MaterialsChemistryandPhysics243(2020)12259)Contents lists available at ScienceDirectMATERIALSCHEMISTRSICSANEMaterials Chemistry and Physics飞ELSEVIERjournalhomepage:www.elsevier.com/locate/matchemphys?opataCore/multi-shell particles based on TiH2, a high-performance thermallyactivated foaming agentM. Romero-Romeroab, C.Dominguez-Rios, R. Torres-Sanchez, A. Aguilar-ElguezabalaCentro de Imvesigacion en Materiales Avanzados S.C and Laboratorio Nacional de Nanotecnologia, Chihuahu, 31136, Mexico Instituto Tecnologico de Chihuchue, Tecnologico Nacional de Mexico, Chihuahue, 31310, MexicoHIGHLIGHTSGRAPHICALABSTRACT.TiH/Ti,O@TiO/SiO,core/multi-shellparticlesweresynthesisedviaLisothermal heating and sol-gel process.+V89.1nm4 It was possible to increase the tempera-72.1 nm89.1 nm72.1 nmture at which hydrogen is released theTiH2TTiH2core from 440to601°Cwhenmulti-shell is formed on TiH2 surface.OTi,o国C These core/multi-shell particles possessTiO2国suitable properties for use as potentialfoaming agents for aluminium alloys.SiO2国TiH2/Ti30@Ti02TiHz/SiO2TiH2/Ti30@TiOz/SiOARTICLEINFOABSTRACTKeywords:The key element in the production of metal foams is the availability of a gas provider, which must be able toTiH2 powderrelease thegas once the metal reaches the liquidus temperature,allowing the homogeneous formation of pores inCore/shell particlea narrow size distribution. TiH2 is by far the most widely used foaming agent for aluminium alloy foaming; Sol-gel processhowever, this compound starts to release hydrogen during heating below 450 °C, and the liquidus temperature ofFoaming agentaluminium alloys is above 600 °c. In this work we studied the formation of a core/shell structure based on TiH2Aluminiumfoamparticles.The best results were obtained with a TiH2/TigO@TiO2/SiO2 core/multi-shell structure, which starts torelease hydrogen around 600 °C.1.Introductiondeveloped to produce AAFs [1-4], the melting and powder metallurgyroutes being the most widely used processes. Both methods require theMetal foams are considered metal and gas composites and, due todecomposition of afoaming agent,which supplies thegas that inducestheir potential applications, are increasingly arousing worldwide inter-the foaming of the aluminium alloy. Powdered titanium hydride (TiH2)est. In particular, aluminium alloy foams (AAFs) have already positivelyis the most used foaming agent [5-11], due mainly to its high efficiencyimpacted the automotive, rail and naval industries, as well as theshown in the volumetric expansion of Al and AiSi, alloys [12].building industry; foam applications have also found uses in acousticOne of the challenges in the production of high quality AAFs is toisolation and industrial machinery[1].Several methods have beenproduce them with uniform pore size distribution and with desirable andCorresponding author.E-mail addresses:carlos.dominguez@cimav.edu.mx, carlos560405@yahoo.com.mx (C,Dominguez-Rios).https://doi.org/10.1016/j.matchemphys.2019.122591Received 21 May 2019; Received in revised form 2 December 2019; Accepted 30 December 2019Available online 2 January 20200254-0584/2020 Elsevier B.V.All rights reserved
Materials Chemistry and Physics 243 (2020) 122591 Available online 2 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved. Core/multi-shell particles based on TiH2, a high-performance thermally activated foaming agent M. Romero-Romero a,b , C. Domínguez-Ríos a,* , R. Torres-Sanchez � a , A. Aguilar-Elguezabal a a Centro de Investigacion � en Materiales Avanzados S.C. and Laboratorio Nacional de Nanotecnología, Chihuahua, 31136, Mexico b Instituto Tecnol� ogico de Chihuahua, Tecnol� ogico Nacional de M�exico, Chihuahua, 31310, Mexico HIGHLIGHTS GRAPHICAL ABSTRACT � TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles were synthesised via isothermal heating and sol-gel process. � It was possible to increase the temperature at which hydrogen is released the TiH2 core from 440 to 601 �C when multi-shell is formed on TiH2 surface. � These core/multi-shell particles possess suitable properties for use as potential foaming agents for aluminium alloys. ARTICLE INFO Keywords: TiH2 powder Core/shell particle Sol-gel process Foaming agent