Lorena Pardo

Álvaro García

Klaus Brebøl

Elisa Mercadelli

Carmen Galassi

Enhanced properties for ultrasonic transduction, phase transitions and thermal depoling in 0.96(Bi 0.5 Na 0.5 )TiO 3 -0.04BaTiO 3 submicron structured ceramic

Keywords: Dielectric materials, oxides, 77.84.Bw, piezoelectric materials, 77.84.-s, electromechanical resonance, 77.65.Fs, materials treatment effects on Microstructure, 81.40.-z, Structure of crystalline solids, 61.66.-f

Submicron structured (grain size≈500nm), dense (Bi 0.5 Na 0.5 ) 0.96 Ba 0.04 TiO 3 ceramics were obtained from sol-gel autocombustion nanopowders by hot-pressing (700-950⁰C) and subsequent recrystallization (1000-1050⁰C). Electromechanical coefficients were obtained by analysis of the resonance spectra of thin disks using Alemany et al. software. The real part of the room temperature set of coefficients of the best performing materials (ε σ 33 =(416 -16 i), d 31 = (-22.68 + 0.55 i)pC/N -1 , k t = 44.5%, k p =21.1% ) can be compared with those of coarsegrained ceramics and d 33 (95 pC.N -1 ) is higher. Shear related coefficients were obtained from thickness poled and length excited plates (ε σ 11 = (402 -89i), d 15 =(108.3 -21.4 i) pC.N -1 and k 15 =39.2%). At the depolarization temperature, T d =153⁰C, the dielectric loss, tanδ(T), of poled samples shows a maximum and the planar resonance virtually vanishes. Shear resonance of thickness-poled plates and weak planar electromechanical resonance are observed above T d .

The relaxor behaviour extends up to the isotropization point, T i =238⁰C. This can be understood as due to the coexistence of the room temperature ferroelectric phase in the stability range of the low temperature non-polar phase at zero field, between T d and T i .

Introduction

Within the expanding area of the study of lead-free piezoelectrics, the (1-x) (Bi 0.5 Na 0.5 )TiO 3 -xBaTiO 3 (BNT-BT) (BNBT100x) solid solution system [START_REF] Takenaka | Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 )TiO 3 -and (Bi 1/2 K 1/2 )TiO 3 -Based Bismuth Perovskite Lead-Free Ferroelectric Ceramics[END_REF] is being an area of high interest due to the peculiarities of its crystal structure [START_REF] Van Eerd | Structural complexity of (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 as revealed by Raman Spectroscopy[END_REF]. Recently [START_REF] Ma | Domain structure-dielectric property relationship in lead-free (1-x)(Bi 1/2 Na 1/2 )TiO 3 -xBaTiO 3 ceramics[END_REF][START_REF] Jo | Evolving morphotropic phase boundary in lead-free (Bi 1/2 Na 1/2 )TiO 3 -BaTiO 3 piezoceramics[END_REF] it has been found that the phase diagram of unpoled ceramics differs from the one of poled ceramics, due to the electric-field induced phase transition observed for compositions near the MPB in BNT-based solid solution systems [START_REF] Hiruma | Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 ) TiO 3 -(Bi 1/2 K 1/2 ) TiO 3 -Ba x TiO 3 LeadFree Piezoelectric Ceramics[END_REF][START_REF] Daniels | Electric-field-induced phase transformation at a lead-free morphotropic phase boundary: Case study in 93%(Bi 0.5 Na 0.5 )TiO 3 -7% BaTiO 3 piezoelectric ceramic[END_REF][START_REF] Pitch | Structural properties of (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 lead-free piezoelectric ceramics[END_REF][START_REF] Royles | Electric-fieldinduced phase switching in the lead-free piezoelectric potassium sodium bismuth titanate[END_REF]. The nearly vertical Morphotropic Phase Boundary (MPB) composition, at x≈0.06 for unpoled ceramics, can be shifted towards lower x values by the action of an electric field [START_REF] Van Eerd | Structural complexity of (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 as revealed by Raman Spectroscopy[END_REF][START_REF] Pitch | Structural properties of (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 lead-free piezoelectric ceramics[END_REF], which induces a ferroelectric phase. Due to these peculiarities, the electromechanical properties at the MPB are not as high as expected by similitude with the Pb(Zr,Ti)O 3 system. Another drawback of the MPB composition for some applications is that depoling temperature, at which most of the electromechanical activity vanishes by a mechanism still unclear, is low (T d ≈100⁰C) [START_REF] Xiao | Temperature stability of Lead-free Piezoelectric Ceramics of Perovskite Bi 0.5 Na 0.5 TiO 3 and K 0.5 Na 0.5 NbO 3 Families[END_REF].

When moving away from the MPB composition towards the (Bi 0.5 Na 0.5 )TiO 3 side of the phase diagram, T d increases approaching that of BNT (T d ≈187⁰C ) [START_REF] Hiruma | Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics[END_REF] and the electromechanical coupling factors show higher anisotropy, i.e., higher ratio k t /k p for the thickness and radial modes of thin disks, thickness poled [START_REF] Chu | Electrical properties of Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 ceramics[END_REF]. In such a range of compositions, the reported electromechanical characterization in the ultrasonic range for x=0.04 (BNBT4) ceramics is incomplete, lacking parameters related to shear modes of resonance. Shear coefficients have a key importance for 3D modeling of piezoceramics by numerical methods [START_REF] Pardo | Shear Resonance Mode Decoupling to Determine the Characteristic Matrix of Piezoceramics for 3-D Modelling[END_REF] due to the increasing number of their applications that are based on shear modes.

An issue in the preparation of BNBT ceramics is the loss of volatiles (Bi , Na) at the high sintering temperatures required (T>1100⁰C), which leads to a.c. losses and d.c. conductivity that limits the poling and, consequently, the piezoelectric performance of the ceramics [START_REF] Hiruma | Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics[END_REF].

Sol-gel combustion synthesis [START_REF] Mercadelli | Sol-gel combustion synthesis of BNBT powders[END_REF] provides highly reactive nanopowder at moderate synthesis temperature (500⁰C). Dense, submicron structured BNBT6 ceramics with good stoichiometry and properties that can be compared with those of coarse grain ceramics have been obtained by hot-pressing powder produced by sol-gel combustion and further recrystallization at T≤1050⁰C [START_REF] Moure | Improvement by Recrystallization of Aurivillius-type Piezoceramics from Mechanochemically Activated Precursors[END_REF][START_REF] Pardo | Piezoelectric properties of leadfree submicron structured (Bi 0.5 Na 0.5 ) 0.94 Ba 0.06 TiO 3 ceramics[END_REF]. Processing of ceramics from nanopowders allows nowadays to obtain fine grain ceramics. These are of interest for both the basic studies of size-effects in ferroelectrics and for their use as high frequency ultrasonic transducer materials, since they allow the miniaturization of the components.

Submicron structured BNBT4 ceramics from nanopowder, prepared by hot-pressing and recrystallization, are studied in this work as candidates for ultrasonic transducer materials.

Electromechanical characterization in the ultrasonic range is carried out from the resonances of thin disks, thickness poled and excited, and thickness-poled shear plates, length excited. Alemany et al. software [START_REF] Alemany | Automatic iterative evaluation of complex material constants in piezoelectric ceramics[END_REF][START_REF] Alemany | Automatic determination of complex constants of piezoelectric lossy materials in the radial mode[END_REF][START_REF] Pardo | A Non-Standard Shear Sample for the Matrix Characterization of Piezoceramics and itsValidation Study by Finite Element analysis[END_REF]] is here used to analyze the impedance spectra at resonance and to calculate complex material coefficients. The thermal evolution of the dielectric permittivity and losses, as well as the one of the planar and shear modes of resonance, is also determined to evaluate the working range of the material.

