**Advanced Techniques for Preparation of Strontium Aluminate Phases**

## **1. Techniques using powder precursors**

The sample prepared with 5 % of gypsum shows the diffraction lines of both, strontium sulfate and ettringite. Higher content of sulfate bearing phase, compared to cement with 2 % of gypsum, is also well visible from scanning electron microscopy images, where large amount of needle-like crystals grow on the surface of clusters consisting of partially hydrated grains

232 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The comparison of Fig.7 and Fig.8 as well as the results of calorimetry (Fig.3) indicate the change in hydration mechanism from the crystallization to the diffusion. There is a way to prepare expansive strontium aluminate cement, but the control of setting time of the paste is necessary in order to reach required rigidity, which enables to handle the expansion stress.

The positive effect was observed for calcined clay (Chapter 7.2).

of strontium aluminate cement.

The solid state synthesis described in Chapter 4.3 is the traditional way for the synthesis of strontium aluminate phase. Small amounts of fluxing agents (Fig.1), such as B2O3 (H3BO3) or LiF can be used [749,810].

**Figure 1.** Flow chart of the preparation of REE doped SrAl2O4 phosphor powders [749].

There are also advanced techniques of the synthesis of strontium aluminate phases including the sol-gel process, the precipitation from solution, the pyrolysis and combustion techniques, the microwave synthesis, the mechano-chemical route, etc. It must be pointed, that the techniques mentioned in this chapter are the sophisticated synthesis routes which are tailored mainly for the processing of nano-scale powders, doped with rare earth elements (REE ions such as Ce4+, Eu2+, Dy3+…), in order to prepare the precursors for strontium aluminate based phosphors for long-persistent luminescence materials, electronic and structural applications (Chapter 10.1). Therefore, the suitability of the techniques mentioned above for the synthesis of strontium aluminate clinker is limited by their complexity, time-consuming preparation process, labor productivity and by the cost of applied chemicals.

**The Sol-gel process** is a method based on the turn of the "*free dispersion system*" with particles of colloidal size (**sol**) into "*bound dispersion system"* (**gel**) with the particles cross-linked into the three-dimensional network. In principle there are two ways of the formation of gel:

**1.** Linking of growing particles into chains, connection of chains into branched chains and then into domains of three-dimensional networks (**microgel**). As the regions of formed

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

gel phase grow and are connected to each other, the continuous network of gel (**lyogel**) is formed. The volume of formed gel is approximately the same as that of the original sol. The process of gradual displacement of liquid phase from gel is termed as **syneresis**.

**Chemical Compound name Function References**

Alkoxides Al(*i*-OC3H7)3 Aluminium isopropoxide [755,760]

Additives 1) C2H5OCH2CH2OH Ethylene glycol monoethyl ether [759]

CH3COOH Acetic acid Chelating

(NH4)2S2O8 Ammonium persulfate Radical

ethylenediamin, TMEDA

2) Dopants enhance nitrates of REE element, the product is termed as ArAl2O4:REE (REE=Rare Earth Elements).

C6H16N2 N,N,N0,N0-tetramethyl-

4) In order to avoid the hydrolysis of alkoxide cations before mixing.

Dopants 2) Dy(NO3)3·xH2O Dysprosium (III) nitrate hydrate Modification

**Table 1.** Initial materials for the preparation of strontium aluminates via advanced techniques.

Sr(NO3)2·4H2O Strontium nitrate tetrahydrate [755,756,760,765]

Al(NO3)3·9H2O Aluminium nitrate heptahydrate [755,756,758]

H2O Water Solvent [759,760] HNO3 (aq) 3) Nitric acid [759,765] C2H5OH Ethanol [765] C3H8O2 2-methoxyethanol [760]

C3H8O3 Glycerol, propan-1,2,3-triol [759] C2nH4n+2On+1 PEG, Polyethylene glycol [755-758] NH4HCO3 Ammonium bicarbonate pH adjusting [755] NH4OH Ammonium hydroxide [757]