Aluminium foam ABSTRACT The key element in the production of metal foams is the availability of a gas provider, which must be able to release the gas once the metal reaches the liquidus temperature, allowing the homogeneous formation of pores in a narrow size distribution. TiH2 is by far the most widely used foaming agent for aluminium alloy foaming; however, this compound starts to release hydrogen during heating below 450 �C, and the liquidus temperature of aluminium alloys is above 600 �C. In this work we studied the formation of a core/shell structure based on TiH2 particles. The best results were obtained with a TiH2/Ti3O@TiO2/SiO2 core/multi-shell structure, which starts to release hydrogen around 600 �C. 1. Introduction Metal foams are considered metal and gas composites and, due to their potential applications, are increasingly arousing worldwide interest. In particular, aluminium alloy foams (AAFs) have already positively impacted the automotive, rail and naval industries, as well as the building industry; foam applications have also found uses in acoustic isolation and industrial machinery [1]. Several methods have been developed to produce AAFs [1–4], the melting and powder metallurgy routes being the most widely used processes. Both methods require the decomposition of a foaming agent, which supplies the gas that induces the foaming of the aluminium alloy. Powdered titanium hydride (TiH2) is the most used foaming agent [5–11], due mainly to its high efficiency shown in the volumetric expansion of Al and AlSi7 alloys [12]. One of the challenges in the production of high quality AAFs is to produce them with uniform pore size distribution and with desirable and * Corresponding author. E-mail addresses: carlos.dominguez@cimav.edu.mx, carlos560405@yahoo.com.mx (C. Domínguez-Ríos). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys https://doi.org/10.1016/j.matchemphys.2019.122591 Received 21 May 2019; Received in revised form 2 December 2019; Accepted 30 December 2019
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 12259121] when compared to untreated TiH2.Table 1Synthesised foaming agents and their descriptionAs an alternative, multi-shell structured particles have been studiedas a way to increase the decomposition temperature of foaming agents.Method of synthesis of shell(s)Description and nomenclatureProa-Flores et al. [22] synthesised pre-oxidised TiH2 powders and theAs-received titanium hydride, TiH2TiH2/TiO2 structure obtained was then covered by a nickel layerCore/shell particles, TiH,/SiO2Sol-gel coatingdeposited by an electroless process. The resulting multi-shell powdersCore/double-shell particles, TiH/Heating of particles in airTigO@TiO2allowed an increaseintemperatureof hydrogenreleaseto525°C.TheCore/multi-shell particles, TiH2/Heating of particles in air and thereafterauthors reported that this multi-shell arrangement on TiH2 allowedTi30@TiO2/Si02sol-gel coatingobtaining a more homogeneous and reproducible pore structure whencompared to untreated TiH2Despite the improvement in pore structure and process controlreproducible properties such as low density, low thermal conductivity(allowing betterreproducibility of foam structure) by usingpre-oxidisedand high capacity to absorb energy by deformation or damping. Toand core/multi-shell TiH2 powders, the temperature difference betweenobtain a uniform porous structure, both melting and powder metallurgyfoaming agent gas release(decompositiontemperature)andliquidusprocesses face the problem of premature hydrogen release from TiH2stemperature of aluminium alloy still is large. Thus, it is fundamental toinasmuch asthere isa wide differencebetweentheTiH2decompositiondevelop foaming agents that are able to liberate the gas ata temperaturetemperature(450C[13])andtheliquidustemperatureofthetypicalasnearaspossibleto,oreveninsidetherangeof,theliquidustemper-aluminiumalloy (~600-650° [14]).In themeltingroute, onceTiH2isature of the aluminium alloy,to increase the opportunities for the suc-mixed with the molten aluminium alloy there is not enough time tocessful development of applications ofaluminium alloy-based foams.distribute the powder uniformly, due to its