Experimental method

(1-x) (Bi 0.5 Na 0.5 )TiO 3 -xBaTiO 3 with x=0.04 (BNBT4) nanometric powder was synthesized by a citrate, nitrate sol-gel auto-combustion method, as explained elsewhere [START_REF] Mercadelli | Sol-gel combustion synthesis of BNBT powders[END_REF]. In this process, the pure perovskite phase is directly obtained by heating the gel at 500ºC. Pellets of the powder of 15 mm diameter and 1 mm thickness were obtained by uniaxial pressure forming followed by cold isostatic pressing at 200 MPa. A combination of hot-pressing at low temperature (700-950⁰C for 2h, at 20MPa in air, using heating and cooling rates of 3⁰C.min -1 ) and subsequent recrystallization at higher temperature (1000 and 1050⁰C for 1-2h in air) has been applied to sinter the green pellets. The BNBT4-1 to 3 ceramics studied were obtained with variable grain size and density to determine the microstructure leading to the optimum properties. Ceramics were characterized using X-ray diffraction (XRD, Siemens D500) and scanning electron microscopy (FEG-SEM, NOVA NANOSEM 230 with beam deceleration module) and their density was obtained by Archimedes method. Computer assisted quantitative microstructural characterization (MIP45 Digital Image System) was carried out by analysis and measurements of images of SEM and optical microscopy (OM, Leitz Laborlux 12 MES/ST). Both mean grain and pore size (<G> and <P>) of the ceramics were determined as the equivalent diameter to a circular shape (D eq =4(S/π) 1/2 , where S=grain or pore surface area) and from the analysis of the measured lognormal distributions of equivalent diameter [START_REF] Ricote | Microstructure-properties relationships in Sm-modified lead titanate piezoceramics. Part I: Quantitative study of the microstructure[END_REF] that also provides the corresponding standard deviations (σ G and σ P ).

Sintered disks were ground to a thickness (t) to diameter (D) ratio below 1/20 (typically t=0.7 mm and D=15 mm) and Ag electrodes were attached at their major faces for the electric poling and for the impedance measurements at the radial and thickness resonance modes.

Rectangular plates of lateral dimensions (L,w) to thickness (t) aspect ratio below 1/10 (typically t=0.7 mm and L,w= 8-9 mm) were also cut from the sintered ceramic disks, electroded and thickness poled. Poling electrodes were mechanically removed and a new pair of electrodes was applied on two of their perpendicular faces for the impedance measurements.

Owing to the reduced d.c. ceramic conductivity, it was possible to carry out poling of disks and plates in a silicone oil bath at T p =150-180⁰C up to fields of 45 kV.cm -1 . Hot-poling took place for 30 minutes and the samples were cooled to room temperature with the field applied.

The d 33 piezoelectric coefficient was measured in a Berlincourt-meter.

Piezoelectric coefficients of ferroelectric ceramics are currently calculated using standard methods [START_REF]IEEE Standard on piezoelectricity[END_REF][START_REF]Piezoelectric properties of ceramic materials and components. Part 2: methods of measurement -Low power[END_REF] from the measurements of complex impedance data at the electromechanical resonances. To overcome the well-known limitations of standard methods when dealing with material losses, alternative methods have been developed and will be used in this work. Fine-tuning of the thickness of the shear plates was carried out to minimize the coupling of the fundamental shear mode with other plate modes [START_REF] Pardo | Shear Resonance Mode Decoupling to Determine the Characteristic Matrix of Piezoceramics for 3-D Modelling[END_REF].

Complex impedance measurements at resonance were carried on using a HP4192A impedance analyzer controlled via a GPIB-PCIIA (National Instruments) interface board.

Alemany et al. software [START_REF] Alemany | Automatic iterative evaluation of complex material constants in piezoelectric ceramics[END_REF][START_REF] Alemany | Automatic determination of complex constants of piezoelectric lossy materials in the radial mode[END_REF][START_REF] Pardo | A Non-Standard Shear Sample for the Matrix Characterization of Piezoceramics and itsValidation Study by Finite Element analysis[END_REF] was used here for the calculation of the complex piezoelectric, elastic and dielectric coefficients in each mode of resonance, as well as for the determination of the corresponding electromechanical coupling factors and frequency numbers. After the calculation of the complex parameters, the resonance spectrum is reconstructed as an accuracy test of the final set of calculated coefficients, quantified by the regression factor (R 2 ) of such reconstruction.

Impedance measurements as a function of the temperature were carried out at various frequencies in the range between 1kHz and 1MHz with a HP4194A analyzer. The heating and cooling rates were 2⁰C/min. From these experimental data and the geometric factor thickness/area, the relative dielectric permittivity ε*= ε´-iε´´, and dielectric losses, tanδ= ε´´/ ε´ were obtained. This experimental set-up was also used for determination of the resonance spectra as a function of temperature.

Experimental results

Fig. 1 shows the XRD patterns of the unpoled ceramics. All of them have a single phase distorted perovskite type structure to the resolution of the technique. Peaks are indexed for a R3c rhombohedral symmetry, BNT-like, in hexagonal description (JCPDS file #70-9850).

However, due to proximity to the MPB, signs of mixed phases [START_REF] Pitch | Structural properties of (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 lead-free piezoelectric ceramics[END_REF] are observed in the unpoled samples. Fig. 2 shows, as an example, two selected 2θ ranges in the XRD patterns of the BNBT4-1 ceramic, around 40°2θ the doublet (006),(202) and around 46.5°2θ the (024) peak, before and after poling. A clear field-induced phase transition with change of crystal symmetry is not observed in any of the ceramics, but the (024) peak becomes sharper, i.e., the rhombohedral character is enhanced. The texture change due to domain orientation during poling is observed by the change in relative intensity in the (006),(202) doublet. These changes are in agreement with recently reported results [START_REF] Pitch | Structural properties of (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 lead-free piezoelectric ceramics[END_REF].

The results of the quantitative characterization of the microstructure of the samples (relative density and number of objects measured in the micrographs, average values and standard deviation of the distributions) are reported in Table I. Fig. 3 shows the SEM images from which the values of average grain size, shown in Table I, were calculated. Fig. 4 shows an example of the intermediate steps in the quantitative microstructural analysis and size distributions of grain (from SEM images) and pore (from OM images). Fig. 5 shows the resonance spectra (resistance and conductance peaks, real parts of the complex impedance, Z * =R+iX, and admittance, Y * =G+iB, respectively) of the radial or planar (Fig. II andIII show also published data for dense BNBT6 and BNBT4 ceramics obtained by different processing routes [START_REF] Chu | Electrical properties of Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 ceramics[END_REF][START_REF] Pardo | Field-induced phase transition and relaxor character in submicron structured lead-free[END_REF][START_REF] Takenaka | Mechanical properties of (Bi 1/2 Na 1/2 ) TiO 3based piezoelectric ceramics[END_REF] and for BNT ceramics [START_REF] Hiruma | Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics[END_REF]. Fig. 6 shows the real part of the relative dielectric permittivity and the dielectric loss as a function of the temperature for the BNBT4-1 sample. Results are shown for the unpoled ceramic, both measuring on heating and cooling, as well as for the poled ceramic measured on heating and for the thermally depoled ceramic measured on cooling. The observed differences can be discussed in terms of the structural changes induced by the poling field. Fig. 7 shows for BNBT4-2 the evolution with the temperature of conductance peak, characteristic frequencies and electromechanical coupling factors of the planar resonance of a thin disk and the shear resonance of a thickness-poled plate.

Discussion

I. Electromechanical properties at room temperature

As shown in Tables I to III, the properties of dense and submicron structured BNBT4-2 and 3 ceramics are in good agreement with previously reported values for coarser grained BNBT4 ceramics prepared at higher temperatures [START_REF] Chu | Electrical properties of Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 ceramics[END_REF]. BNBT4-1 ceramic shows similar grain size, but lower relative density (Table I) and, therefore, higher porosity, than BNBT4-2 and 3. This results in higher compliance and mechanical and dielectric losses, as well as lower coupling coefficients (Table II). The optimum set of properties is found for the combination of submicron grain size and low porosity of BNBT4-2 (Tables II andIII). This can be understood as a consequence of a good stoichiometry, arising from the low processing temperatures, and homogeneous microstructure (Figs. 1 and3) that reduces conductivity and losses and allows efficient poling at relatively high temperature.