C6H8O7 Citric acid [756,757] C6H10O3 Ethyl acetoacetate 4) [760] (C6H9NO)n PVP, Polyvinylpyrrolidone [765] C3H5NO Acrylamide [761] C7H10N2O2 Methylene bisacrylamide [761]

Eu(NO3)3·xH2O Europium(III) nitrate hydrate [755,760,756]

Ce(NO3)3·6H2O Cerium(III) nitrate hexahydrate [758]

1) These chemicals form the polymeric network (Pechini and modified Pechnini methods), avoiding undesirable precipitation via the formation of chelates with metal ions (complexing agent), initiators or chemicals maintaining the

SrCO3 Strontium carbonate [757]

chemical

Advanced Techniques for Preparation of Strontium Aluminate Phases 235

agent

initiator

of luminescent properties [758]

[761]

[761]

[755,756]

[758,759]

Salts Sr(CH3(CO2))2 Strontium acetate monohydrate Starting

Water and organic liquids

pH in the system.

3) Dissolution of oxides.

The particles in sol may lose the stability (**DLVO theory**<sup>1</sup> [750]) due to ageing of sol (sponta‐ neously) or by the change of pH or temperature, addition of electrolytes and mechanic action (mixing, ultrasound) and the formation of flocs takes place. The process of formation of these clusters is termed as **flocculation** (**agglomeration**, **aggregation** and more generally as **coagulation**). The gel (**coagel**) is formed by the connections between aggregates after sedi‐ mentation

The capillary forces generate the strain stress in gel during the evaporation of solvent and the solid framework falls into pieces of **xerogel**<sup>2</sup> . Subsequent mechanical (grinding) and thermal treatment (calcination) lead to the powder precursor. According to the applied chemical the alkoxide-, semialkoxide-, Pechini-[751] and modified Pechini-route were recognized.

The sol–gel process is an efficient technique for the syntheses of phosphors due to good mixing of initial materials and relatively low reaction temperature resulting in more homogeneous products than those obtained by solid-state reaction synthesis method. The technique uses alkoxides and salts listed in Table 1. There are numerous modifications in applied solvent, pH, chelating agent as well as in the temperature and time of treatment, but the most general scheme of the synthesis includes these steps:


<sup>1</sup> Theory is named after Derjaguin B, Landau L, Verwey E and Overbeek J.

<sup>2</sup> There is an option to avoid these changes by drying gel under critical conditions in order to prepare aerogel.


1) These chemicals form the polymeric network (Pechini and modified Pechnini methods), avoiding undesirable precipitation via the formation of chelates with metal ions (complexing agent), initiators or chemicals maintaining the pH in the system.

2) Dopants enhance nitrates of REE element, the product is termed as ArAl2O4:REE (REE=Rare Earth Elements).

3) Dissolution of oxides.

gel phase grow and are connected to each other, the continuous network of gel (**lyogel**) is formed. The volume of formed gel is approximately the same as that of the original sol. The process of gradual displacement of liquid phase from gel is termed as **syneresis**. The particles in sol may lose the stability (**DLVO theory**<sup>1</sup> [750]) due to ageing of sol (sponta‐ neously) or by the change of pH or temperature, addition of electrolytes and mechanic action (mixing, ultrasound) and the formation of flocs takes place. The process of formation of these clusters is termed as **flocculation** (**agglomeration**, **aggregation** and more generally as **coagulation**). The gel (**coagel**) is formed by the connections between aggregates after sedi‐

234 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The capillary forces generate the strain stress in gel during the evaporation of solvent and the

treatment (calcination) lead to the powder precursor. According to the applied chemical the

The sol–gel process is an efficient technique for the syntheses of phosphors due to good mixing of initial materials and relatively low reaction temperature resulting in more homogeneous products than those obtained by solid-state reaction synthesis method. The technique uses alkoxides and salts listed in Table 1. There are numerous modifications in applied solvent, pH, chelating agent as well as in the temperature and time of treatment, but the most general

