Ultrasound assisted salts–metal reaction for synthesizing TiB2 particle

Ultrasound assisted salts–metal reaction for synthesizing TiB 2particles at lowtemperatureZhiwei Liu ⇑,Milan Rakita,Wilson Xu,Xiaoming Wang,Qingyou Han ⇑School of Engineering Technology,Purdue University,401North Grant Street,West Lafayette,IN 47906,USAh i g h l i g h t sA high yield of TiB 2particles (90.4%)can be obtained via ultrasound assisted salts–metal reaction at 700°C. Most of TiB 2particles (95%)are smaller than 300nm in size.The effects of ultrasound on the formation of TiB 2particles at low temperature are investigated.a r t i c l e i n f o Article history:Received 19August 2014Received in revised form 22October 2014Accepted 6November 2014Available online 15November 2014Keywords:TiB 2Salts–metal reaction Ultrasounda b s t r a c tIn situ TiB 2particles can be synthesized via salts–metal reaction by adding K 2TiF 6and KBF 4mixed salts into the Al melt.In this research,a novel ultrasound assisted technique was proposed with the aim of synthesizing TiB 2particles in the Al melt at low temperature by means of introducing high-intensity ultrasound into the salts–metal reaction at 700°C,which was at least 100°C lower than those in the conventional methods.The effects of ultrasound on the synthesis of TiB 2particles were investigated by comparing two groups of experiments with and without ultrasound.The results showed that the yield of TiB 2particles in the ultrasonically treated sample could reach 90.4%,whereas the value of which was around 28%in the control sample without ultrasound after 10min of reaction,indicating that ultra-sound was able to promote significantly the formation of TiB 2particles at low temperature.Most of in situ formed TiB 2particles (95%)were smaller than 300nm in size.Our research showed that significantly enhanced formation of small TiB 2particles in ultrasonic field can be attributed to faster dissolution of mixed salts powders into Al melt,accelerated mass transfer of solutes,as well as the promoted nucleation and growth of TiB 2particles due to the effects of ultrasonic cavitation.Ó2014Elsevier B.V.All rights reserved.1.IntroductionIn situ particulate reinforced Al composites have attracted a great deal of attentions in the aerospace and automotive indus-tries,as well as other structural applications due to their excellent properties,such as high specific strength and low weight [1].Recently,in situ TiB 2/Al composites have been studied further since TiB 2is particularly useful due to its excellent properties,such as high hardness and chemical inertness in Al matrix at elevated temperature [2]It is well known that in situ TiB 2particles can be synthesized via salts–metal reaction [3],which involves adding mixed Ti and B bearing salts (i.e.K 2TiF 6and KBF 4)to molten Al at high melting temperature,leading to the formation of TiB 2particles in Al matrix.Plenty of researchers have fabricated in situ formed TiB 2/Al alloy composites by using Al–K 2TiF 6–KBF 4system via salts–metal reaction.Kumar et al.[4]synthesized TiB 2particles by adding the mixed salts of K 2TiF 6and KBF 4into molten A356alloy at 800°C,and the size of in situ formed TiB 2particles was in the range of 0.5–1.5l m.Han et al.[5]also obtained in situ TiB 2particles in Al–Si alloy at 800°C,and the size of particles were smaller than 1l m.Xue et al.[6]fabricated in situ TiB 2particles reinforced 2014Al alloy composite at 870°C by using mixed salts as the main reactants,and the size of TiB 2particles was below 2l m.In their research,in order to obtain the final product of TiB 2phase the melting temperatures of aluminum were higher than 800°C.Generally,low melting temperature can lead to the formation of Al 3Ti phase,which decreases the yield of TiB 2phase in Al matrix [7].Based on the above researchers’work,it is clear that higher melting temperature can result in the formation of some TiB 2particles with the size exceeding 1l m.However,the existence of/10.1016/j.cej.2014.11.0431385-8947/Ó2014Elsevier B.V.All rights reserved.