BNBT4-2 and 3 ceramics show values of the properties in between those of BNBT6 and BNT composition. The dielectric permittivity (ε σ 33 ) is lower than in BNBT6 and higher than in BNT; the thickness coupling factor and electromechanical anisotropy are higher (k t /k p = 2.2) than in BNBT6 (k t /k p = 1.5), approaching the value of BNT, whereas d 33 piezoelectric coefficient is higher than in BNT, approaching the value of BNT6 (Table II).

The comparison of the data obtained in submicron structured ceramics from nanopowder evidences that the dielectric losses, as well as the piezoelectric e 33 , d 31 , g 31 are lower for the BNBT4 composition than for the BNBT6. Also, when comparing real values of all elastic s E ij coefficients and the frequency numbers it is observed that BNBT4 ceramic is similar to BNBT6 from the mechanical point of view, but the losses are also lower.

Table III shows the characterization for the shear mode in which, again, we found that values of BNBT4 properties are intermediate from those of BNBT6 and BNT. The free permittivity (ε σ 11 ), the shear coupling factor and the d 15 piezoelectric coefficient of BNBT4 are higher than those of BNT, approaching the values of BNBT6.

Besides these interesting properties and just as an example among the many studies on doping BNBT ceramics to enhance electromechanical performance, CaO/MnO co-doping [START_REF] Yoon | Effects of co-doped CaO/MnO on the piezoelectric /dielectric properties and phase transition of lead-Free (Bi 0.5 Na 0[END_REF] has shown efficiency in increasing d 33 , k p and k t . At the same time the depolarization temperature is almost unchanged, the sinterability increases and the grain size of the ceramics is reduced. Therefore, routes for further improvement of the properties of pure BNBT4 ceramics for ultrasonic transduction purpose by doping are avaliable.

II. Temperature evolution of the permittivity and electromechanical properties

The curves of the real part of the dielectric permittivity as a function of temperature have a maximum at T m = 275⁰C and are dispersive up to ~238⁰C on heating and ~225⁰C on cooling (Fig. 6). This upper limit temperature for the dispersive region is often ascribed in BNTbased compositions [START_REF] Takenaka | Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 )TiO 3 -and (Bi 1/2 K 1/2 )TiO 3 -Based Bismuth Perovskite Lead-Free Ferroelectric Ceramics[END_REF] to T R-T , the transition temperature from the room temperature ferroelectric (rhombohedral) (FE(R)) phase to the antiferroelectric (tetragonal) (AFE(T)) phase by similitude with BNT. However, it resembles more to the isotropization point (T i ) of the optical properties of BNT (T R-T <T i < T m ), which is associated to a local maximum in the first derivative of the real part of the dielectric permittivity vs. temperature curve dε´/dT [START_REF] Isupov | Ferroelectric Na 0.5 Bi 0.5 TiO 3 and K 0.5 Bi 0.5 TiO 3 perovskites and their solid solutions[END_REF]. Early interpretation of T i on cooling involved the percolation phenomena of the AFE regions that begin to form at T m within the ferroelastic twins formed in the paraelectric-ferroelastic (PE-Felas) phase, stable above T m . This ferroelastic twins originates on cooling from the paraelectric-paraelastic (PE-Pelast) cubic phase, stable above 540⁰C for pure BNT. The percolation of the AFE regions creates long-range AFE order in the crystal. On heating, the complete separation of AFE regions at T i can only take place when a critical density of PE-Felast interlayer takes place. This requires a higher temperature, thus the observed thermal hysteresis of T i . It is clear that the temperature of formation on cooling and destruction on heating of the long range ordered AFE phase will correspond to the most unstable state of the crystal and thus to a dε´/dT local maximum. The AFE character of the BNT-based compositions is under discussion. The term Low Temperature Non-Polar at Zero Field phase (LTNPZF), which would account with the pinched hysteresis loops characteristic of the phase taking place above T R-T , seems more appropriated [START_REF] Vakhrushev | Phase transitions and soft modes in sodium bismuth titanate[END_REF]. The depolarization temperature for BNBT4 is the temperature at which the ferroelectric long-range order and macroscopic ferroelectric properties disappear, T R-T or, more properly, T FE-LTNPZF . A recent work [START_REF] Ma | Domain structure-dielectric property relationship in lead-free (1-x)(Bi 1/2 Na 1/2 )TiO 3 -xBaTiO 3 ceramics[END_REF] has coined the term antiferroelectric-relaxor, which will account with the dielectric dispersion found in the range of stability of this phase, between T FE-LTNPZF and T i .

On heating of the poled sample, T d = T FE-LTNPZF is revealed by the sharp jump in the real part of the permittivity and the maxima of the losses at ~153⁰C for BNBT4 (Fig. 6), in good agreement with previous results from depolarization current [START_REF] Chu | Electrical properties of Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 ceramics[END_REF]. Whereas there are not differences in the curves of the real part of the permittivity versus temperature measured on cooling , the difference between measurements on heating of the samples before and after poling is clear, being ε σ 33 ´(poled) > ε´(unpoled) in the interval between T d = T FE-LTNPF and T i (Fig. 6).

As shown in Fig. 7, for T d ≈150⁰C we found f s ≈ f p in the resonance spectrum of the planar mode as a function of the temperature. The electromechanical coupling factor calculated using standard methods vanishes at T d . However, a weak resonance spectrum was observed up to higher temperatures and it is possible to obtain k p ≠0 up to T≈230⁰C using Alemany et al. software (Fig. 7). This residual macroscopic piezoelectricity indicates that, although the longrange FE order disappear at T d , it may exist a medium-range order [START_REF] Shen | Electric-field-induced dielectric anomalies in c-oriented 0.955Pb(Zn 1/3 Nb 2/3 )O 3 -0.045PbTiO 3 single crystals[END_REF] by coexistence of FE clusters in the LTNPZF phase above T d , also reflected in the differences of the permittivity before and after poling between T FE-LTNPF and T i . Such a medium-range order progressively disappears as the temperature increases towards T i . For BNT [START_REF] Roleder | The piezoelectric effect in NaBiTiO 3 ceramics[END_REF] and BNT-BKT-BT [START_REF] Hiruma | Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 ) TiO 3 -(Bi 1/2 K 1/2 ) TiO 3 -Ba x TiO 3 LeadFree Piezoelectric Ceramics[END_REF] ceramics a weak piezoelectric activity above T FE-LTNPZF =200⁰C and 177⁰C, respectively, was also previously reported.

When the shear resonance is studied in a similar manner, the observed depolarization temperature, as defined by the point in which f s ≈ f p , is found at a higher temperature (T d SH ≈ 175⁰C) (Fig. 7). The electromechanical anisotropy here observed reveals the complexity of the depolarization process of these samples. This anisotropy could arise from a texture developed during hot-pressing or by a mechanism of progressive dissapearance of the FE clusters that reduces first the polarization extension mechanisms and, at higher temperature, the polarization rotation mechanisms associated to the shear mode. Dielectric anisotropy has been reported for BNT ceramics [START_REF] Hiruma | Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics[END_REF]. The full explanation of the mechanism underlying these effects exceeds the purpose of this work and demands additional structural information. It is clear, however, from the results in Fig. 7 that the working range of BNBT4 ceramics extends to higher temperatures than the currently reported T d when the applications are based on shear resonances.

Conclusions

Submicron structured, dense (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 ceramics with x=0.04 were obtained and studied as candidates for lead-free ultrasonic transducer materials. For BNBT4 ceramics, the free permittivity (ε σ 33 , ε σ 11 ), the k t and k 15 coupling factors and electromechanical anisotropy (k t /k p ) are higher than for BNBT6. The d 33 , d 31 and d 15 piezoelectric coefficient are higher in BNBT4 than in BNT, approaching the values for BNT6. The comparison of ceramics from sol-gel auto-combustion nanopowder also evidences that BNBT4 ceramics have lower dielectric, mechanical and piezoelectric losses than BNBT6 ceramics. BNBT4 has higher depolarization temperature than BNBT6, which is T d ≈150⁰C when determined from dielectric loss measurements in poled samples and thermal evolution of the radial mode spectrum.

