Cent. Eur. J. Phys. • 9(2) • 2011 • 417-422DOI: 10.2478/s11534-010-0121-x

Central European Journal of Physics

Synthesis and characteristics of nanocrystallinematerials in Ti, B, C and N containing system

Research Article

Anna Biedunkiewicz∗, Paweł Figiel, Urszula Gabriel, Marta Sabara, Stanisław Lenart

Institute of Materials Science and Engineering, West Pomeranian University of Technology in Szczecin,Av. Piastow 19, 70-310 Szczecin, Poland

Received 27 July 2010; accepted 1 November 2010

Abstract: In this work, the results of investigations of manufacturing ceramic materials on the basis of Ti, B, C and Ncontaining systems are presented. The nanocrystalline ceramics were synthesized using a non-hydrolyticsol-gel method. The process was carried out in two stages. In the first low-temperature stage the precursorwas obtained. The synthesis of ceramic phases, however, was conducted in the second high-temperaturestage, in an argon atmosphere. Depending on the initial composition of the mixtures, the temperature andthe time, the following products were obtained: TiCx, TiCxN1−x, TiB2 and B4C.The course of the process was investigated by thermogravimetric and differential scanning calorimetrymethods (TG-DSC) coupled with mass spectrometry (MS). The solid state products were identified with useof X-ray diffraction (XRD). The size of the crystallites was estimated by the Scherrer method. The structureand morphology images of nanocrystalline powders were obtained using scanning electron microscopy(SEM) and transmission electron microscopy (TEM).

PACS (2008): 81.05.Je, 81.07.Bc, 81.07.Wx, 81.20.Fw

Keywords: powders • nanocrystallite • carbides • borides • sol-gel© Versita Sp. z o.o.

1. Introduction

Titanium diboride (TiB2) has attracted great interest dueto its excellent mechanical properties, chemical resistanceand good thermal and electrical conductivities. Thus, itis widely used for cutting tool composites, wear resis-tant parts, metal melting crucibles and electrode materials[1–5]. Exceptional properties were shown for fine-grainedTiB2, which is used during the manufacturing of tools usedin steel machining technologies [2, 5].

Fracture and wear resistance of TiC-TiB2 composites ob-

∗E-mail: Anna.Biedunkiewicz@zut.edu.pl

tained from TiB2 and TiC powders is higher than that ofsingle TiB2 and TiC phases [5]. The hardness of this com-posite was observed to increase with increasing tempera-ture [6, 7]. For this reason, these composites are used forthe manufacturing of matrices and cutting tools, heat ex-changer elements, and combustion engine elements. Thewall lining plates of nuclear reactors and constructionalelements of vacuum deposition installation are also man-ufactured from these powders [8, 9].Boron carbide (B4C) ceramics have some excellent phys-ical and chemical properties [10]. Its ultrahigh hardness,high wear and impact resistance and low specific weight,and good chemical stability make it suitable for applica-tion in ball mills, blasting nozzles, wheel dressing tools,wire drawing dies, rocket propellant, light weight armour

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Synthesis and characteristics of nanocrystalline materials in Ti, B, C and N containing system

plates and mechanical seal faces, as well as for grindingand cutting tools etc [11]. It is an excellent neutron ab-sorption material used in the nuclear industry due to itshigh neutron absorption coefficient [12, 13]. Densificationof monolithic B4C powder requires high temperatures, usu-ally above 2160°C, or application of high pressures owingto the covalent bonds [14, 15]. Another major limitation toits widespread use arises from its extreme susceptibilityto brittle fracture [16]. The tendency towards improvingits low strength and fracture toughness and reduce thesintering temperature have been observed. It has beenshown that a secondary phase, such as borides, added inthe B4C matrix can serve as a reinforcing and tougheningphase, which can also reduce the sintering temperature[13]. TiB2 has a relatively low crystalline boundary dif-fusion coefficient, which promotes slow densification andthus requires prolonged sintering times.Several methods have been utilized in the synthesis ofTiB2 materials. Commercially TiB2 powder is producedby reduction of titanium oxide and boron oxide with eithercarbon or an alkali metal. Among the above mentionedprocessing techniques, the carbothermal reduction processis commercially used and is by far the cheapest becauseof the inexpensive raw materials and simple process itrequires [16].Boron carbide can be prepared by reaction of elemen-tal boron and carbon powder, in a self-propagatinghigh-temperature synthesis (SHS), the carbothermalroute, reduction of BCl3 by CH4 at 1500°C by laser andlow temperature synthesis from a CCl4, BBr3 and Na mix-ture [10, 12, 17].The advantages of all phases are usually obtained in thecomposite. Overall combination of all phases brings outthe latent advantage of each while concurrently eliminat-ing shortcomings of the individual phases. In order toovercome the inherent limitations of the monolithic ce-ramic constituents, the primary aim of this research isto prepare nanocrystalline B4C/TiB2 and TiC/TiB2 com-posite powders. These powders were synthesized by anon-hydrolytic sol-gel method. The synthesis was car-ried out in two stages. In the first, low-temperature stagethe precursor was obtained [18, 19]. However, the syn-thesis of ceramic phases was carried out in the secondhigh-temperature stage under argon atmosphere. In thissystem TiCxN1−x, TiB2 and B4C were obtained, dependingon initial precursor composition, temperature and time.