premature thermal decom-Taking into account the information mentioned above, this workposition.Ontheotherhand,inthepowdermetallurgyprocess,whenthefocuses on the development of a foaming agent structured as a core/compacted mix of aluminium alloy and TiH2 powders are heated themulti-shell based on TiH2 by the formation of three oxide layers. Thehydrogen gas is released when the aluminium alloy is barely melted,multi-shell was synthesised to increase substantiallytheTiH2decom-which leads to a heterogeneous pore distribution.position temperature up to the liquidus temperature range of typicalIn order to improve the foaming of aluminium alloys, the tempera-aluminium alloys, allowing these core/multi-shell particles to becometure of gas release from the TiH2 must be increased to near 600C; thus,better homogeneity of pore size distribution can be obtained duringfoaming.OnewaytodelaythereleaseofhydrogenfromTiH,istocover00the TiH2 particles with barriers to increase the temperature of thermal000.-000.0d)decomposition. A core/shell arrangement being necessary for this pur-pose, such that the shell is able to delay the heat transfer from thea*aluminium alloy to the TiH2 core. Another mechanism to delay gas*(e) aa*c)0xoxxoAXarelease isto have a sealedshell structure thatrestrains theprematureTiH,release of gas.Theeasiest way to obtain the core/shell structure isby thecontrolled oxidation of the surface of the TiH2 particles to form a TiO2oTiO,*tshell. For this purpose, TiH2 particles are heated in air, thus promotingxATi,ob) o山tothe formation of a surface layer of TiO2.The use of surface-oxidised TiH2 (TiH2/TiO2) in the melting routehas been reported for metal foaming, where the advantage of the delay4**in gas release is used to improve pore size homogeneity, based on大a)+increasing the mixing time [13,15-17]. A detailed study of the thermal儿八decomposition of TiH2/TiO2 and its impact on the structure and prop-erties of AAFswascarriedoutbyRomero-Romero etal.[16],wherean7020304050809060onset of release of hydrogen at540°C was achieved, instead of 440C,20 ()which was the temperature observed for the gas release from TiH2.Pre-oxidised TiH, has also been used in powder metallurgy, and severalFig. 2. X-ray diffraction patterns of a) TiH2, b) TiH2/TigO@TiO2, c) TiH/workshavereportedanimprovementinporehomogeneity[14,18,19]Tig0@TiO2/SiO2treated at 450°C and d)TiH:/Tig0@TiO2/SiO2treated at1100°℃.as well asanincrease inmechanical propertiesandprocesscontrol [20,b)8ra)T61(%) aunoA15I14王IANHHIIH*2-1fIHom0.151010010001μmParticle size (μm)Fig. 1. a) SEM micrograph of as-received TiH2 and b) particle size distribution of TiH2 powders2
Materials Chemistry and Physics 243 (2020) 122591 2 reproducible properties such as low density, low thermal conductivity and high capacity to absorb energy by deformation or damping. To obtain a uniform porous structure, both melting and powder metallurgy processes face the problem of premature hydrogen release from TiH2, inasmuch as there is a wide difference between the TiH2 decomposition temperature (~450 �C [13]) and the liquidus temperature of the typical aluminium alloy (~600–650 �C [14]). In the melting route, once TiH2 is mixed with the molten aluminium alloy there is not enough time to distribute the powder uniformly, due to its premature thermal decomposition. On the other hand, in the powder metallurgy process, when the compacted mix of aluminium alloy and TiH2 powders are heated the hydrogen gas is released when the aluminium alloy is barely melted, which leads to a heterogeneous pore distribution. In order to improve the foaming of aluminium alloys, the temperature of gas release from the TiH2 must be increased to near 600 �C; thus, better homogeneity of pore size distribution can be obtained during foaming. One way to delay the release of hydrogen from TiH2 is to cover the TiH2 particles with barriers to increase the temperature of thermal decomposition. A core/shell arrangement being necessary for this purpose, such that the shell is able to delay the heat transfer from the aluminium alloy to the TiH2 core. Another mechanism to delay gas release is to have a sealed shell structure that restrains the