Host materials

Due to the special characteristics of the laser emission process and the parasitic non-radiative de-excitation, it is necessary to carefully select the laser materials, including both the active ions and host materials. In addition, the characteristics of dopants and the states of doping have also played a crucial role in determining the performances of laser materials and thus the solid-state lasers. The efficiency and effectiveness of doping is mainly determined by the degree of matching in ionic radii between the dopant ions and substituted cations. The Shannon ionic radii of the ions in condensed state with anionic coordination number of 6 and 8 are rs = 0.103-0.115 nm and rg = 0.113-0.128 nm, respectively. In both cases, the radius decreases with increasing atomic number [79]. These ions can substitute for host cations with similar ionic radius, such as Ca , La , Gd , Y , Lu ,  

Sc and so on. The efficiency of the substitution is closely related to the conditions of fabrication and processing of the laser materials, such as properties of starting materials, synthesis methods, processing parameters, and so on. 

There are also other requirements for the host materials, such as mechanical strength, chemical stability, fabrication processibUily, and so on, which should be taken into account during the selection of host materials. All raw materials and procedures involved should be cost-effective, especially for large-scale production, otherwise no industry will adopt. 

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A semiconductor laser takes advantage of the properties of a junction between a p-type and an n-type semiconductor made from the same host material. Such an n-p combination is called a semiconductor diode. Doping concentrations are quite high and, as a result, the conduction and valence band energies of the host are shifted in the two semiconductors, as shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy E. ... 

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

Solid-State Lasers. Sohd-state lasers (37) use glassy or crystalline host materials containing some active species. The term soHd-state as used in connection with lasers does not imply semiconductors rather it appHes to soHd materials containing impurity ions. The impurity ions are typically ions of the transition metals, such as chromium, or ions of the rare-earth series, such as neodymium (see Lanthanides). Most often, the soHd material is in the form of a cylindrical rod with the ends poHshed flat and parallel, but a variety of other forms have been used, including slabs and cylindrical rods with the ends cut at Brewster s angle. 

Phosphors usually contain activator ions in addition to the host material. These ions are dehberately added in the proper proportion during the synthesis. The activators and their surrounding ions form the active optical centers. Table 1 Hsts some commonly used activator ions. Some soflds, made up of complexes such as calcium tungstate [7790-75-2] CaWO, are self-activated. Also in many photolurninescence phosphors, the primary activator does not efficiently absorb the exciting radiation and a second impurity ion is introduced known as the sensitizer. The sensitizer, which is an activator ion itself, absorbs the exciting radiation and transfers this energy to the primary activator. 

The principal use of titanium sulfides is as a cathode material ia high efficieacy batteries (11). In these appHcations, the titanium disulfide acts as a host material for various alkafl or alkaline-earth elements. 

Gupta and his students have developed procedures for determining the elastic and plastic contributions to shock-deformed metals. The work explicitly recognizes that the metal sample is an inclusion in a host material which may act to cause local deformation unique to the particular host [83G01, 87G01]. 

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

During electrochemical reduction (charge) of the carbon host, lithium cations from the electrolyte penetrate into the carbon and form a lithiated carbon Li rCn. The corresponding negative charges are accepted by the carbon host lattice. As for any other electrochemical insertion process, the prerequisite for the formation of lithiated carbons is a host material that exhibits mixed (electronic and ionic) conductance. 

The quality and quantity of sites which are capable of reversible lithium accommodation depend in a complex manner on the crystallinity, the texture, the (mi-cro)structure, and the (micro)morphology of the carbonaceous host material [7, 19, 22, 40-57]. The type of carbon determines the current/potential characteristics of the electrochemical intercalation reaction and also potential side-reactions. Carbonaceous materials suitable for lithium intercalation are commercially available in many types and qualities [19, 43, 58-61], Many exotic carbons have been specially synthesized on a laboratory scale by pyrolysis of various precursors, e.g., carbons with a remarkably high lithium storage capacity (see Secs. 

A critical review on the foundation and earlier results on metal intercalates of the transition metal dichalcogenides and related host materials can be found in the seminal paper of Whittingham [53]. The electrochemical and transport properties... 

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, as it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems, while in contrast to Pb and Cd, magnesium is inexpensive, environmentally friendly, and safe to handle. However, the development of Mg-ion batteries has so far been limited by the kinetics of Mg " " diffusion and the lack of suitable electrolytes. Actually, in spite of an expected general similarity between the processes of Li and Mg ion insertion into inorganic host materials, most of the compounds that exhibit fast and reversible Li ion insertion perform very poorly in Mg " ions. Hence, there... 

Protons are not the sole species that can be incorporated into the lattices of different host materials. At the beginning of the 1960s, Boris N. Kabanov showed that during cathodic polarization of different metals in alkaline solutions, intercalation of atoms of the corresponding alkali metal is possible. As a result of such an electrochemical intercalation, either homogeneous alloys are formed (solid solutions) or heterogeneous polyphase systems, or even intermetallic compounds, are formed. 

The properties of lithium intercalation compounds depend in a decisive manner on the nature of the host material. If oxides of metals with varying valency are used instead of TiSj, the potential of the electrode (consequently also, the battery voltage) will increase to 3 V. Even higher values (up to 4 V) are obtained when as host material... 

At present, intercalation compounds are used widely in various electrochemical devices (batteries, fuel cells, electrochromic devices, etc.). At the same time, many fundamental problems in this field do not yet have an explanation (e.g., the influence of ion solvation, the influence of defects in the host structure and/or in the host stoichiometry on the kinetic and thermodynamic properties of intercalation compounds). Optimization of the host stoichiometry of high-voltage intercalation compounds into oxide host materials is of prime importance for their practical application. Intercalation processes into organic polymer host materials are discussed in Chapter 26. 

In those experiments, the solvent is distinguished from the host material by the huge difference in the transverse relaxation times. The technique to be described here monitors interdiffusion between two sample compartments initially filled with deuterated and undeuterated liquids (or gels) of the same chemical species. Bringing the compartments into contact initiates interdiffusion. Mapping of the proton spin density thus permits the evolution of the corresponding concentration profiles to be followed. 

Boulon G (2004) Optical Transitions of Trivalent Neodymium and Chromium Centres in LiNb03 Crystal Host Material 107 1-25... 

Insertion (intercalation) compounds. Insertion compounds are defined as products of a reversible reaction of suitable crystalline host materials with guest molecules (ions). Guests are introduced into the host lattice, whose structure is virtually intact except for a possible increase of some lattice constants. This reaction is called topotactic. A special case of topotactic insertion is reaction with host crystals possessing stacked layered structure. In this case, we speak about intercalation (from the Latin verb intercalare, used originally for inserting an extra month, mensis intercalarius, into the calendar). 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

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