2. ExperimentalTo investigate Ti-B-C-N systems, the precursor ob-tained from a mixture composed of 5 wt. % boron and

95 wt % gel containing polyacrylonitril, dimethylfor-mamid and titanium chloride was used [18, 19]. Allthe processes were carried out in an argon atmosphere.High-purity argon ‘Alphagaz 2 Ar’ from Air Liquide(H2O<ppm/mol, O2 < 0.1 ppm/mol, CnHm < 0.1 ppm/mol,CO< 0.1 ppm/mol, CO2 < 0.1 ppm/mol, H2 <0.1 ppm/mol) was used during the experiments. Aftergelling, the precursor with boron was held at a tem-perature of 110°C for 1 hour, followed by a tempera-ture of 400°C for 3 hours. These operations finished thefirst low-temperature stage. After this stage an adequateamount of titanium and boron in powder form was addedto the precursor. Metallic titanium powder 100 mesh (Ti,Aldrich) and amorphous boron powder (B, Aldrich) wereused. Titanium and boron were added to the metalorganicprecursor in amounts corresponding to 50 or 100 wt % ofthe precursor. After the low-temperature stage I, the pre-cursor, with addition of titanium and/or boron, was held attemperatures of 850, 1100 and 1300°C for 3 and 5 hours.The course of the processes was investigated using theTG-DSC method. The sample temperature and TG,DTG, and HF functions were registered. The inves-tigations were carried out using TG-DSC (SDT Q600,TA) coupled with MS (Thermostar GDS 301 Pfeif-fer Vacuum) for the identification of gaseous products.The argon purge flow rate during measurements wasset at 100 cm3/min. Measurements were performed innon-isothermal (β = 10 Kmin−1) and isothermal (3 and5 h) conditions. Samples weights below 30 mg were used.On the basis of thermoanalytical measurements, the tem-perature ranges required for the synthesis of TiCxN1−x,TiB2 and B4C powders in a Ti-B-C-N system, were de-termined, along with the temperature and time of sampleheating.The solid products were investigated by the XRD method.Nanoparticle size, and chemical and phase composi-tion were determined with the following techniques:XRD (Philips PW3040/x0 X’Pert Pro), TEM (JEOL JEM1200EX), SEM (JEOL JSM 6100), and EDX (Oxford In-struments, ISIS 300). The average crystallite size wasdetermined using the Scherrer equation with the Scherrerconstant (k) equal to around 0.90, following widespreadpractice [20, 21].

3. Results

In stage I, during the heating of the precursor at a temper-ature of 400°C, cyclization and carbonization of polyacry-lonitrile took place [22]. The synthesis of ceramic phaseswas carried out in the high-temperature stage. The heat-ing of the precursor with titanium and/or boron under ar-

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Anna Biedunkiewicz, Paweł Figiel, Urszula Gabriel, Marta Sabara,Stanisław Lenart

gon, at a temperature of 1300°C for 3-5 hours, resulted inthe formation of TiCxN1−x, TiB2 and B4C. The morphology

of the powder and a mapping of the Ti distribution in thepowder, are presented in Figure 1.

a

b c d

Figure 1. Scanning electron microscope images of powders: (a) SEM image, magnification ×10000, (b) SEI image, magnification ×100, (c) BEIimage, (d) and mapping of the Ti distribution in the powder.