premature release of gas. The easiest way to obtain the core/shell structure is by the controlled oxidation of the surface of the TiH2 particles to form a TiO2 shell. For this purpose, TiH2 particles are heated in air, thus promoting the formation of a surface layer of TiO2. The use of surface-oxidised TiH2 (TiH2/TiO2) in the melting route has been reported for metal foaming, where the advantage of the delay in gas release is used to improve pore size homogeneity, based on increasing the mixing time [13,15–17]. A detailed study of the thermal decomposition of TiH2/TiO2 and its impact on the structure and properties of AAFs was carried out by Romero-Romero et al. [16], where an onset of release of hydrogen at 540 �C was achieved, instead of 440 �C, which was the temperature observed for the gas release from TiH2. Pre-oxidised TiH2 has also been used in powder metallurgy, and several works have reported an improvement in pore homogeneity [14,18,19], as well as an increase in mechanical properties and process control [20, 21] when compared to untreated TiH2. As an alternative, multi-shell structured particles have been studied as a way to increase the decomposition temperature of foaming agents. Proa-Flores et al. [22] synthesised pre-oxidised TiH2 powders and the TiH2/TiO2 structure obtained was then covered by a nickel layer deposited by an electroless process. The resulting multi-shell powders allowed an increase in temperature of hydrogen release to 525 �C. The authors reported that this multi-shell arrangement on TiH2 allowed obtaining a more homogeneous and reproducible pore structure when compared to untreated TiH2. Despite the improvement in pore structure and process control (allowing better reproducibility of foam structure) by using pre-oxidised and core/multi-shell TiH2 powders, the temperature difference between foaming agent gas release (decomposition temperature) and liquidus temperature of aluminium alloy still is large. Thus, it is fundamental to develop foaming agents that are able to liberate the gas at a temperature as near as possible to, or even inside the range of, the liquidus temperature of the aluminium alloy, to increase the opportunities for the successful development of applications of aluminium alloy-based foams. Taking into account the information mentioned above, this work focuses on the development of a foaming agent structured as a core/ multi-shell based on TiH2 by the formation of three oxide layers. The multi-shell was synthesised to increase substantially the TiH2 decomposition temperature up to the liquidus temperature range of typical aluminium alloys, allowing these core/multi-shell particles to become Table 1 Synthesised foaming agents and their description. Description and nomenclature Method of synthesis of shell(s) As-received titanium hydride, TiH2 – Core/shell particles, TiH2/SiO2 Sol-gel coating Core/double-shell particles, TiH2/ Ti3O@TiO2 Heating of particles in air Core/multi-shell particles, TiH2/ Ti3O@TiO2/SiO2 Heating of particles in air and thereafter sol-gel coating Fig. 1. a) SEM micrograph of as-received TiH2 and b) particle size distribution of TiH2 powders. Fig. 2. X-ray diffraction patterns of a) TiH2, b) TiH2/Ti3O@TiO2, c) TiH2/ Ti3O@TiO2/SiO2 treated at 450 �C and d) TiH2/Ti3O@TiO2/SiO2 treated at 1100 �C. M. Romero-Romero et al
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 122591b)Pa)727('ne)()xTi,oTiH,oTiO26052004006008001000120020040060080010001200Raman shift (cm-)Ramanshift(cm-l)c)d)40fman signal()(cne) isaTiH,OTiH,/TiO,@Ti,oTi,oTiO,2004001200200400600800100012006008001000Ramanshift (cm-l)Raman shift(cm-1)Fig. 3. Raman spectra of a) TiH2 powder, b) TigO@TiO2 shells, c) TiH2/TigO@TiO2 sample and d) deconvolution of TiH2/TigO@TiO2-a)440b)443TiH,/SiO,A[1070sio,1035()aoo(ne)aoeosq95218975-9358041200100080060040012001000800600400Wavenumber(cm")Wavenumber(cm-)Fig. 4. FTIR spectra of a) SiO2 particles and b) TiH2/SiO2 sample.promising foaming agents for aluminium alloys.A study of the thermal2.2.1.Synthesis ofTiH2/SiO2core/shell particlesperformance of foaming agents is presented, as well as a detailed studyThe TiH/SiO2 foaming agent was synthesised by the Stober processof the morphology and phase compositions of the layers on TiH2.