Finally, an electromechanical anisotropy was observed during the thermal depolarization. The depolarization temperature from the evolution of the shear mode spectrum is found to be T d SH ≈175⁰C.

Figure captions

Table captions

Table I. Results of the quantitative characterization of the ceramic microstructure: density and characteristics of the lognormal distributions of equivalent diameters of grains (from SEM images) and pores (from OM images).

5

  (a)) and thickness (Fig. 5(b)) modes of, thickness poled and excited, thin disk and the shear mode of, thickness-poled and length excited, shear plate (Fig. 5(c)) of BNBT4-2 ceramic. Both the experimental (symbols) and reconstructed spectrum (lines) are shown for each resonance.

Fig. 1 .Fig. 2 .

 12 Fig. 1. XRD pattern of BNBT4 ceramics.

Fig. 3 .

 3 Fig. 3. SEM micrographs of ceramic surfaces polished and quenched from 800⁰C to room temperature: (a) BNBT4-1 (HP700⁰C-2h+1050⁰C-1h), (b) BNBT4-2 (HP800⁰C-2h+1050⁰C-1h) and (c) BNBT4-3 (HP950⁰C-2h+1000⁰C-2h).

Fig. 4 .Fig. 5 .

 45 Fig. 4. Steps of the process of quantitative microstructural characterization of the BNBT4-2 (HP800⁰C-2h+1050⁰C-1h) ceramics: original grey level images from microscopy, objects identified in the corresponding digital images for the measurement of their areas and lognormal distributions of grain (Figs. (a)-(c)) and pore (Figs. (d)-(f)) equivalent diameters.

Fig. 6 .

 6 Fig. 6. BNBT4-1 ceramic: dielectric permittivity, real part and losses, of unpoled samples on heating and cooling (Figs. (a) and (b)) and after poling (Figs. (c) and (d)) on heating and on cooling of the thermally depoled sample. Arrows indicate the increasing measurement frequency (1, 2, 5, 10, 50, 100, 200, 500 and 1000 kHz).

Fig. 7 .

 7 Fig. 7. Thermal dependence of the conductance peaks (Figs. (a) and (b)) and of the characteristic frequencies and electromechanical coupling factors for the radial mode of a thickness poled and excited thin disk (Fig. (c)) and the shear mode of a thickness-poled and length excited shear plate (Fig. (d)) of BNBT4-2.

Fig. 2 .

 2 Fig. 2.

  Fig. 5

Table II

 II all losses, which are calculated from the measured radial and thickness resonances of thin disks. The Poisson ratio, electromechanical coupling factors (k p , k t ) and frequency numbers (N p , N t ) are shown in Table II. Similarly, Table III summarizes the material data (e 15, d 15 , h 15 , g 15 , c 11

	summarizes the relative density, range of grain size, d 33 from Berlincourt-meter
	and the piezoelectric (h 33 , e 33 , d 31 , g 31 ), elastic (c 33 D , c 11	D , s 11 E , s 12 E , s 66 E ) and dielectric (ε S 33, ε σ	33 )
	linear coefficients, including D ,s 55	E , ε S	11, ε σ	11 , k 15 , N 15 ) obtained from the shear resonance of
	thickness poled plates. Tables

Table II .

 II Piezoelectric, elastic and dielectric parameters, electromechanical coupling factors and frequency numbers, obtained from measurement of the planar (P) and thickness (T) resonances of thin disks of BNBT4 ceramics, poled and excited in thickness. Data from the literature are also shown for comparison.

Table III .

 III Piezoelectric, elastic and dielectric parameters, as well as electromechanical coupling factors and frequency numbers, obtained from the shear resonance of plates of BNBT4 ceramics, thickness poled and length excited. Data from the literature are also shown for comparison.

		BNT	mixed oxides	sintering	1100⁰C/2h	[10]		72.9	NA	NA	16.8	3260	10.2	45.5	2780	343 (1MHz)	0.028 (1MHz)	---	---	---	-15.0	---	---	---	0.263	7.07	---	---
		BNBT6	mixed oxides	sintering	1160-1180⁰C/1h BNT [11] mixed oxides	sintering	122 1100⁰C 2h ref. [10]	NA NA	NA 36.2 (104) 29 3000 1650 ---(110)	---367(1MHz)	40 ---	2522 87.3	601 (1KHz) ---	0.018 (1KHz) ---	---17.9	---	---	---	---	---	---	0.25	---	---	---
	Table I. Table II. Table III.	Composition, HP + recrystallization recrystallization BNBT4 sol-gel autocombustion Processing conditions HP + recrystallization Synthesis route, BNBT4 sol-gel autocombustion Composition, BNBT6 sol-gel autocombustion HP 800⁰C-2h + 1050⁰C-1h [22] Synthesis route, BNBT4-1 BNBT4-2 BNBT4-3 BNBT4 mixed oxides sintering 1160-1180⁰C/1h [11] BNBT4-1 700⁰C-2h/ 1050⁰C-1h Processing conditions 700⁰C-2h/ 800⁰C-2h/ 950⁰C-2h/ BNBT4-2 800⁰C-2h/ 1050⁰C-1h BNBT4-3 950⁰C-2h/ 1000⁰C-2h Berlincourt 1050⁰C-1h 1050⁰C-1h 1000⁰C-2h d relative density (%) 93% 95% 98% 33 (10 -12 C.N -1 ) 100 95 99 148 87 R 2 mean size <G> (nm) (520 ± 10) SEM (560 ± 10) (510 ± 10) P 0.9997 0.9998 0.9996 0.9999 NA R 2 T 0.9734 0.8987 0.8460 0.9548 NA k σ G (nm) SEM (550 ± 20) (600 ± 20) (500 ± 20) p (%) P 19.6 21.1 21.1 26.8 21 N Number of measured grains 722 SEM 695 789 p (kHz.mm) P 2980 3054 3125 2933 3000 k 31 (%) P 11.8 12.6 12.6 15.74 ---k mean size <P> (nm) (2600 ± 40) OM (3850 ± 30) (630 ± 40) (1830 ± 9) t (%) T 41.3 44.5 44.3 40.4 45 N t (kHz.mm) T 2268 2288 2348 2281 2570 ε σ (600 ± 60) σ P (nm) OM (400 ± 10) (470 ± 6) (250 ±4) 33 P 435 -18i 416 -16 i 475 -20i 641 -31i 445 (1KHz) tan δ (ε σ 33 ) P 0.041 0.037 0.042 0.048 0.021 ε S Number of measured pores 2022 OM 842 33 T 330 -15i 318 -10 i 360 -15i 456-31 i ---h 33 (10 8 V/m) T 27.8 +.26i 31.8 + 0.14 i 30.3 +0.46i 23.4 -0.07 i ---e 33 (C.m 2 ) T 8.13 -0.30i 8.95 -0.25 i 9.68 -0.25i 9.46-0.67i ---Composition, Synthesis route, Processing conditions BNBT4 sol-gel autocombustion HP + recrystallization BNBT4-2 800⁰C-2h/1050⁰C-1h R 2 0.7939 K 15 (%) 39.2 N 15 (kHz.mm) 1423 ε S 11 343 -67 i ε σ 11 402 -89 i e 15 (C.m 2 ) 5.00 -0.80 i d 15 (10 -12 C.N -1 ) 108.3 -21.4 i h 15 (10 8 V.m -1 ) 16.37 + 0.55 i g 15 (10 -3 m.V.N -1 ) 30.30 + 0.66 i s E 55 (10 -12 m 2 N -1 ) 21.80 -0.80 i Fig. 1. 2θ (degrees) d 31 (10 -12 C.N -1 ) P -22.4 + 0.53i -22.7 + 0.55 i -23.7 + 0.54i -37.0 + 1.33i ---BNBT6 sol-gel autocombustion HP + recrystallization [22] 0.9847 40.2 1373 609 -29i 726 -40i 6.55 -0.23i 158.3 -8.31i 12.16 + 0.16i 24.64 + 0.05i 24.17 -0.42i 20 30 40 50 HP800ºC-2h+1050ºC-1h g 31 (10 -3 m.V.N -1 ) P -5.82 -0.10i -6.16 -0.08 i -5.63 -0.11i -6.51 -0.08i ---c D 33 (10 10 N.m 2 ) T 13.26 +0.48i 12.77 + 0.39 i 14.98 + 0.73i 13.62 +0.45i ---c D 11 (10 10 N.m 2 ) T 11.87 + 0.06 i 13.16 + 0.06 i 13.30 + 0.07i 11.94 + 0.08i ---s E 11 (10 -12 s E 12 (10 -12 s E 66 (10 -12 Poisson ratio P 0.28 0.29 0.30 0.31 0.25 m 2 N -1 ) P 9.36 -0.05i 8.86 -0.04 i 8.49 -0.04i 9.73 -0.07i ---m 2 N -1 ) P -2.59 + 0.01i -2.59 + 0.01 i -2.51 +0.01i -3.02 + 0.02i ---m 2 N -1 ) P 23.90 -0.12i 22.89 -0.11 i 21.99 -0.11i 25.52-0.19i ---60 HP700ºC-2h+1050ºC-1h 800⁰C-2h /1050⁰C-1h BNBT6 mixed oxides sintering 1200⁰C 2h [23] NA 49.8 1568 ---373 (1KHz) ---194 ------23.3 (116) (122) HP950ºC-2h+1000ºC-2h (024) (018)(214) Intensity (a.u.) (006)(202) (300) (208) 70 (012) (220)