⇑Corresponding authors.Tel.:+17654945866;fax:+17654946219.E-mail addresses:lzw_6260@ (Z.Liu),hanq@ (Q.Han).large-sized particles in the matrix can result in the reduction tensile properties of materials,especially for the ductility and strength[8].Thereby,decreasing the formation of large-sized particles is desired in most cases.In addition,two issues areally associated with the high melting temperature.One is the manufacturing cost,for higher temperature demands moreThe other one is the burning loss of some alloying elements such Mg and Zn in the Al alloy.Although high temperature isfor the formation of TiB2phase,exploring an approach tosize TiB2particles with the size less than1l m at low melting perature is greatly meaningful for the industrial production.High-intensity ultrasound has been widely used in organicsynthesis,materials and organometallic chemistry,and industrial manufacturing processes[9–11],for the intensification of chemi-cal/physical processing applications can be promoted significantly in ultrasonicfields.Suslick[12]has reported that ultrasonic irradi-ation could increase reactivities by nearly a millionfold in chemical reactions.The effects of ultrasound arise from acoustic cavitation: the formation,growth,and implosive collapse of bubbles coupled to the ultrasonicfield[13].The above process can occur simulta-neously at millions of locations in a reactor within a few microsec-onds[14],which can achieve temperatures above5000K, pressures exceeding105kPa,and heating and cooling rates in excess of1010K/s[15–18].These extreme,transient conditions produced during acoustic cavitation can promote the reactions which need high temperature,high pressures,or long reaction times.Moreover,acoustic cavitation can generate some unique effects,such as shock waves,micro-jets,and acoustic streaming, which can increase mass transport to accelerate chemical reactions [19].Till now,the study of synthesis of TiB2particles via salts–metal reaction by using K2TiF6and KBF4as the reagents at low melting temperatures(as low as700°C)has never been reported.In addi-tion,the study of introducing high-intensity ultrasound in the salts–metal reaction is also limited.Thereby,this research extends the application of ultrasound in the production of materials.In this research,K2TiF6and KBF4salts are used as the reagents.A compar-ison of two experiments with and without ultrasound will be carried out to explore the effects of ultrasound on the synthesis of TiB2particles at low melting temperature.2.Experimental2.1.Raw materials and preparationPotassium hexafluorotitanate(K2TiF6,98%purity)and potas-sium tetrafluoroboride(KBF4,98%purity)were used as the reagents.Pure Al ingot was used as the matrix.Solid salt powders blends with a molar ratio of Ti/B=1/2were mixed sufficiently using a glass mortar.The amount of mixed salt powders added corresponded to the composition of Al-5wt.%TiB2.2.2.Synthesis of TiB2particles via salts–metal reactionA500g pure Al ingot was melted in a graphite crucible (U8Â13cm)in an electrical resistance furnace.An ultrasonic Nb probe with a cross sectional area of about5.06cm2was immersed into the melt and the temperature of the melt was remained stable at around700°C.The mixed salt powders were added into the melt.In the meantime,high-intensity ultrasound was introduced the melt.The experimental setup is illustrated in Fig.1.In the fabricating process,the Nb probe tip was immersed into molten mixed salts by less than1cm.The melt was treated by ultrasound for10min,and the slagfloated on the top of the melt was removed before pouring the melt into a steel mold to form an ingot.This sample was referred to as an ultrasonically treated sample(UT sample).The power of the ultrasonic generator was1.5kW and the frequency was20kHz.In order to investigate the effects of ultrasound on the synthesis of TiB2particles,a control sample was also fabricated at700°C without ultrasonic treatment.2.3.Extraction experiment for obtaining TiB2particlesIn this research,some TiB2particles were extracted from the samples for the further studies.By extraction experiment,the yields of TiB2particles in the salts–metal reaction with and without ultrasonic treatment can be calculated,and the size distri-butions of TiB2particles in both two samples can be analyzed as well.The detailed extraction procedure for obtaining TiB2particles was given below.Firstly,a small ingot was cut from the sample, and the surface of which was cleaned by sand paper.Secondly, the small ingot was dissolved in a15vol.