[23] with modifications, in order to obtain a layer of SiO2 on the TiH2particles. For this purpose, 260 mL of a solution of ethanol (99.75%),2.Materials and methodsammoniumhydroxide(28.8wt%of NH3)and deionisedwater inavolume ratio 6.55:3.66:1, was prepared.The measured pH value was11-12,whichwascontrolled during the wholeprocess.Asuitable2.1.StartingmaterialsamountofSiO2precursor(TEOS)wasusedtoobtainashellofaround100 nm thickness on the TiH2 particles.Foaming agents were prepared using TiH2 powder (Sigma-Aldrich,Inthefirststage,2.5gofTiH2particlesweresonicatedfor30minin98% purity,<44μm)as core and TEOS (tetraethoxysilane,Alfa Aesar,ethanol,and1/6ofthetotalamountofammoniumhydroxidewas99%purity)toform Si02layers.added to the TiH2 suspension under vigorous conventional stirring. Thestirring was maintained for 15min.Then 1/6 of the total amount of2.2.Synthesis of foaming agents with core/shell structureTEOS was added ata rate of 0.05mLmin-1,thetemperature setat40Cand the solution kept under stirring for 24 h. In the second stage, theSeveral foaming agents were prepared using TiH2 starting particles,remaining ammonium hydroxide solution was added, followed by thewhich were treated to obtain particles surrounded by shells. Table 1addition of the deionised water. After 15 min stirring, the remainingsummarises the prepared materials and their description, as well as theamountof TEOSwas addedata rateof 0.05mLmin-and theresultantnomenclature used throughoutthis work.3
Materials Chemistry and Physics 243 (2020) 122591 3 promising foaming agents for aluminium alloys. A study of the thermal performance of foaming agents is presented, as well as a detailed study of the morphology and phase compositions of the layers on TiH2. 2. Materials and methods 2.1. Starting materials Foaming agents were prepared using TiH2 powder (Sigma-Aldrich, 98% purity, < 44 μm) as core and TEOS (tetraethoxysilane, Alfa Aesar, 99% purity) to form SiO2 layers. 2.2. Synthesis of foaming agents with core/shell structure Several foaming agents were prepared using TiH2 starting particles, which were treated to obtain particles surrounded by shells. Table 1 summarises the prepared materials and their description, as well as the nomenclature used throughout this work. 2.2.1. Synthesis of TiH2/SiO2 core/shell particles The TiH2/SiO2 foaming agent was synthesised by the Stober € process [23] with modifications, in order to obtain a layer of SiO2 on the TiH2 particles. For this purpose, 260 mL of a solution of ethanol (99.75%), ammonium hydroxide (28.8 wt % of NH3) and deionised water in a volume ratio 6.55:3.66:1, was prepared. The measured pH value was 11–12, which was controlled during the whole process. A suitable amount of SiO2 precursor (TEOS) was used to obtain a shell of around 100 nm thickness on the TiH2 particles. In the first stage, 2.5 g of TiH2 particles were sonicated for 30 min in ethanol, and 1/6 of the total amount of ammonium hydroxide was added to the TiH2 suspension under vigorous conventional stirring. The stirring was maintained for 15 min. Then 1/6 of the total amount of TEOS was added at a rate of 0.05 mL min 1 , the temperature set at 40 �C and the solution kept under stirring for 24 h. In the second stage, the remaining ammonium hydroxide solution was added, followed by the addition of the deionised water. After 15 min stirring, the remaining amount of TEOS was added at a rate of 0.05 mL min 1 and the resultant Fig. 3. Raman spectra of a) TiH2 powder, b) Ti3O@TiO2 shells, c) TiH2/Ti3O@TiO2 sample and d) deconvolution of TiH2/Ti3O@TiO2. Fig. 4. FTIR spectra of a) SiO2 particles and b) TiH2/SiO2 sample. M. Romero-Romero et al