Acknowledgements

This work was funded by Spanish CSIC project #201060E069 and supported by the European Network of Excellence (NoE-MIND CE FP6 515757-2), that established the European Institute of Piezoelectric Materials and Devices (Piezoinstitute). Authors wish to thank Mr. José Luis Millán (FINNOVA 2010 grantee) for his technical assistance in the quantitative microstructural characterization.


Lourens ; Heres
email: lourens.heres@vionfood.com

Ron Hoogenboom

Rik Herbes

Wim Traag

Bert Urlings

Tracing and analytical results of the dioxin contamination incident in 2008 originating from the Republic of Ireland

Keywords: Traceability, Risk assessment, Screening assays, Regulations Additives/Contaminants: Dioxins, Dioxins -TEQs, PCBs Food Types: Meat, Animal feed, Animal products -meat, Ingredients dioxins, PCBs, incident, tracing, food chain Dioxin incident pork 2008

High levels of dioxins (PCDD/Fs) in pork were discovered in France and the Netherlands at the end of 2008. The contamination was rapidly traced back to a feed stock in the Republic of Ireland (RoI). Burning oil, used for drying of bakery waste, appeared to be contaminated with PCBs. As a consequence, very high levels up to 500 pg TEQ g -1 fat were found in pork. The congener pattern clearly pointed to PCB-oil as a source but the ratio between the non-dioxin-

Introduction

Dioxins, i.e. polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), and dioxin-like polychlorinated biphenyls (dl-PCBs) remain a major threat for the consumer and as a result for the food chain. This is due to their accumulation in the body and large array of potential effects to human health. To provide guidance, limits for chronic exposure of consumers (Tolerable Weekly Intake, TWI) has been derived by the Scientific Committee on Food [START_REF] Scf | Opinion of the Scientific Committee on Food on the risk assessment of dioxins and dioxin-like PCBs in food[END_REF]. Within the legislation maximum levels in food and feed products were set to reduce exposure of consumers below the TWI (EC 2006a(EC , 2006b)). PCDD/Fs may be derived from a large number of different sources, like the incineration of waste, production of certain chlorinated compounds but also the burning or heating of polychlorinated biphenyls (PCBs).

For example, during the past decade incidents were observed with citrus pulp using lime from a plant producing polyvinyl chloride (PVC-) [START_REF] Malisch | Increase of the PCDD/F-contamination of milk, butter and meat samples by use of contaminated citrus pulp[END_REF], feed fat contaminated with PCB-oil in Belgium [START_REF] Bernard | Food contamination by PCBs and dioxins; an isolated episode in Belgium is unlikely to have affected public health[END_REF][START_REF] Hoogenboom | The Belgian dioxin crisis; involvement of RIKILT[END_REF][START_REF] Larebeke N Van | The Belgian PCB and dioxin incident of January-June 1999: exposure data and potential impact on health[END_REF], cholin chloride mixed with pentachlorophenol treated sawdust [START_REF] Llerena | An episode of dioxin contamination in feedingstuff: the choline chloride case[END_REF], sequestered minerals produced from seaweed, bakery waste dried over fires from painted wood (Hoogenboom et al. 2004a), peels from potatoes sorted in a bath containing kaolinic clay with high levels of dioxins from an ancient source [START_REF] Hoogenboom | Kaolinic clay derived PCDD/Fs in the feed chain from a sorting process for potatoes[END_REF], and feed fat derived from a gelatine production plant using hydrochloric acid (HCl) contaminated by a still unrecovered source (Hoogenboom et al. 2006). In 2008 there was an incident with contaminated zinc-oxide in Chile that caused the long-term closure of the borders of Korea and Japan for Chilean pork [START_REF] Kim | Dioxin contamination of Chilean pork from zinc oxide in feed[END_REF]. This long list of incidents shows that frequent monitoring of food and feed is required to detect gaps and weaknesses in chain control and to detect incidents and their sources as early as possible in order to protect human health and prevent large economic losses. At the same time, this is hampered by the large number of different sources that make a targeted monitoring approach for dioxins rather difficult. The present paper describes how private food quality systems led to the discovery of another dioxin incident at the end of 2008. Surveillance by the food business operator (FBO) led to the rapid identification of the source. The incident is described from the stage where the contamination was detected at a meat producer and where the origin was fully unknown, towards a stage where all evidence, including characterisation of the PCDD/F profiles, pointed to the Republic of Ireland (RoI) as the origin of the contamination and PCBs as the source. An estimate is made about the contamination levels that occurred. Besides describing the incident, this paper aims to show the effect of efficient public and private sector cooperation that led in this case to a rapid identification of the origin. Finally opportunities for improvement in HACCP and surveillance systems for chemical contaminants are identified.

Materials and methods

Case description

Onset

A French meat processor (Company A) reported the contamination of pork with dioxins to their supplier (Company B) on Monday November 24 th 2010. At this point Company B reported the incedent to Company C, Company B´s parent Company. The tested product was a deboned loin from a batch of 1557 kg, received in France on October 14 th . On November 25 th in the late afternoon Company A sent a detailed analytical report to company C . This report included the PCDD/F congeners and showed a contamination level of 433 pg dioxin toxic equivalents (TEQ)/g fat. In addition the sample also contained an elevated level of dioxin-like PCBs (16 pg TEQ g -1 fat). Indicator PCBs (PCBs 28,52,101,138,153,180) were not determined but the typical furan pattern pointed to PCBs as the source.

Follow-up actions

Immediately after receiving the first information about the contaminated pork sample, several actions were taken by Companies B and C. Tracking and tracing was started, a surveillance of the contamination was initiated, the competent authority (CA) was immediately informed, and information was acquired from plants processing the slaughter by-products.

Tracking and tracing

Company B is a meat production company in the south of The Netherlands that slaughtered about 600 sows per day, mainly from Dutch and partly from Belgium origin. Additionally, it received external supplies (mainly carcasses) from other companies on a daily basis.

Approximately 1000 carcasses were cut and deboned daily.

Like in many deboning facilities tracking and tracing in the deboning area was based on production day batches. From each day it is know which carcasses are deboned, which and how many products are cut from these carcasses, and where and when these products are sold or stored. Tracing showed that the contaminated loin was produced on October 13 th . On this day, 1049 carcasses were deboned originating from 7 countries of the European Union (EU) and 9 slaughterhouses (Netherlands (2), Belgium, Germany (2), England, Northern Ireland, Republic of Ireland (RoI), and Italy). The sows were slaughtered on different days (Table 1).