On the basis of TG-DSC measurements the course of thesynthesis process of ceramic phases was determined. InFigure 2, TGu and HF functions are presented, regis-tered for the sample obtained after addition of 50 wt %Ti, and the sample obtained after addition of 50 wt % Tiand 50 wt % B, heated under argon at 1300°C for 3 hours.Initially, sample mass loss connected with the desorptionof volatile products was observed. Later, mass loss con-nected to the pyrolysis of PAN took place. Carbon ma-terial was formed. At temperatures above 850°C, the firstceramic phase i.e. (TiCxN1−x) was observed and, at higher

temperatures, i.e. 1100°C and 1300°C, TiB2 and B4C re-spectively, were observed. The processes proceeded withsample mass loss. In Figure 3, a diffractogram is shown ofthe product obtained in the Ti-B-C-N system after heattreatment at 1300°C for 3 h in argon.

Among the gaseous products formed during the heating ofthe sample at 1300°C under argon CH, C2H3, O2, CH3,C2H2 and C2H4 or/and N2 and H2 were identified. Themass spectra of C2H4 or/and N2 and H2 are presented inFigure 4.

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Synthesis and characteristics of nanocrystalline materials in Ti, B, C and N containing system

Figure 2. TGu and HF functions depending on sample temperature after addition of 50 wt % Ti to precursor and after addition of 50 wt % Ti and50 wt % B.

Figure 3. X-Ray diffraction of the product of the precursor with 50 wt. % Ti and 50 wt. % B transformation.

In Table 1 the composition of precursors are listed withthe values of the average sizes of TiB2 and TiCxN1−x

nanocrystallites obtained after heating under argon ata temperature of 1300°C for 3 hours. The average sizeof TiB2 (d112 line), TiCxN1−x (d311 line) and B4C (d021

line) nanocrystallites was determined based on XRD peakbroadening using the Scherrer formula.

Increasing the heating time from 3 to 5 hours affected thegrowth of the average crystallite sizes by about 40%. Thedetermined average sizes of TiB2, TiCxN1−x and B4C crys-tallites were in good accordance with the sizes observedin TEM images. TEM image and electron diffraction ofthe obtained nc-TiB2 particles are shown in Figure 5. Itwas stated that addition of Ti resulted in higher conver-sion of free carbon and boron. Additionally the growth

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Anna Biedunkiewicz, Paweł Figiel, Urszula Gabriel, Marta Sabara,Stanisław Lenart

a

b

Figure 4. DTG curves and mass spectra for C2H4/N2 and H2 identi-fied in gaseous products formed during the heating of thesample obtained after addition of 50 wt % Ti and 50 wt %B at 1300°C under argon. a) CH2=CH2/N2, b) H2.

Table 1. Listing of Ti-B-C-N system precursors composition with av-erage sizes of TiB2, TiCxN1−x and B4C nanocrystallites.

Id Addition to precursor [wt %] Average crystals size [nm]TiB2 TiCxN1−x B4C

1. 50 B 41 10 212. 50 Ti 43 16 273. 50 B + 50 Ti 51 8 284. 30 B + 70 Ti 48 11 265. 100 Ti 57 24 27

in interplanar spacing of the crystal lattice was observed,probably due to the increase in carbon content resultingfrom carbonization of TiCxN1−x. Along with the growth of

the boron fraction in proportion to the precursor, the frac-tion of TiB2 and B4C phases in comparison to TiCxN1−x,and the amount of unconverted boron, increased.

a

b

Figure 5. a) TEM image of the TiB2 nanocrystals, b) electron diffrac-tion of the nc-TiB2.

The initial composition of the samples obtained afteradding 50 wt % titanium and 50 wt % boron to the precur-sor, was assumed as the appropriate composition for car-rying out the synthesis of nanocrystalline ceramics. Thehighest fraction of TiB2 and B4C phases in comparison toTiCxN1−x was obtained.

4. SummaryUsing powders of pure titanium, boron and a metalorganicprecursor, a Ti-B-C-N system was investigated in orderto determine the synthesis conditions of ceramics contain-

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Synthesis and characteristics of nanocrystalline materials in Ti, B, C and N containing system

ing TiB2, B4C and TiCxN1−x. The phase composition ofthe precursor pyrolysis product was modified by the ad-dition of boron and titanium. The influence of precursorcomposition, and the heating time for the composition ofnanostructural ceramic obtained by its heating at a tem-perature of 1300°C, was investigated. The longer heatingtime favoured larger crystallite sizes. The increase in theTiB2 and B4C fractions in proportion to TiCxN1−x was ob-tained by addition of excess boron. The powders obtainedat a temperature of 1300°C after 3 hours were character-ized by large fractions of nc-B4C and nc-TiB2 crystallitessmaller than 100 nm.

AcknowledgementsThis work was financed by the Ministry of Science andHigher Education in the frame of realization of researchproject no. N N507 444334, 2008-2011.

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