**a. Dissolving of initial materials:** the kind of solvent or liquid depends on initial materials. Oxides are usually dissolved in HNO3 of various concentrations. Water or water slightly acidified by the addition of several HNO3 drops (in order to avoid the hydrolysis) is used for the dissolution of nitrates. Alkoxides are mixed with organic solvent (methanol, ethanol, propanol, ethylene glycol…) which enables to control the process of hydrolysis via defined amount of water (water to alkoxide ratio). Other important parameters are

**b. Mixing of constituents:** chelating agents and pH adjusting chemicals are often used in order to avoid the undesirable precipitation and to improve the system homogeneity. For the Pechini and modified Pechini method [751,756] the ratio between chelating agent and metal cations is one of the most important parameters. Usually the value higher than one

**c. Concentrating of the solution and the formation of gel by the evaporation of solvent under stirring or by condensation reaction:** the prepared system is stirred for several hours. The required temperature (mostly in the range from 60 to 80 °C) and pH are kept constant during this time. The condensation reaction between chelating agent (most usually citric acid) and glycol requires an increase in temperature (100 – 150 °C), which is necessary for Pechini in order to obtain clear transparent color of viscous polymer

2 There is an option to avoid these changes by drying gel under critical conditions in order to prepare aerogel.

the catalysis, the temperature, the time and intensity of stirring.

1 Theory is named after Derjaguin B, Landau L, Verwey E and Overbeek J.

alkoxide-, semialkoxide-, Pechini-[751] and modified Pechini-route were recognized.

. Subsequent mechanical (grinding) and thermal

mentation

is used.

[67,752,753]:

solid framework falls into pieces of **xerogel**<sup>2</sup>

scheme of the synthesis includes these steps:

4) In order to avoid the hydrolysis of alkoxide cations before mixing.

**Table 1.** Initial materials for the preparation of strontium aluminates via advanced techniques.

Modified Pechini methods uses EDTA [754] as chelating agent or does not apply glycol.


electro-spinning combined with the sol-gel process is being widely employed to fabricate the

Advanced Techniques for Preparation of Strontium Aluminate Phases 237

The **precipitation method** or the chemical precipitation method exhibits some advantages, such as low processing temperature, high homogeneity and the purity of products. By this method, the nano-sized particles which are uniformly distributed, could be prepared [766]. In this method, a solution of the precursor reactants is mixed with dopants in an acid solution. Once the desired compound precipitates, the sample is centrifuged and washed repeatedly. The precipitate is treated at high temperature, then cooled and ground to the precursor of desired grain size [767]. The solvothermal co-precipitation synthesis uses an autoclave in order

The **combustion synthesis** techniques are classified according to the physical nature of the

**•** Solution-combustion synthesis (SCS) where the initial reaction medium is aqueous solution.

**•** Conventional SHS method where the reactants are in solid state (condensed phase com‐

The **solution-combustion method** is based on the preparation of mixture of nitrates of metal cations (Sr(NO3)2⋅4H2O, Al(NO3)3⋅9H2O) and dopants such as REE(NO3)3 with organic fuel (urea, CO(NH2)2). The temperature ranging from 60 to 80 °C is mostly used for this operation. The continuous stirring, lasting from several minutes to one hour, ensures the homogenization of mixture. The evaporation and recrystallization of the product provide the solution, gel, foam or fine powder for subsequent thermal treatment. The combustion is fast process (a few

to prepare the precipitate under hydrothermal conditions [768,769].

ceramics fibers.

bustion).

initial reaction medium [776]:

**Figure 2.** General scheme of the sol-gel process.

**•** Flame synthesis, i.e. gas phase combustion.

**f. Mechanical treatment of xerogel into the fine powder of precursor:** the calcined product is ground into fine powder of the precursor. This operation does not require any extensive force as porous and very brittle product is prepared. Some times the hand grinding in porcelain or agate dish is performed.