Tracking showed that 70 customers obtained meat from production of October 13 th . Meat produced on this date was in several shipments summing up to 400 tonnes, due to combination of products from earlier and later production dates.

Following the initial notification, products from October 13 th that were still in possession of company B were tracked and placed on hold. Frozen stock was shipped back to the cutting and deboning facility and samples were taken from the remaining products. From the pallets second set of samples was collected on November 26 th and tested for PDCC/Fs and dl-PCBs using gas chromatography / mass spectrometry (GC/MS). These two sets of samples were negative. In a third sampling of December 2 nd with 10 pooled samples, one sample showed a positive test result in the so-called CALUX-bioassay (Chemical Activated LUciferase gen eXpression), a screening assay for dioxin-like compounds. This sample showed a confirmed PCDD/F content of 1.9 pg TEQ g -1 fat. This is relatively low but higher than the normal background of 0.1 pg TEQ g -1 fat.

Private Surveillance of new shipments

Following the notification, there was a possibility that the contaminated pork originated from Dutch farms. Therefore, a surveillance program was started by company C to investigate this possibility. The 175 Dutch farms that were identified as the possible origin of the contaminated supply were listed for surveillance. Samples were taken from the next deliveries coming from these farms of up to 5 pigs. Both sows and fattening pigs were sampled. This Surveillance was started on November 27 th . In total 149 supplies to Dutch slaughterhouses were sampled within 7 days, i.e. 646 pigs and sows. Herds were released when no sample showed a positive result and at least 3 samples were tested on PCB's.

At the same time, the supply of carcasses from foreign origin was continued and all carcasses were sampled by pooling 10 carcasses in one sample. Nine freights with on average 138 carcasses were supplied, 6 from England, 1 from Northern Ireland and 2 from the RoI.

The two German slaughterhouses that supplied carcasses for the production on October 13 th to company B belonged to the same parent company. Therefore an equivalent surveillance system was introduced into these slaughterhouses. December 5 th from samples from the shipment of carcasses from RoI on December 2 nd . After the initial signal of possible positive PCB results, the samples were sent to the Dutch national reference laboratory (RIKILT) for confirmation of PCDD/Fs with high resolution GC / high resolution MS (HRGC/HRMS). Samples were confirmed on Saturday December 6 th . In the afternoon of December 5 th , five other samples were positive in the PCB analysis, which were confirmed by RIKILT early on Monday 8 th . The private surveillance program was stopped that Monday following the positive results from Irish origin

Information and cooperation with authorities

The CA of the Netherlands (Food and Consumer Products Safety Authority, VWA) were informed during the morning of November 25 th , with regard to the oral incident report of the dioxin finding by company A at the previous day. Company C and the VWA, together with an expert of RIKILT had a meeting on December 3 rd . The actions taken by company B and C were discussed with respect to appropriateness and extend of the implemented measures. In this meeting it was concluded that the PCDD/F congener profile in this case was to a certain level comparable with the Belgium dioxin case of 1999, where PCDD/Fs and PCBs were both elevated. The ratio of indicator PCBs and PCDD/Fs was however much lower (Table 3). The approach of the company to test for indicator PCBs for the further unravelling of this case was approved, as the participants agreed that high levels of dioxins were to be expected and as such also clearly elevated levels of indicator PCBs.

The VWA stayed in close contact with company C and was informed about the surveillance, tracking and tracing actions of the company, and supervised the actions taken.

The authorities were immediately informed about the suspected positives in the PCB test of 8 RoI origin on December 5 th . As soon as these samples were confirmed by the RIKILT to contain high levels of PCDD/Fs, the VWA informed their colleagues of the Irish Food Safety Authority (FSAI), as well as the European Rapid Alert System for Food and Feed (RASSF) on Saturday December 6 th . At this stage it turned out that the authorities from RoI had detected elevated indicator PCB levels in pork and as a result had quarantined a number of pig farms [START_REF] Tlustos | The dioxin contamination incident in Ireland 2008[END_REF].

Officials from the EU, the RoI and the Netherlands had a telephone conference to discuss all findings, including all test results of Company B on December 6 th . During that evening a full recall of RoI pork was announced by the RoI competent authority (Tlustos 2009a,b).

Collection of supply chain data

On December 3 rd , increased concentrations of dioxins in pork fat above legal limits were reported by a gelatine processing plant in Ghent, Belgium. This company processes pig skins for the production of gelatine and produced pig fat for animal feed as a by-product. Since the incident in 2006 [START_REF] Hoogenboom | A novel source for dioxins present in recycled fat from gelatin production[END_REF] this fat had to be checked for dioxins. The level of PCDD/Fs in fat increased from the second half of September and onwards (figure 1A) but exceeded the legal limit only in week 48. Tracing showed that the raw material for this byproduct weighed around 600 metric tonnes for each batch, and originated from various countries from several pig slaughter and deboning sites (table 2).

The first increased dioxin level was seen in a sample of September 12 th . This was from pig skins obtained in the first weeks of September. This sample showed a PCDD/Fs level around 1.4 pg TEQ g -1 fat, i.e. below the legal limit of 2 pg TEQ g -1 fat. However, in normal situations the levels are below the detection limit of 0.3 pg TEQ g -1 fat. At the end of November (week 48) a sample contained PCDD/Fs above the legal limit of 2 pg TEQ/g -1 fat for feed materials, i.e. the level was 3.7 pg TEQ/g -1 fat. This simultaneous occurrence of The tracing results of the fat from this gelatine plant were analyzed afterwards, during evaluation of the incident. From most batches with elevated dioxin levels the tracing results were known (see figure 1). Based on this data Poland, Belgium and Austria could be excluded as source of the contamination with the results of end October, because suppliers of pig skin from these countries had not supplied skin for two or more batches of fat with more than 1 pg TEQ g -1 (see figure 1B). The United Kingdom (UK) seemed a less probable source as it was absent as supplying country from the tank sampled at October 13 th with elevated dioxin levels, and most peaks in UK supplies did not coincide with peaks of dioxins. However a definite conclusion about UK could not be made. Firstly because in all tanks with elevated dioxin levels, skin fat from UK pigs was present. Secondly the levels of dioxins decreased at the beginning of November, a period in which the supplies from UK and RoI were lower than average. And finally in the sample from November 24 th with a high dioxin level supplies from UK and RoI were twice as high as normal (15%).

German and Dutch supplies were present in all fat samples and could therefore not be excluded as source. The amount of raw material from the RoI correlates well with the levels of dioxin in the fat samples (figure 1B), but this is only a causal relationship in hindsight.

Henceforth, a fully conclusive result about the country of origin was not possible on the supply of the gelatine plant. The main cause is that the amount of skin used for the production of the batches with fat is extremely large, and the amounts supplied are relatively constant for all suppliers. An intensified targeted surveillance stratified on country or regions of origin (Germany, The Netherlands, UK and RoI) could have been initiated at the end of October On November 27 th another fat melting company was informed about the dioxin case, and was asked to test stored fat samples from October from by-products partly originating of company B. In one plant within the firm that processed bones, elevated PCB levels were observed in fat samples collected in the last two weeks of November, but not in the preceding weeks. The sum of PCBs was <64 ng g -1 , which is higher than normal but not exceeding current legal limits. Results were available after identification of RoI as the source of the contamination and therefore no further sourcing was done.

Contamination levels

From the observed levels of dioxins in positive samples, the contamination levels in tainted pigs can be deduced. In the Index pork loin analysed in France on behalf of company A, a dioxin content of 433 pg TEQ g -1 fat was reported. This sample was derived from 1 contaminated animal.