%aqueous HCl solution in a beaker at room temperature.After dissolution,a layer of par-ticles were deposited at the bottom of the beaker,and then the HCl solution was decaned.Thirdly,TiB2particles were washed with water for several times until the supernatant displayed natural pH.In the end,these particles were washed with ethanol,and then dried by using an electric hair dryer.Since this research mainly concerned the synthesis of TiB2particles,aqueous HCl solution was used as the etchant based on the following considerations. TiB2can exist stably in HCl solution,whereas other compounds, such as Al3Ti can be dissolved.By which,the interference of other phases can be eliminated in the following calculation of yield of TiB2particles and analysis of their size distributions.2.4.Examination of samples2.4.1.XRD analysisThe phases formed in the two samples were both examined by X-ray diffraction(XRD,Bruker D8)using Cu K a radiation at40kV and40mA and a scan rate of0.10°/s.The extracted particles from two samples and the slags produced in both experiments were also analyzed by XRD to examine the purity of TiB2particles and the compositions of slags.2.4.2.Microstructural analysisThe microstructures of two samples deeply etched using0.05% HF solution were analyzed by scanning electron microscopy(SEM, s4800)equipped with an energy dispersive spectroscope(EDS) device for identifying the components in the samples.The ethanol solution containing TiB2particles in a baker was treated in an ultrasonic cleaning bath for several minutes to break up the clusters of particles,and then some solution was drop-casted on a transmission electron microscopy(TEM)grid.The morphology of TiB2particle was observed by using Titan80–300kV environ-mental electron microscope(Titan_80_300)operated at200kV.Schematic diagram of ultrasound assisted salts–metal reactionparticles.318Z.Liu et al./Chemical2.4.3.Calculating the yields of TiB2particlesIn order to evaluate the efficiency of ultrasound on the forma-tion of TiB2particles in the salts–metal reaction,the yields of TiB2particles in the control and UT samples were calculated, respectively.Three small ingots with a weight of about8g cut from each sample were treated by a completed extraction process, respectively.Three groups of extracted powders were weighted by an electric balance.And then the actual weight percentage of TiB2particles in the Al matrix could be calculated.Since the sample was fabricated corresponding to the composition of Al-5wt.%TiB2, the yield of TiB2particles could be further obtained.In the end,an average value of the yield was computed in order to decrease the experimental error.2.4.4.Measuring the size distributions of TiB2particlesZetasizer Nano ZS(Malvern)was used to measure the size dis-tributions of TiB2particles extracted from both samples,which could measure the particles with the size ranging from0.3nm to 5l m.Before the test,a small amount of TiB2powders were diluted with DI water.10measurements were conducted for each sample, and statistical analyses regarding to the results were also provided.3.Results and discussionAfter adding the mixed K2TiF6and KBF4powders into the pure Al melt irradiated with high-intensity ultrasound,the solid powders were dissolved into the melt in a very short time.Shortly, the melt surface turned red,indicating that plenty of heat was gen-erated during the salts–metal reaction;whereas,the dissolution rate of which became slower in the Al melt without ultrasound, suggesting that ultrasound was able to accelerate the dissolution of salts into the melt effectively.The temperature changes of the Al melt in both experiments were measured in one minute intervals for10min with a thermocouple which was immersed into the melt with a depth of about3cm,as shown in Fig.2.TheMoreover,some gases were also generated during the salts–metal reaction due to the direct dissociation of the salts which may occur as follows[20]:K2TiF6!2KFþTiF4ðgÞð1ÞKBF4!KFþBF3ðgÞð2Þ3.1.XRD analysis of samplesFig.3shows XRD patterns of the UT and control samples.The existence of obvious diffraction peaks of TiB2phase indicate that TiB2phase was in situ formed in ultrasonically treated sample. However,for the control sample,a rather weak TiB2phase peak was found,suggesting the amount of synthesized TiB2phase was much lower than that in the UT sample(as shown in the inset graph).This result indicates that high-intensity ultrasound could accelerate synthesis of TiB2particles;its effects on the formation of TiB2particles will be discussed later in this paper.Fig.4shows XRD patterns of the slags produced in the fabrica-tion of two samples.KAlF4and K3AlF6were the two main phases in both slags.One important feature should be mentioned is that a weak XRD diffraction peak corresponding to a phase containing B element was observed in the control sample slag,indicating that the transfer of B element from salts into the reaction interface has notfinished.Based on the XRD result of ultrasonically treated sample,the formation of TiB2phase could follow the reaction below[20]:3K2TiF6þ6KBF4þ10Al!3TiB2þK3AlF6þ9KAlF4ð3ÞMoldovan et al.