The prepared powder of precursor can be shaped by pressing, cold-isotactic pressing [762] or hot-isotactic pressing (HIP) [371]. Another option is the preparation of paste or suspension which can be deposited onto the substrate (e.g. by dip-or spin-counting [763,764], electropho‐ retic deposition, slip-or solution casting [761,764]…) or casted into the mound. The subsequent thermal treatment including the synthesis of required phases and the sintering can be per‐ formed in the furnace or by special sintering techniques. The method is schematically shown in Fig.2.

The possibility to prepare pure and fine product of good homogeneity under low processing temperature is the main advantage of the sol-gel method. The synthesis of SrAl2O4:REE fibers via the combination of electro-spinning with the sol-gel process were described by Cheng at al [765]. Ceramics can not be electrospun directly to fibers from the solution. Recently, the

**Figure 2.** General scheme of the sol-gel process.

(1)

Modified Pechini methods uses EDTA [754] as chelating agent or does not apply glycol.

are often used. The time of drying varies from several hours to a few days.

236 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

ly used for the preparation of strontium-aluminate precursor.

porcelain or agate dish is performed.

in Fig.2.

**d. Drying of gel to xerogel:** the temperatures in the temperature range from 100 to 200 °C

**e. Thermal treatment:** the pyrolysis of organic compounds and the calcination are two main processes which take place during this step. The temperatures higher than 400 °C applied for several hours are usually necessary in order to complete the removal of organics, but the course of process is highly affected by the nature of applied chemicals, by heating rate and by kiln atmosphere (static, inert, oxidative or reduc‐ tive). The temperatures about 600 °C are usually applied. The removal of carbon formed under the reductive conditions (or site reduction conditions) often requires much higher temperatures and affects the oxidation state of metal cations via the *p*CO/*p*CO2 ratio. Low partial pressure of oxygen also supports the formation of various organic com‐ pounds during the pyrolysis. At sufficient temperature, the solid-state reaction takes place during the process of calcination. Much higher temperature may cause the material to reach an undesirable degree of sintering which complicates the prepara‐ tion of fine powder of the precursor. The calcination temperature of 900 °C is usual‐

**f. Mechanical treatment of xerogel into the fine powder of precursor:** the calcined product is ground into fine powder of the precursor. This operation does not require any extensive force as porous and very brittle product is prepared. Some times the hand grinding in

The prepared powder of precursor can be shaped by pressing, cold-isotactic pressing [762] or hot-isotactic pressing (HIP) [371]. Another option is the preparation of paste or suspension which can be deposited onto the substrate (e.g. by dip-or spin-counting [763,764], electropho‐ retic deposition, slip-or solution casting [761,764]…) or casted into the mound. The subsequent thermal treatment including the synthesis of required phases and the sintering can be per‐ formed in the furnace or by special sintering techniques. The method is schematically shown

The possibility to prepare pure and fine product of good homogeneity under low processing temperature is the main advantage of the sol-gel method. The synthesis of SrAl2O4:REE fibers via the combination of electro-spinning with the sol-gel process were described by Cheng at al [765]. Ceramics can not be electrospun directly to fibers from the solution. Recently, the electro-spinning combined with the sol-gel process is being widely employed to fabricate the ceramics fibers.

The **precipitation method** or the chemical precipitation method exhibits some advantages, such as low processing temperature, high homogeneity and the purity of products. By this method, the nano-sized particles which are uniformly distributed, could be prepared [766]. In this method, a solution of the precursor reactants is mixed with dopants in an acid solution. Once the desired compound precipitates, the sample is centrifuged and washed repeatedly. The precipitate is treated at high temperature, then cooled and ground to the precursor of desired grain size [767]. The solvothermal co-precipitation synthesis uses an autoclave in order to prepare the precipitate under hydrothermal conditions [768,769].