Materials Chemistry and Physics 243 (2020) 122591M,Romero-Romero et al.TiKad)279248217OK186155TiL12493AIK62TiKCK2um4.026.030.000.671.342.012.683.354.695.36280.OKe)252224196168140112SiKTiL8456TiK28umO0.000.470.942.352.823.293.764.231.411.88TiKOKf)O261232203174145116SiKTIL8758AIKTiK292μm0.000.671.342.012.683.354.024.695.366.03Fig.5. SEM micrographs of a) TiH/Tig@TiO2, b) TiH2/SiO2, c) TiH2/Tig@TiO2/SiO2. Elemental EDS microanalysis for samples d) TiH2/TigO@TiO2, e) TiH2/SiO2 and ) TiH/TigO@TiO,/SiO2.Aluminium signal is due to the material of the holder used to support the samples.solution maintained under stirring for 24 h.2.2.2.SynthesisofTiH2/Tig0@TiO2core/double-shell particlesThe obtained TiH2/SiO2 particles were recovered by centrifugation.To synthesise the TiH2/TigO@TiO2foamingagent,TiH2particlesThe washing of the recovered particles was carried out with ethanolwere heated isothermally to oxidise the particle surfaces. The isothermalunder sonication. Particles were recovered by centrifugation andheating was carried out at 500 °C for 60 min in air.For this purpose,particles were spread on an alumina plate, which was previously heatedwashed by sonication once again.Finally, the solution was keptrestingovernight, the ethanol decanted and the particles dried at 100 °C for 60at 500 C. Synthesis conditions were defined based on results describedh.elsewhere [16], where it was demonstrated that TiH2 powders heatedisothermally at 500 c presented the best thermal decomposition
Materials Chemistry and Physics 243 (2020) 122591 4 solution maintained under stirring for 24 h. The obtained TiH2/SiO2 particles were recovered by centrifugation. The washing of the recovered particles was carried out with ethanol under sonication. Particles were recovered by centrifugation and washed by sonication once again. Finally, the solution was kept resting overnight, the ethanol decanted and the particles dried at 100 �C for 60 h. 2.2.2. Synthesis of TiH2/Ti3O@TiO2 core/double-shell particles To synthesise the TiH2/Ti3O@TiO2 foaming agent, TiH2 particles were heated isothermally to oxidise the particle surfaces. The isothermal heating was carried out at 500 �C for 60 min in air. For this purpose, particles were spread on an alumina plate, which was previously heated at 500 �C. Synthesis conditions were defined based on results described elsewhere [16], where it was demonstrated that TiH2 powders heated isothermally at 500 �C presented the best thermal decomposition Fig. 5. SEM micrographs of a) TiH2/Ti3O@TiO2, b) TiH2/SiO2, c) TiH2/Ti3O@TiO2/SiO2. Elemental EDS microanalysis for samples d) TiH2/Ti3O@TiO2, e) TiH2/ SiO2 and f) TiH2/Ti3O@TiO2/SiO2. Aluminium signal is due to the material of the holder used to support the samples. M. Romero-Romero et al
M. Romero-Romero et al.Materials Chemistry and Physics 243 (2020) 122591b)aTi2umc)d)si0Fig. 6. a) SEM micrograph of a TiH2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, e) O and d) Si signals.attributes compared to othertreatments.SiO2-TiH2, synthesised foaming agents and core free-shells were analysedby Raman spectroscopy using Horiba LabRam HR VIS-633equipment2.2.3.SynthesisofTiH2/Ti3O@TiO2/SiO2core/multi-shellparticlesThe foaming agent TiH2/TigO@TiO2/SiO2 was synthesised byequipped with a He-Ne laser (632.8 nm).isothermal heating as described above (Section 2.2.2) followed by theWhile synthesis ofTiH2/SiO2andTiH/Tig0@TiO2/SiO2foamingSiO2 sol-gel coating process described in Section 2.2.1.agentswas carried out,theSiO2particles suspended intheupper partofthe solution were recovered prior to the centrifugation step.Theseparticles and TiH2/SiO2 were analysed by FTIR spectroscopy using a2.3.SiO2andTigO@TiO,shellsPerkinElmerSpectrumGXFTIR spectrometer.In order to characterise the SiO2 and TigO@TiO2 shells on the TiH2/2.4.3.ScanningelectronmicroscopySiO2and TiH2/Tig0@TiO2particles,~6g ofcore/shell particleswereAnalyses by scanning electron microscopy (SEM) were made with akeptunderstirringinasolutionofHCl/H2Oina1/1volratiountil thefieldemission JEOLJSM-7401Fmicroscope to studythe morphology ofcore(TiH2)was dissolved.The obtained shells were recovered by vac-the TiH2 and the synthesised foaming agents, as well as SiO2 and theuum filtration and washed with ethanol.Ti30@TiO2 shells.TheTiH2/Tig0@TiO2/SiO2particles were encapsu-lated in a polymeric matrix and subsequently prepared using the con-2.4.Characterisation of thefoaming agents and shellsventional metallographic process with the purpose of transversallyexposing the particles for shell thickness measurement. The samples2.4.1.Particle size distributionwere analysed by energy dispersive x-ray