[21]also proposed their reaction formula to describe the synthesis of TiB2phase:3K2TiF6þ6KBF4þ10Al!3TiB2þ12KFþ10AlF3ð4ÞIn both reaction formulas,TiB2phase was the only product2.Temperature changes of the Al melt with time during the fabrication of control samples.3.XRD patterns of ultrasonically treated and control samples and XRD patterns (inset)with2-Theta from26to44degrees.Z.Liu et al./Chemical Engineering Journal263(2015)317–3243193.2.Microstructural features of samplesFig.5a shows the typical microstructure of the deep-etched control sample.It is clear that two types of reinforcements were formed in the sample.One type was clusters of particles,and the other one was blocky particles,with the length beyond several micrometers.A more detailed view of the microstructure is shown in Fig.5b.A very small amount of TiB2particles identified by EDS existed in the matrix as clusters.On the other hand,a blocky Al3Ti particle identified by EDS was also found in the matrix,and due to the low content of Al3Ti phase,the related XRD test could not detect its existence.The abovefindings indicated that the salts–metal reaction was not complete in the molten aluminum at 700°C.Fig.6shows the microstructural features of deep-etched UT sample.The newly formed phase was located at the boundaries of a-Al grains with a network structure,as shown in Fig.6a.After magnification,we found that the reticular phase was composed of plenty of tiny in situ formed TiB2particles which were identified by EDS,and these TiB2particles had different sizes,as shown in Fig.6b and c.Due to the effect of particle pushing,in situ formed TiB2particles were pushed by a-Al dendrites to the grain boundaries during the solidification process.In addition,Fig.6d shows the typ-ical morphology of one of in situ formed TiB2particles,which was hexagonal in shape,and was about100nm in size.3.3.XRD analysis of the extracted TiB2particlesPhase compositions of the extracted TiB2particles from UV and purity,as shown in Fig.7.Only TiB2phase was detected in both samples,indicating that Al3Ti phase existed in the control sample had dissolved into the aqueous HCl solution completely.Based on the XRD results,it is clear that no any other phases would influ-ence the following analyses about the yields and size distributions of TiB2particles.3.4.Yields of TiB2particlesFig.8shows the measured weight percentages of in situ formed TiB2particles in the UT and control samples,as well as their yields by three groups of calculations.The average measured weight percentage of TiB2particles in the UT sample could reach4.52%, and the corresponding yield of TiB2was90.4%;whereas,the values were about1.40%and28%for the control sample.These statistic results are in good agreement with the above XRD and SEM analyses(Figs.3,5and6).It is clear that ultrasound was able to significantly improve the formation of TiB2particles in the salts–metal reaction.3.5.Size distributions of TiB2particlesThe size distributions of TiB2particles extracted from the con-trol and UT samples were shown in Figs.9and10,respectively, in which the detailed statistical analyses of the particles size were also attached.Overall,in situ formed TiB2particles in the control sample were smaller than700nm,in which approximate95% TiB2particles were smaller than300nm.The result clearly showed that low reaction temperature can decrease the TiB2particles size remarkably.However,a low reaction temperature can lead to a rather low yield of TiB2,which limits the fabrication of TiB2parti-cles at low temperatures.For the UT sample,the size distribution of in situ formed TiB2particles was similar with the control sample. Almost all TiB2particles were smaller than700nm,and around 94.6%TiB2particles were smaller than300nm.It is obvious that the use of ultrasound in the salts–metals reaction at low tempera-ture have its unique advantages.On the one hand,a high yield of TiB2particles can be obtained.On the other hand,the size of most of in situ formed TiB2particles can be controlled under300nm.3.6.Formation of TiB2particles in the salts–metal reactionBefore understanding the effects of ultrasound on the synthesis4.XRD patterns of slags of the UT and control samples,and XRD patterns(inset)with2-Theta from40to54degrees.5.