The **combustion synthesis** techniques are classified according to the physical nature of the initial reaction medium [776]:


The **solution-combustion method** is based on the preparation of mixture of nitrates of metal cations (Sr(NO3)2⋅4H2O, Al(NO3)3⋅9H2O) and dopants such as REE(NO3)3 with organic fuel (urea, CO(NH2)2). The temperature ranging from 60 to 80 °C is mostly used for this operation. The continuous stirring, lasting from several minutes to one hour, ensures the homogenization of mixture. The evaporation and recrystallization of the product provide the solution, gel, foam or fine powder for subsequent thermal treatment. The combustion is fast process (a few minutes) which takes place after introducing the sample into the furnace heated to the temperatures from 400 to 600 °C. The temperature during the combustion may reach 1100 °C. The voluminous foamy ash after the combustion is then ground and used as a precursor for the flame-pyrolysis of oxide powder for the synthesis of strontium aluminates [770-773].

22 2 2 2 2 2 CO(NH ) +3 O 2 CO +2 N +4 H O ® (3)

Advanced Techniques for Preparation of Strontium Aluminate Phases 239

3663 2 2 2 2 2 C H N O +3 O 6 H O+6 N +6 CO ® (4)

It is believed that the detonation products do not have enough time to grow into large and perfect crystallites, when the mixture is subjected to shock waves. The detonation products mainly consist of particles with very small sizes containing high density of defects and

**Figure 4.** Schematic presentation of detonation method for the synthesis of strontium aluminate [774].

The **self-propagating high-temperature synthesis** (SHS) is one of the combustion methods [776-779] which uses strong exothermic reactions for the preparation of oxide ceramics [780], non-oxide ceramics [778,781-785] and cermets [786-789]. After mixing the powder of initial solid reactants and shaping (pressing, isotactic pressing) it the exothermic self-catalysed reaction is initiated. Once initiated, the combustion front becomes self-sustained, traveling through the sample due to the sufficient liberation of reaction heat and the final product is formed progressively with no additional heat required. The method can be applied whenever

For example, strong affinity of aluminium to oxygen causes, that the formation of aluminium oxide is a strong exothermic reaction. The reaction is well known and used for the alumino‐

welding of rail tracks. A tempting option arises to obtain the heat for the synthesis of strontiumaluminate via self-propagating high-temperature synthesis. SrAl2O4 was prepared by SHS

Milling is known as one of important unit operations and is widely used in various processing of materials such as minerals, food, medicine, chemicals and building materials. As an

method by Sathaporn at al [790] according to the general scheme in Fig.5.

, which are of huge practical demand for the production of ferroalloys and

dislocations [774,775].

thermic reactions3

3 Discovered by Nikolay Beketov.

a sufficiently exothermic chemical reaction is available.

**Figure 3.** Scheme of the combustion method.

Final thermal treatment in a weak reductive atmosphere leads to the SrAl2O4:REE phosphor [772]. It is obvious that the preparation is much faster if compared to the sol-gel process, but there is only a limited chance to control the conditions of thermal treatment. Fast formation of large amount of gases and the swelling of material may cause the losses of precursor. The method is schematically shown in Fig.3.

The **Detonation method** of the synthesis of SrAl2O4 was described by Li at al [774]. The detonation method is an efficient technique for the preparation of nanometer powders because of good mixing of the initial materials. At the same time, the size of prepared particles can be controlled to a certain extent by adjusting the detonation parameters; the production cost is lower compared to sol-gel process and the process can be finished in a short interval of time. During the detonation synthesis process, the mixtures composed of raw materials can undergo complex physical and chemical reactions and have series of changes, such as the conglomer‐ ation, the crystallization and the phase-translation at high temperature.