The Irish carcasses detected as positive by company B were from pigs slaughtered on November 28 th . The 230 carcasses were sent with two trucks. All samples from the first truck were negative (11 pooled samples). From the second truck, 7 out of 12 pooled samples were positive. The Real Prevalence can be estimated from the pool prevalence by the following equation: P estimated = 1-(1-P pool )^(1/ number in pool) [START_REF] Cowling | Comparison of methods for estimation of individual-level prevalence based on pooled samples[END_REF]. A pool prevalence of 7 out of 23 gives an estimated fraction of contaminated carcasses of 3.6%, or about 8 tainted carcasses in the shipment of 230. If so, there was on average one positive carcass per positive pool, and a limited number of pools with 2 or 3 positive carcasses. Since the pooled samples were derived from 10 different carcasses from RoI sows, the actual level in an individual carcass must be higher than that from the pooled sample. With varying 11 concentrations between 3 and 138 pg TEQ g -1 fat in the pools, the data suggest that the contamination level in the contaminated sows was on average 300 pg TEQ g -1 fat. This was more or less confirmed by follow-up analysis of non-pooled pork samples from RoI at the RIKILT, showing a highest level of 540 pg TEQ g -1 fat.

The average contribution of RoI pig skin in the gelatine production was 7%. When the concentration in the fat after gelatine production is recalculated by dividing the concentration by the percentage of RoI skin, a steady but fluctuating increase in concentration is seen towards 25 pg TEQ g -1 fat. Assuming an average of 10% contaminated pigs in the RoI cohort (as was stated by the RoI authorities, [START_REF] Tlustos | The dioxin contamination incident in Ireland 2008[END_REF], the average dioxin concentration in fat from contaminated pig would have been around 250 pg TEQ g -1 fat. This is relatively close to the contamination level found in the index pork loin, the levels estimated from the pooled samples and the levels measured later in individual pigs.

The RoI government announced the observation of elevated levels of dioxins and PCBs in beef as well, but levels were not published [START_REF] Tlustos | The dioxin contamination incident in Ireland 2008[END_REF]. During the incident the highest level however, was detected in pork liver in Poland and confirmed by the Community Reference Laboratory (CRL) on dioxins (annual report 2008, CVUA, Freiburg), showing a level of 16,000 pg TEQ g fat -1 .

Source of the RoI incident

The RoI authorities reported in consultation with the VWA that the use of contaminated bread crumbs in animal feed was identified to be the source. The bread crumbs were produced from bakery waste, like biscuits out of date, which were dried using a direct heating process. In such a process, the combustion gasses are in direct contact with the feed material to be dried.

The fuel used in this process was contaminated with PCB transformer oil. In June/July 2008 the management of the factory that produced dried bread crumbs had decided to switch to 12 cheaper fuel in response to the increasing fuel prices. After burning, this resulted in high levels of dioxins in the combustion gasses which were deposited on the material to be dried.

The fuel oil was originating from UK (Northern Ireland) but it was not disclosed thus far at which stage the PCB transformer oil was added to the fuel.

All possibly contaminated feed was traced and placed on hold. Also all animals on the affected farms in RoI and UK (Northern Ireland) were held for being disposed of safely (source SCoFCAH 19-20 January 2009). All pork produced between 1 st September and the 6 th of December was recalled from the shops.

PCB and dioxin profiles in pork, bread crumbs and oil

In the following part of this paper the congeners profiles of different positive samples from this dioxin incident are compared. Suspected samples from company B and from the gelatine production plant were sent to WUR-RIKILT, the Dutch Reference laboratory for dioxins and PCBs. Later on RIKILT received one sample of contaminated bread crumbs and one sample of oil used from drying the crumbs from the Irish authorities. Samples were analyzed as described previously [START_REF] Hoogenboom | A novel source for dioxins present in recycled fat from gelatin production[END_REF]), based on the extraction of fat with ASE (bread crumbs), the purification on a Powerprep TM system (FMS Inc.) and the analysis by HRGC/HRMS.

Pig meat and fat

RIKILT analysed the 7 pooled RoI carcass samples by GC/MS and measured levels between 3 and 138 pg TEQ g -1 fat (Table 3). Figure 2A shows a pattern of the 17 dioxin congeners. In terms of contribution to the TEQ-level, 2,3,4,7,8-PeCDF was by far the most important congener (about 85-90%). The samples also contained elevated dl-PCBs, contributing about 3-5% to the total TEQ levels, and also indicator PCB-levels. The ratio between the sum of the the ratio of 50,000 observed during the Belgian dioxin and PCB incident in 1999 (Hoogenboom et al. 2004b). In that incident 200 kg of PCB-oil had been mixed directly with 60 tonnes of fat that was used for the production of animal feed. It seems likely that the burning process increased the amount of PCDFs and as such decreased the ratio of indicator PCBs and PCDD/Fs.

Figures 2B and2C show patterns of the dl-and indicator PCBs. The relative contribution of PCB 180 is higher than in the feed in the '99 Belgian incident (30 versus 10% (Hoogenboom et al. 2004b)). Also PCB 156 contributed to a higher extent in comparison to PCB 118. This data indicates that the PCB-oil in this case was higher chlorinated than the Arochlor 1254/1260 mixture in the Belgian incident, pointing to an Arochlor 1260 mixture.

This information was passed to the RoI authorities shortly after confirmation that RoI was the origin of the contaminated pork.

Fat from a gelatine plant

Since the gelatine incident in 2006 [START_REF] Hoogenboom | A novel source for dioxins present in recycled fat from gelatin production[END_REF]), caused by hydrochloric acid contaminated with dioxin, the fat should be checked for dioxins. Levels are normally below 0.3 pg TEQ/g fat. Several of the samples from the Belgium gelatine plant were reanalyzed by RIKILT and showed a congener pattern similar to the pattern observed in the meat samples (Figure 2).

Bread crumbs

The sample of contaminated bread crumbs showed a dioxin level of 8500 ng TEQ kg -1 , in addition to 60 ng TEQ kg -1 of the dl-PCBs and 1.4 mg/kg of the six indicator PCBs (Table 3).

The ratio between indicator PCBs and dioxin-TEQs in this material was only 166. The pattern 14 of the congeners is included in Figure 2. A remarkable point was the relatively high contribution of TCDF and 1,2,3,7,8-PeCDF to the TEQ, as compared to the pork meat. These congeners, however, are known to be rather unstable in pigs (Hoogenboom et al. 2004b).

Contaminated oil

The real source of the contamination was the oil used for the drying of the bread crumbs. A sample of contaminated oil showed a level of the six indicator PCBs of 306 mg/kg. Since these congeners make up only part of the mixture, the actual PCB level in the oil may be 2-4x higher. The PCB pattern in this sample was very similar to that of the bread crumbs (Figure 2C). The dioxin level was below 22 µg TEQ kg -1 . Although the detection limit was rather high, due to the small sample intake, it is clear that the relative amount of PCDD/Fs is much lower than in the bread crumbs (ratio > 14,000) and that most of the dioxins may have been formed during the burning of the oil resulting in transformation of the PCBs in the oil.

Discussion

Practical lessons

As in this case, most of the time food business operators are looking in the dark when they receive a report indicating high levels of chemical substances in one of their products, especially when there is no other information in the supply chain indicating a contamination accident somewhere. In this case the batch sizes were large, there were several slaughterhouses of possible origin and the supplying farms were a multitude. Introduction of limited batch sizes and batch-wise processing of meat from different origin would facilitate traceability in these kinds of incidents markedly. The parallel identification of dioxin contaminations in fat produced at the gelatine production site helped to conclude that the finding was not an incident and that RoI could be the country of origin, but also indicated that the actual onset of the contamination started in September 2008. When the levels of dioxins increased and exceeded the EU action limit for PCDD/Fs of 1 pg TEQ g -1 fat, the company had started to investigate the source, for example by testing the added salt. But an earlier report to other parties in the supply chain could have initiated broader awareness. Identifying the country of origin at this big gelatine production side was complicated, because the large batches of fat produced originate from raw materials from different collectors, from different countries, and from different production days.

Consequently, most of the suppliers supplied to every batch. Nevertheless, at the end of October some supplying countries could be excluded and a targeted surveillance at regional level could have identified RoI, possibly by the beginning of November.