(a)Typical microstructure of the contrast sample,and(b)higher magnification of image in the marked area inJournal263(2015)317–324and no TiB 2was obtained either.The above indicate that TiB 2could not be formed without Al phase,suggesting that Al as the medium formation of TiB 2phase in salts–metal reaction.findings,Feng et al.suggested that the formation the reactions below:3K 2TiF 6þ13Al !3Al 3Ti þ3KAlF 4þK 3AlF 2KBF 4þ3Al !AlB 2þ2KAlF 4Al 3Ti þAlB 2!TiB 2þ4AlOnce Al 3Ti and AlB 2formed in the Al melt (5)and (6),both of them immediately react Al matrix according to reaction (7).Besides just described formation mechanism,been proposed to explain the formation of TiB itation-growth mechanism [24]based on the The detailed process for synthesis of TiB 2metal reaction can be described as follow.After solve in the molten aluminum,Ti and B released salts and diffused into Al melt across the salts/Al rather small sized Al 3Ti and AlB 2by reduction phases can dissolve in the Al melt yer liquid Al containing Ti and B adjacent formed.These processes can be expressed Al 3Ti !dissolving½Ti Al þ3AlAlB 2!dissolving2½B Al þAlWhen the solutes Ti and B in liquid Al reached might be separated out as the compounds according to the following reactions:½Ti Al þ3½Al !Al 3Ti 2½B Al þ½Al !AlB 2½Ti þ2½B !TiB microstructure of the ultrasonic treated sample,(b)higher magnification of image in the marked area in (a),(c)more feature formed TiB 2particle.Fig.7.XRD patterns of extracted TiB 2particles from the UT and control samples.Fig.8.Yields of in situ formed TiB 2particles in the UT and control samples.Among these three compounds,TiB2is the most thermodynam-ically stable phase due to its lowest free energy formation[25].The formation of products is influenced not only by the thermody-namic factor,but also by the kinetic factor at a certain temperature. As well known that reaction(12)can occur to form TiB2phase, thereby thefinal formation of TiB2phase is controlled by the transfer of Ti and B from molten salts to liquid Al.At a certain tem-perature,after the concentrations of Ti and B reach saturation in the Al melt,TiB2nuclei are formed,and then TiB2particles start to grow up due to the deposition of more Ti and B.In our research, in situ formed TiB2particles had a rather small size.We suggest that the precipitation-growth mechanism might be more suitable to explain the formation of TiB2particles during the salts–metal reaction.The following analysis of the effects of ultrasound on the synthesis of TiB2particles will be mainly carried out according to this mechanism.During the precipitation-growth process for synthesizing TiB2 particles,temperature plays a key factor in the salts–metal reaction.Higher temperature can make the molten salts more unstable,and Ti and B atoms are able to break away more easily from the salts.Thereby,the diffusion rates of Ti and B are increased as a result of the increase of diffusion coefficient,and more Ti and B can cross the salts/Al interface,leading to a faster saturation of Ti and B in the Al melt.Consequently,TiB2nuclei can be formed more easily.In contrast,when the melting temperature is low,the transfer of Ti and B from molten salts to the Al melt is hindered,leading to a limitation of the synthesis of TiB2phase[26].Moreover,more K2TiF6and KBF4salts will decompose according to reactions(1) and(2),which reduce Ti-and B-transfer efficiencies.Since the decompositions of both salts are inevitable,the actual yield of TiB2phase in Al matrix is always less than the theoretical yield.Also,it has been reported that the reaction between K2TiF6and Al could take place at a higher rate than that between KBF4and Al [27].In addition,KBF4is less stable than K2TiF6in the temperature interval of750–950°C[19],suggesting that reaction(2)will be conducted more easily compared to reaction(1),and more B will be lost in the form of BF3gas.Thereby,the ratio of Ti and B entered into the Al melt is higher than½,resulting in the formation of Al3Ti phase.The existence of Al3Ti phase in the control sample in this research agrees with the above analysis.3.7.Effects of ultrasound on the synthesis of in situ TiB2particlesIn the present research,it is demonstrated that high-intensity ultrasound was able to promote the synthesis of TiB2phase effec-tively via the salts–metal reaction at low melting temperature.It is well known that ultrasonicfields in a liquid can give rise to nonlin-ear effects,such as acoustic cavitation and acoustic streaming[28], both of which can influence the process of chemical reactions.AFig.9.Size distribution of TiB2particles extracted from the control sample. Fig.10.Size distribution of TiB2particles extracted from the UT sample.developed cavitation generated by ultrasonic vibration in the melt can be evaluated by two ultrasonic parameters:one is the frequency and the other is the acoustic intensity.Generally,the frequency of high-intensity ultrasound might be influenced by the temperature of the melt.High temperature can result in the decrease of the ultrasonic