spectroscopy (EDS)AMastersizer20o0laserparticlesizeanalvserfromMalvernInstruments was used to determine the particle size distribution of TiH22.4.4.TransmissionelectronmicroscopyA study of themorphology of SiO2and TigO@TiO2 shells wasmadepowders.Themeasurementofthesamplewasperformedthreetimes.bytransmissionelectronmicroscopy(TEM)usingaHitachi7700mi-2.4.2. Phase analysiscroscope.Thespecimensof TEMwerepreparedasfollows:eachsampleTiH2powders,as well as thesynthesised foamingagents,werewas dispersed in ethanol by sonication, afterwards two drops of theanalysed by X-ray diffraction (XRD) using a Panalytical Xpert'PROresulting suspensionweredeposited in aTEMgrid byusingacapillarydiffractometer. The XRD patterns were obtained under the followingtube, and finally, moderate heating was applied to evaporate theconditions: Cu Ka radiation, 20 scanning angle varied from 20° to 90°,ethanol.step size of 0.033° along with a 40 s step time. In other experiments,TiH2/Ti30@Ti02/Si02washeatedisothermallyat450Cfor120minand1100Cfor120min inordertoobtaincrystallinephasesofTi02and
Materials Chemistry and Physics 243 (2020) 122591 5 attributes compared to other treatments. 2.2.3. Synthesis of TiH2/Ti3O@TiO2/SiO2 core/multi-shell particles The foaming agent TiH2/Ti3O@TiO2/SiO2 was synthesised by isothermal heating as described above (Section 2.2.2) followed by the SiO2 sol-gel coating process described in Section 2.2.1. 2.3. SiO2 and Ti3O@TiO2 shells In order to characterise the SiO2 and Ti3O@TiO2 shells on the TiH2/ SiO2 and TiH2/Ti3O@TiO2 particles, ~6 g of core/shell particles were kept under stirring in a solution of HCl/H2O in a 1/1 vol ratio until the core (TiH2) was dissolved. The obtained shells were recovered by vacuum filtration and washed with ethanol. 2.4. Characterisation of the foaming agents and shells 2.4.1. Particle size distribution A Mastersizer 2000 laser particle size analyser from Malvern Instruments was used to determine the particle size distribution of TiH2 powders. The measurement of the sample was performed three times. 2.4.2. Phase analysis TiH2 powders, as well as the synthesised foaming agents, were analysed by X-ray diffraction (XRD) using a Panalytical Xpert’PRO diffractometer. The XRD patterns were obtained under the following conditions: Cu Kα radiation, 2θ scanning angle varied from 20� to 90�, step size of 0.033� along with a 40 s step time. In other experiments, TiH2/Ti3O@TiO2/SiO2 was heated isothermally at 450 �C for 120 min and 1100 �C for 120 min in order to obtain crystalline phases of TiO2 and SiO2. TiH2, synthesised foaming agents and core free-shells were analysed by Raman spectroscopy using Horiba LabRam HR VIS-633 equipment equipped with a He–Ne laser (632.8 nm). While synthesis of TiH2/SiO2 and TiH2/Ti3O@TiO2/SiO2 foaming agents was carried out, the SiO2 particles suspended in the upper part of the solution were recovered prior to the centrifugation step. These particles and TiH2/SiO2 were analysed by FTIR spectroscopy using a PerkinElmer Spectrum GX FTIR spectrometer. 2.4.3. Scanning electron microscopy Analyses by scanning electron microscopy (SEM) were made with a field emission JEOL JSM-7401F microscope to study the morphology of the TiH2 and the synthesised foaming agents, as well as SiO2 and the Ti3O@TiO2 shells. The TiH2/Ti3O@TiO2/SiO2 particles were encapsulated in a polymeric matrix and subsequently prepared using the conventional metallographic process with the purpose of transversally exposing the particles for shell thickness measurement. The samples were analysed by energy dispersive x-ray spectroscopy (EDS). 2.4.4. Transmission electron microscopy A study of the morphology of SiO2 and Ti3O@TiO2 shells was made by transmission electron microscopy (TEM) using a Hitachi 7700 microscope. The specimens of TEM were prepared as follows: each sample was dispersed in ethanol by sonication, afterwards two drops of the resulting suspension were deposited in a TEM grid by using a capillary tube, and finally, moderate heating was applied to evaporate the ethanol. Fig. 6. a) SEM micrograph of a TiH2/SiO2 particle. Elemental mapping obtained by SEM-EDS: b) Ti, c) O and d) Si signals. M. Romero-Romero et al