functional group incorporated changes the properties of the ?nal material, for example ?uoro-substituted compounds can create hydrophobic and lipophobic materials, additional reactive functional groups can be introduced to allow further reactions such as amino, epoxy or vinyl groups (Scheme 1.7). Beside molecules with a single trialkoxysilane group also multifunctional organic mole-cules can be used, which are discussed in more detail in Chapter 6.

by the incorporation of OH-groups that interact with, for example, hydroxyl groups formed during the sol–gel process or by ionic modi?cations of the organic poly-mer. Covalent linkages can be formed if functional groups that undergo hydroly-sis and condensation reactions are covalently attached to the organic monomers.Some typically used monomers that are applied in homo- or copolymerizations are shown in Scheme 1.8.

201

Introduction to Hybrid Materials

Scheme 1.8Organic monomers typically applied in the

formation of sol–gel/organic polymer hybrid materials.

1.2.2

Formation of Organic Polymers in Presence of Preformed Inorganic Materials

If the organic polymerization occurs in the presence of an inorganic material to form the hybrid material one has to distinguish between several possibilities to overcome the incompatibilty of the two species. The inorganic material can either have no surface functionalization but the bare material surface; it can be modi?ed with nonreactive organic groups (e.g. alkyl chains); or it can contain reactive sur-face groups such as polymerizable functionalities. Depending on these prerequi-sites the material can be pretreated, for example a pure inorganic surface can be treated with surfactants or silane coupling agents to make it compatible with the organic monomers, or functional monomers can be added that react with the sur-face of the inorganic material. If the inorganic component has nonreactive organic groups attached to its surface and it can be dissolved in a monomer which is sub-sequently polymerized, the resulting material after the organic polymerization, is a blend. In this case the inorganic component interact only weakly or not at all with the organic polymer; hence, a class I material is formed. Homogeneous materials are only obtained in this case if agglomeration of the inorganic compo-

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C.定点数表示的是整数 D.二进制数据表示在计算机中容易实现 6.浮点数0.00100011B×2-1的规格化表示是() A.0.1000110B×2-11B B.0.0100011B×2-10B C.0.0100011B×20B D.0.1000110B×21B 7.两个定点数作补码加法运算,对相加后最高位出现进位1的处理是() A.判为溢出B.AC中不保留 C.寄存在AC中D.循环加到末位 8.运算器中通用寄存器的长度一般取() A.8位B.16位 C.32位D.等于计算机字长 9.目前在大多数微型机上广泛使用宽度为32/64位的高速总线是() A.ISA B.EISA C.PCI D.VESA 10.某计算机指令的操作码有8个二进位,这种计算机的指令系统中的指令条数至多为 ()A.8 B.64 C.128 D.256 11.间接访内指令LDA @Ad的指令周期包含CPU周期至少有() A.一个B.二个 C.三个D.四个 12.在程序中,可用转移指令实现跳过后续的3条指令继续执行。这种指令的寻址方式是() A.变址寻址方式B.相对寻址方式

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2.2.2 工艺流程简述 2.2.2.1 压缩、预冷 原料空气通过空气过滤系统,去除灰尘和机械杂质。过滤后的空气由多级压缩机压缩到工艺所需压力,然后进入空冷塔进行冷却。压缩过程中产生的冷凝疏水在厂房内凝液罐中汇集后,由凝液泵加压送入循环回水管线。 空气自下而上穿过空冷塔,以对流形式被循环冷却水和低温冷冻水分段冷却,同时也得到了清洗。 在空冷塔底部,空气被由冷却水泵送入的循环冷却水预冷。 在顶部,空气由冷冻水泵送入的冷冻水进一步冷却。 低温冷冻水是在水冷塔中产生,其产生的原理是利用从冷箱来的干燥的污氮气汽化小部分循环冷却水,水在汽化过程中吸收热量,同时使冷却水的温度降低。 空气离开空冷塔的温度越低,对于下游空气纯化单元的负荷就越小。 空气中的少量化学杂质也被冷却水吸收。 空冷塔和水冷塔为填料塔,在空冷塔顶部设置有除沫器以去除空气中的水雾。 2.2.2.2 吸附净化 空气纯化单元包括两台交替运行的分子筛吸附器,压缩空气通过吸附器时,水、CO、氮氧化合物和绝大多数碳氢化合物都被吸附。 吸附器交替循环,即一只吸附器吸附杂质而另一只吸附器被再生。吸附和再生过程顺序自动控制以保证装置连续运行。采用来自冷箱的污氮对吸附器进行再生。再生时吸附器与吸附流程隔离,再生气放空。与吸附流程隔离的吸附器先卸压,然后先用经蒸汽加热器加热的低压污氮进行再生,然后用从蒸汽加热器旁路来的冷低温氮气对吸附器进行冷却,之后再用吸附后的空气对吸附器升压并返

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计算机组成原理试题及答案

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空分工艺流程

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分集接收技术

题目:多径衰落信道下分集接收技术性能仿 真 学科门类(文、理、工、医):工 院 系:信息工程学院 专 业:通信与信息系统 初 审: 评 审: 2014年郑州大学第九届研究生论文大赛