When exploding and decomposing, nitrates can release great quantum of oxygen. So they can be used as oxidizers or accessorial oxidizers in dynamite. The main equations of the reaction of detonation can be expressed as follows:

$$\text{Sr(NO}\_3\text{)}\_2 + 2\text{ Al(NO}\_3\text{)}\_3 \cdot 9\text{H}\_2\text{O} \rightarrow \text{SrAl}\_2\text{O}\_4 + 4\text{ N}\_2 + 18\text{ H}\_2\text{O} + 10\text{ O}\_2\tag{2}$$

$$2\text{ CO}(\text{NH}\_2)\_2 + 3\text{ O}\_2 \rightarrow 2\text{ CO}\_2 + 2\text{ N}\_2 + 4\text{ H}\_2\text{O}\tag{3}$$

$$2\,\mathrm{C}\_{3}\mathrm{H}\_{6}\mathrm{N}\_{6}\mathrm{O}\_{3} + 3\,\mathrm{O}\_{2} \to 6\,\mathrm{H}\_{2}\mathrm{O} + 6\,\mathrm{N}\_{2} + 6\,\mathrm{CO}\_{2} \tag{4}$$

It is believed that the detonation products do not have enough time to grow into large and perfect crystallites, when the mixture is subjected to shock waves. The detonation products mainly consist of particles with very small sizes containing high density of defects and dislocations [774,775].

The **self-propagating high-temperature synthesis** (SHS) is one of the combustion methods [776-779] which uses strong exothermic reactions for the preparation of oxide ceramics [780], non-oxide ceramics [778,781-785] and cermets [786-789]. After mixing the powder of initial solid reactants and shaping (pressing, isotactic pressing) it the exothermic self-catalysed reaction is initiated. Once initiated, the combustion front becomes self-sustained, traveling through the sample due to the sufficient liberation of reaction heat and the final product is formed progressively with no additional heat required. The method can be applied whenever a sufficiently exothermic chemical reaction is available.

For example, strong affinity of aluminium to oxygen causes, that the formation of aluminium oxide is a strong exothermic reaction. The reaction is well known and used for the alumino‐ thermic reactions3 , which are of huge practical demand for the production of ferroalloys and welding of rail tracks. A tempting option arises to obtain the heat for the synthesis of strontiumaluminate via self-propagating high-temperature synthesis. SrAl2O4 was prepared by SHS method by Sathaporn at al [790] according to the general scheme in Fig.5.

Milling is known as one of important unit operations and is widely used in various processing of materials such as minerals, food, medicine, chemicals and building materials. As an

minutes) which takes place after introducing the sample into the furnace heated to the temperatures from 400 to 600 °C. The temperature during the combustion may reach 1100 °C. The voluminous foamy ash after the combustion is then ground and used as a precursor for the flame-pyrolysis of oxide powder for the synthesis of strontium aluminates [770-773].

238 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

Final thermal treatment in a weak reductive atmosphere leads to the SrAl2O4:REE phosphor [772]. It is obvious that the preparation is much faster if compared to the sol-gel process, but there is only a limited chance to control the conditions of thermal treatment. Fast formation of large amount of gases and the swelling of material may cause the losses of precursor. The

The **Detonation method** of the synthesis of SrAl2O4 was described by Li at al [774]. The detonation method is an efficient technique for the preparation of nanometer powders because of good mixing of the initial materials. At the same time, the size of prepared particles can be controlled to a certain extent by adjusting the detonation parameters; the production cost is lower compared to sol-gel process and the process can be finished in a short interval of time. During the detonation synthesis process, the mixtures composed of raw materials can undergo complex physical and chemical reactions and have series of changes, such as the conglomer‐

When exploding and decomposing, nitrates can release great quantum of oxygen. So they can be used as oxidizers or accessorial oxidizers in dynamite. The main equations of the reaction

3 2 33 2 24 2 2 2 Sr(NO ) + 2 Al(NO ) 9H O SrAl O + 4 N + 18 H O+ 10 O × ® (2)

ation, the crystallization and the phase-translation at high temperature.