frequency.In this research,the fre-quency change of ultrasound in the Al melt was measured in one minute intervals for10min with a frequency meter.The related results were shown in Fig.11.Before immersing the Nb probe into the melt,the ultrasonic frequency was20.01kHz.After10min,the value of frequency was around19.84kHz.It is clear that the ultra-sonic frequency decreased a little bit,but its attenuation was rather limited.Thereby,the actual working frequency of intro-duced into the melt in this research was able to arouse ultrasound.The acoustic intensity pkin the melt can be expressed by the following expression[29]:p k ¼2P q L C L12ð13Þcavitation.Based on this calculation,it is reasonable to believe thatthe cavitation was able to reach the developed stage in molten salts.The synthesis of TiB2particles undergoes the processes ofnucleation and growth,in which the saturability of Ti and B atomsand their diffusion rates from the molten mixed salts to Al meltboth influence the formation of TiB2particles.Thereby,the effectsof ultrasound on promoting the formation of TiB2particles can beinvestigated in terms of the above aspects.In order to explain thesynthesis of TiB2in ultrasonicfield,a schematic illustration aboutwas shown in Fig.12.As mentioned above,a rapid dissolution of solid mixed salts inthe Al melt was obtained in our research,which was attributed tothe effects of ultrasonic cavitation.For Al–K2TiF6–KBF4system,ultrasound can generate microjet impact and shock wave damagein front of the surfaces of solid salts[9].These two physical effectsresulted in the localized erosion which can greatly promote thedissolution rate of solid salts.The time when ultrasound is of intro-duced to the Al melt is denoted as t1.Generally,with the presenceof molten salts Ti and B atoms start to diffuse into Al melt acrossthe interface of salts and Al melt.As the time increases,the concen-trations of Ti and B adjacent to the interface increase due to the Fig.11.The working frequency of ultrasound in the salts–metal reaction.Fig.12.Schematic illustration showing the synthesis mechanism of small size TiB2particles in ultrasonicfield(t:the holdingZ.Liu et al./Chemical Engineering Journal263(2015)317–324323of time.Due to the strong turbulent mixing and acoustic streaming, TiB2particles are pushed far away from the solute-rich region,and the growth of TiB2particles can be hindered effectively.Thereby, the size of in situ formed TiB2particles was smaller than300nm in our research.The mechanical disturbances created by both cavitation and ultrasonic streaming alter thefluid dynamics and increase bulk-phase mass transfer.On the one hand,supersaturation of Ti and B in the Al melt could be obtained easily,and Ti and B had more chance to contact in ultrasonicfield,leading to the formation of more effective TiB2nuclei in the Al melt.On the other hand,rapid mass transfer of the solute could provide more Ti and B on the surface of the growing TiB2particles.In addition,extremely high temperature and pressure created by bubble collapse in ultrasonicfield can give rise to the following effects[33]:(1)subsequent rapid local cooling rates,calculated at 107–1010K/s,play a significant role in increasing supersaturation;(2)localized pressure increases reduce the crystallization temperature,and(3)the cavitation events allow the excitation energy barriers associated with nucleation to be surmounted.The above effects can accelerate the nucleation and growth rates of TiB2particles obviously.4.ConclusionsHigh-intensity ultrasound was introduced during the synthesis of in situ TiB2particles from the Al–K2TiF6–KBF4system via the salts–metal reaction.The effects of ultrasound on the formation of TiB2particles at low melting temperature were studied.The following conclusions are drawn.(1)Due to the effect of ultrasonic cavitation,TiB2particles canbe in situ synthesized in the Al melt at700°C.The yield of TiB2particles synthesized could be promoted remarkably from28%(without ultrasound)to90.4%by using ultrasound.(2)The size of in situ formed TiB2particles was decreasedsignificantly due to the low melting temperature.And most of TiB2particles(around95%)were smaller than300nm in size.(3)Ultrasound assisted the salts–metal reaction,as a simpleapproach was proposed for synthesizing TiB2ceramic parti-cles with small size,which expands the use of ultrasound in the fabrication of materials.References[1]Q.Zhang,W.H.Wu,G.Q.Chen,L.T.Jiang,B.F.Luan,The thermal expansion andmechanical properties of high reinforcement content SiCp/Al composites 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