多径衰落信道下分集接收技术性能仿真 摘要:随着信息时代的到来,近几年来,在通信领域,很多的技术都得到了发展和应用,通信质量问题也得到越来越多的关注,当信号在实际的无线通信系统中传输时,多径传输的存在会而使信号产生衰落,衰落会影响通信的质量,多径效应是影响无线通信质量的一个重要因素,多径效应通常会影响信号的传输,然而分集技术可以有效的减弱多径效应带给无线信道的不良的影响。使用分集技术可以获得分集增益,通过获得分集增益来提高通信的质量。 本设计介绍了有关通信系统仿真的方法和概念,也对多径衰落信道做了详细的介绍,论文的最后一章用MATLAB仿真了多径衰落信道,通过仿真可以比较直观的看出此信道的特点,论文详细的介绍了几种分集合并技术,并对这几种技术做了简单的分析和比较,仿真了信号在不同的分集接收技术上的BER。 关键词:信号;多径效应;分集技术;通信仿真 The performance simulation of diversity reception technology on Rayleigh fading channel Abstract:With the information age coming, in recent years, in the field of communication, many techniques are making a big development, the communication quality issues have been more and more attention, when the signal transmit in a real communication system .In multipath transmission signal will be fading, fading affects the quality of the communication .multipath effect is an important factor affecting the quality of the radio communication, multipath effects usually affect the signal transmission, however, the diversity technique can be effectively reduced multipath effects bring the adverse effects of the radio channel. Diversity gain can be obtained by obtaining the diversity gain to improve the quality of the communication using the diversity technique. This topic provides information communication system simulation methods and concepts. In this paper, I make a detailed introduction about multipath fading channel .In final chapter, MATLAB is used to simulate the multipath fading channel, Through the simulation we can see this channel characteristics more intuitive, the paper describes in detail several diversity combining techniques, and these types of technology to do a simple analysis and comparison of simulated signals in different diversity reception technology BER. Keywords: signal Multi-path effects diversity reception technology 1 绪论 1.1 引言 达接收端的信号路径不只有一条。即存在多径传输。多径传输会给信号带来多径衰落。多径衰落会使到达接收机的信号在实际的无线通信中,信号在传输过程中存在反射、折射、绕射等现象使到与原来的发射信号相差较大,造成错码,因此,怎样提高信号的输出信噪比,提高信道特性是现在通信领域的重要研究课题。 分集技术的关键是“分”,“分”的含义就是要使信号通过多个信道,这里所说的信道可以是空间的,可以是时间的,也可以是频率的。通过多个信道传送同一信号,然后在接收端会接收到多个信道信息,因为每个信道的特性不可能完全相同,在不同信道上传输的多路信号的衰落就不尽相同。多径信号叠加后在每个时间点上的信号就会减少衰落,多条信号叠加后所包含的的信息比较接近原来的信号,这样接收机就能比较准确的恢复原来的信号。因此分集技术可以降低衰落,如果不用分集技术,在这种情况下要想获得比较高的输出信号信噪比,发射机必须要有较高的信号发射功率,信号的发射功率较小会使到达接收端的衰落信号

计算机组成原理答案

计算机组成原理答案文档编制序号:[KKIDT-LLE0828-LLETD298-POI08]

第5章习题参考答案 1.请在括号内填入适当答案。在CPU中: (1)保存当前正在执行的指令的寄存器是( IR ); (2)保存当前正在执行的指令地址的寄存器是( AR ) (3)算术逻辑运算结果通常放在( DR )和(通用寄存器)。 2.参见图的数据通路。画出存数指令“STO Rl,(R2)”的指令周期流程图,其含义是将寄存器Rl的内容传送至(R2)为地址的主存单元中。标出各微操作信号序列。 解: STO R1, (R2)的指令流程图及微操作信号序列如下: 3.参见图的数据通路,画出取数指令“LAD (R3),R0”的指令周期流程图,其含义是将(R3)为地址主存单元的内容取至寄存器R2中,标出各微操作控制信号序列。 解: LAD R3, (R0)的指令流程图及为操作信号序列如下: 4.假设主脉冲源频率为10MHz,要求产生5个等间隔的节拍脉冲,试画出时序产生器的逻辑图。 解: 5.如果在一个CPU周期中要产生3个节拍脉冲;T l=200ns,T2=400ns,T3=200ns,试画出时序产生器逻辑图。 解:取节拍脉冲T l、T2、T3的宽度为时钟周期或者是时钟周期的倍数即可。所以取时钟源提供的时钟周期为200ns,即,其频率为5MHz.;由于要输出3个节拍脉冲信号,而T3的宽度为2个时钟周期,也就是一个节拍电位的时间是4个时钟周期,所以除了C4外,还需要3个触发器——C l、C2、C3;并令

211C C T *=;321C C T *=;313C C T =,由此可画出逻辑电路图如下: 6.假设某机器有80条指令,平均每条指令由4条微指令组成,其中有一条取指微指令是所有指令公用的。已知微指令长度为32位,请估算控制存储器容量。 解:80条指令,平均每条指令由4条微指令组成,其中有一条公用微指令,所以总微指令条数为80 (4-1)+1=241条微指令,每条微指令32位,所以控存容量为:24132位 7.某ALU 器件是用模式控制码M S 3 S 2 S 1 C 来控制执行不同的算术运算和逻辑操作。下表列出各条指令所要求的模式控制码,其中y 为二进制变量,φ为0或l 任选。 试以指令码(A ,B ,H ,D ,E ,F ,G)为输入变量,写出控制参数M ,S 3,S 2,S l ,C 的逻辑表达式。 解: 由表可列如下逻辑方程 M=G S 3=H+D+F S 2=A+B+D+H+E+F+G S 1=A+B+F+G C=H+D+Ey+Fy 8.某机有8条微指令I 1—I 8,每条微指令所包含的微命令控制信号如下表所示。 a —j 分别对应10种不同性质的微命令信号。假设一条微指令的控制字段仅限为8位,请安排微指令的控制字段格式。

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