**Figure 3.** Scheme of the combustion method.

method is schematically shown in Fig.3.

of detonation can be expressed as follows:

<sup>3</sup> Discovered by Nikolay Beketov.

The preparation of SrAl2O4 nanoparticles by CO2 laser vaporization (LAVA) technology and laser melting method is described in works [794,795]. The LAVA technique is suitable for the preparation of a variety of ceramic nano-powders from coarse initial powders (usually oxides) although no specially designed precursors are required. The great advantage of the laser synthesis is the possibility to one-step, fast synthesis of these materials in air at the atmospheric

Advanced Techniques for Preparation of Strontium Aluminate Phases 241

The bulk single crystals of strontium aluminate can be prepared by the floating zone (FZ) or

Thin film phosphors have several advantages over powders, such as higher lateral resolution from smaller grains, better thermal stability, reduced out gassing and better adhesion to solid

**i. The Pulsed laser ablation (PLD):** is a preparatory technique with several attractive

**ii. Pulsed ion beam evaporation (IBE):** the method uses the high-power pulsed ion

beam which is focused on the target. Formed high density ablation plasma enables

features, including the stoichiometric transfer of target material, the generation of quality plume of energetic species, the hyper thermal reaction between ablated cations and molecular oxygen in the ablation plasma and the compatibility with background pressures ranging from UHV to 100 Pa. The plasma produced during the pulsed laser ablation is very energetic and its mobility can be easily controlled by changing the processing parameters [799-804]. The process involves various phe‐ nomena such as the target heating, the material removal and the plasma shielding. The absorption of laser radiation heats the target and the vaporization of target material takes place. Plasma shielding causes the drop in the laser intensity reaching

substrates [799]. The following methods can be used for the preparation of thin films:

laser floating zone (LZF) method [796,797] or by the Czochralsky technique [26,798]

pressure.

**2. Preparation of crystals**

**3. Preparation of thin layers**

the target surface [805].

to prepare thin film of material [806].

**iii. Electron beam bombardment (EBB**) [807].

**iv. RF magnetron sputtering** [808,809].

**Figure 5.** The flow diagram for the SHS synthesis of strontium aluminate.

extended branch in milling operations, the field of mechanochemistry has attracted much attention in recent years and the research papers on this topic have been increasing. One of unique phenomena in mechanochemistry is the solid state reaction among two or more multicomponents without heating, to produce a constituent compound. A potential application for this solid state reaction is not only the material synthesis but also the separation and recovery of chemical species and components from minerals and waste materials treated by the reaction through another chemical and/or physical operation [791].

The boundary of two or multi-components of solid material may be activated by the following reasons: When the sample powders are trapped and crushed between two balls colliding inside a ball mill pot, they undergo plastic deformation, and are repeatedly flattened, cold-welded, fractured and rewelded. The force of the impact acts on the powder particles, leading to the breakage of crystallographic bonds and new surface is produced. The new created surfaces enable the particles to weld together easily and this leads to an increase in the rate of dissolution of solid material. Mechanical deformation produces new surface via the formation of frag‐ ments and the increase of surface energy of the material. Other profound changes affecting the surface as well as the chemical, physico-chemical and structural properties may also take place. This is proved by the presence of variety of crystal defects such as increased number of grain boundaries, dislocations, vacancies and interstitial atoms, stacking faults, and deformed and ruptured chemical bonds. The presence of such defect structure enhances the diffusivity of solute elements. Consequently, grinding a mixture of two or more solids substances results in the micro-homogenization of initial components, and sometimes, it induces the formation and synthesis of new fine powders [791-793].

The preparation of SrAl2O4 nanoparticles by CO2 laser vaporization (LAVA) technology and laser melting method is described in works [794,795]. The LAVA technique is suitable for the preparation of a variety of ceramic nano-powders from coarse initial powders (usually oxides) although no specially designed precursors are required. The great advantage of the laser synthesis is the possibility to one-step, fast synthesis of these materials in air at the atmospheric pressure.

## **2. Preparation of crystals**

The bulk single crystals of strontium aluminate can be prepared by the floating zone (FZ) or laser floating zone (LZF) method [796,797] or by the Czochralsky technique [26,798]

## **3. Preparation of thin layers**

extended branch in milling operations, the field of mechanochemistry has attracted much attention in recent years and the research papers on this topic have been increasing. One of unique phenomena in mechanochemistry is the solid state reaction among two or more multicomponents without heating, to produce a constituent compound. A potential application for this solid state reaction is not only the material synthesis but also the separation and recovery of chemical species and components from minerals and waste materials treated by the reaction

240 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The boundary of two or multi-components of solid material may be activated by the following reasons: When the sample powders are trapped and crushed between two balls colliding inside a ball mill pot, they undergo plastic deformation, and are repeatedly flattened, cold-welded, fractured and rewelded. The force of the impact acts on the powder particles, leading to the breakage of crystallographic bonds and new surface is produced. The new created surfaces enable the particles to weld together easily and this leads to an increase in the rate of dissolution of solid material. Mechanical deformation produces new surface via the formation of frag‐ ments and the increase of surface energy of the material. Other profound changes affecting the surface as well as the chemical, physico-chemical and structural properties may also take place. This is proved by the presence of variety of crystal defects such as increased number of grain boundaries, dislocations, vacancies and interstitial atoms, stacking faults, and deformed and ruptured chemical bonds. The presence of such defect structure enhances the diffusivity of solute elements. Consequently, grinding a mixture of two or more solids substances results in the micro-homogenization of initial components, and sometimes, it induces the formation and

through another chemical and/or physical operation [791].

**Figure 5.** The flow diagram for the SHS synthesis of strontium aluminate.

synthesis of new fine powders [791-793].

Thin film phosphors have several advantages over powders, such as higher lateral resolution from smaller grains, better thermal stability, reduced out gassing and better adhesion to solid substrates [799]. The following methods can be used for the preparation of thin films:


**Chapter 10**

**Other Technical Applications of Strontium Containing**

The properties and application fields of other strontium containing materials are shortly described in this chapter. Although these material and substances shouldn't be directly related to strontium aluminate cements or strontium aluminates, this survey may be illustrative for the importance and mutual relationships between individual strontium bearing materials.

Some amorphous calcium aluminates are photosensitive and thus are potential candidates for optical information storage devices [128,790,810-813], hence the research on the strontium aluminates is recently very intensive. Strontium based aluminate phosphors (Table 1) are well known for their high quantum efficiency, long-lived afterglow, good chemical stability, and other excellent luminescent features, which make them appropriate candidate to replace the traditional II–VI based phosphors. These materials are successfully used in various applica‐ tions like luminous paints for highways, airports, buildings, ceramic products, textile industry, dial plates of glow watches, warning signs, escape routes, etc. [814]. The following properties

**• Phosphorescence:** when the substances slowly re-emit absorbed electromagnetic radiation (usually UV) in the form of visible light. The same effect, but absorbed energy is re-emitted

**• Thermoluminescence:** is a process at which the substance releases high-energy radiation in the form of visible light upon heating, which enables electrons to return to their positions.

**• Mechanoluminescence:** is a process at which the mechanical action causes emitting of light. In order to investigate their properties, various strontium aluminate phases occurring in the SrO – Al2O3 system were synthesized using the solid-state synthesis as well as nontraditional

According to the **Blasse's theory** [827] on the energy transfer mechanism in oxide phosphors, the critical energy transfer distance (*R*c) can be calculated from the concentration quenching

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are often mentioned with regard to these compounds:

immediately, is termed as **fluorescence**.

synthesis routes described in Chapter 9.

data using the following equation:

**Materials**

**1. Phosphors**