In the surveillance applied in this incident, indicator PCBs were used for screening and samples were pooled. In the dioxin crisis in Belgium in 1999 the indicator PCB/dioxin TEQ ratio was 50.000 to 1. In this RoI situation it was 2000 -4000 to 1, at least in the pigs. In the positive loin (Company A) a contamination level of 433 pg TEQ dioxins g -1 fat was found. At this high level, also the PCB levels (1200 ng g -1 fat) were far above legal limits. At lower dioxin levels, for example 5 pg TEQ g -1 (which is 5 times above the legal limit), PCB levels could be at, or below 15 ng g -1 , which is the newly proposed EU limit of 15 ng g -1 fat but far below existing national legal limits. More important, such levels are below the limit of detection for many routine laboratories and cannot prove that products are compliant with the The contamination was shown to be originating from uncontrolled feed production by burning PCB contaminated oil. Since 2006 all feed business operators are legally obliged to incorporate HACCP-requirements in their quality systems. Looking at the hazards and risks of direct combustion and the use of unspecified oil, a monitoring system on PCBs and dioxins should be integrated in the companies own quality system. In assured GMP based feed production the risk of such contamination scenario's are minimized. Such a food safety system was however not in place at the feed producer of concern.

Risk assessment

In order to decide on the appropriate risk management actions and to inform and reassure consumers, it is essential to evaluate the potential health risks arising from the consumption of contaminated products. On December 10 th 2008, The European Food Safety Authority [START_REF] Efsa | Statement of EFSA on the risks for public health due to the presence of dioxins in pork from Ireland[END_REF] published a risk assessment on the incident. The main conclusion was that the additional exposure due to dioxin contamination was a minimal risk. Consumption of contaminated pork clearly resulted in a high temporary exceeding of the exposure limit (TWI) of 14 pg TEQ per kg bw per week, as established by the SCF (2001). However, the TWI is set for a chronic rather than an acute exposure, aiming at keeping the levels in the body (body burden) below the critical levels. Therefore, EFSA also estimated the possible effect of the exposure on the already existing dioxin and dl-PCB levels in the body. It was concluded that continuous consumption of contaminated pork would contribute significantly to the existing body burden. However, "in a more likely scenario with a daily consumption of 10% contaminated RoI pork for an average consumer of pork fat for the respective period of the fat), the body burden would increase by approximately 10%". EFSA considered this increase in body burden of no concern for this single event.

Based on data on the number of affected farms (Tlustos 2009a), the 10% contaminated RoI pork seems a realistic assumption. The levels of 50, 100 and 200 pg TEQ g -1 , used for the assessment, however, may be a clear underestimation, because higher levels were found in the index loin (433 pg TEQ/g) and also the pooled samples in The Netherlands indicated higher individual levels (around 300 pg TEQ g -1 fat). Moreover, the high levels in the meat also pointed to the possibility of high levels in the liver, an effect well-known in rats exposed to high levels of dioxins (Hurst et al. 2000a, Bell et al. 2007c). Own data in pork from one of the latest dioxin incidents showed e.g. a ratio of 4 to 8 between the dioxin levels in the liver and in back fat, expressed per g of fat. This possibility was confirmed by CVUA ( 2008) reporting one liver sample containing 16,400 pg TEQ g -1 fat. Based on 5% fat this would amount to about 800 pg TEQ g -1 liver.

Another important issue that was not covered in the EFSA assessment is the potential effect of a peak exposure resulting from the consumption of a highly contaminated food product. Normally, levels expressed on a fat base are in equilibrium between adipose tissue and blood. As a result most of the dioxins and dl-PCBs are stored in the body fat and less than 1% may be circulating in the blood. At a background body burden of 10-20 pg TEQ g -1 fat, the blood will contain an absolute amount of about 250-500 pg TEQ. A piece of 150 g pork meat with 10% fat and 600 pg TEQ g -1 would contain 9000 pg TEQ, i.e. 18 times as much as in the blood. For 100 gram of liver with 800 pg TEQ g -1 this would be even a 160 to 320-fold higher. Following consumption, these dioxins may therefore cause an increased level in the blood and as a result the temporarily increased exposure of sensitive tissues. This was confirmed by Hurst et al. (2000aHurst et al. ( , 2000b) ) showing that an acute exposure results in relatively higher levels in the foetus than a chronic exposure. Since fat tissue is a slowly perfused tissue, this situation may even last for a longer period of time (Hurst et al. 2000a). These data were used by the SCF (2001) to correct data from rat studies with single exposures in order to derive the TWI for chronic low exposure by increasing the critical body burden level by a factor 2.6. It is also appears that adverse effects in prenatal rats appear to occur from exposure during a relatively short time window [START_REF] Scf | Opinion of the Scientific Committee on Food on the risk assessment of dioxins and dioxin-like PCBs in food[END_REF]. Bell et al. (2007aBell et al. ( , 2007bBell et al. ( , 2007c) also compared an acute exposure with a chronic exposure in pregnant rats and confirmed that an acute exposure results in relatively higher exposure of the foetus. However, in these studies the chronic exposure resulted in a delayed puberty in male off-spring at much lower doses than in the pups of mothers exposed to a single high dose. In a recent review of the data, the authors argued that they might have missed the critical window in the single exposure or that the delayed puberty is actually caused postnatal, due to exposure through the milk [START_REF] Bell | Interpretation of studies on the developmental reproductive toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin in male offspring[END_REF]. In summary, it cannot be excluded that a peak exposure is relevant for the unborn exposed in utero. A re-evaluation of the existing data is required to evaluate the risk of an incidental high exposure. At this stage it seems wise to declare such high levels in future a clear food safety risk and to avoid such high exposures by preventive actions and adequate control of feed and food.

Private and Public Co-operation

The Dutch FBO (Company B) directly informed the positive dioxin finding of company A to their CA. The CA decided not to alert the public, but waited for more evidence and facts about the source of the contamination. In good co-operation and keeping the right responsibilities between CA and FBO, the source of the contamination could be identified. In this case it was very valuable to have an open exchange on data, both analytical data as well as knowledge about previous dioxin contaminations. In this regard it was important to have During this incident there was a very close contact between the Dutch and the RoI CAs from the first positive finding in The Netherlands (out of the monitoring of company B). With the available information from the Netherlands, the RoI CA could act faster since at that stage no dioxin results were available in RoI. Also sharing the information in an early stage with the European Commission was important to have a common strategy concerning the results and the situation. Normally the Rapid Alert System for Feed and Food is used to inform the EC and the other Member States of the EU, but this system is and should be used with objective data only. On the other hand the exchange of (fragmented) data outside this RASFF showed to be very valuable.

Conclusions

The incident with dioxin contaminated pork from the Republic of Ireland once again shows that serious dioxin incidents still occur, which may impose a heavy burden on the food chain.

Very high levels of 300 up to 800 pg TEQ g -1 fat in pork and pork liver were the result of the contamination of a relatively minor feed ingredient.

The described surveillance effort by the FBO demonstrates that good cooperation and exchange of data between food and feed business operators at different stages in the supply chain, and the official authorities and its bodies can lead to a fast tracing of the source of the contamination. The private surveillance systems in this case were able to identify the origin of contamination within 10 days after the first signal of increased dioxin levels in a product. This full responsive behaviour of FBOs is what is envisaged in the European legislation, where the This crisis nevertheless underlines that private quality systems might still have their weaknesses. The CAs should therefore enforce that every company implements an effective food safety system where this is not yet the case. And in times of crises they need to supervise whether the food producer is able to cope with these complex matters, and whether the HACCP-procedures are effective.

The small PCBs/dioxins ratio in this incident showed that analysis of indicator PCBs may not always be a valuable tool to identify dioxin contamination, even if PCBs are the source.

Therefore a test detecting dioxins and dioxin-like PCBs must be considered as the preferred test for the detection of these compounds. However, the capacity of the official GC/HRMS method is still expensive and the capacity limited. When for that reason a PCB-tests and pooling of samples is applied the limitations of the alternatives should be taken into account.

Routine screening with the CALUX ® assay might be a better alternative, especially at low contamination levels.

Several other lessons to be learned were identified. The interval between the sampling of the meat, mid October, and the testing end November, could have been shorter, and could have reduced the exposure of consumers to dioxins. Secondly, the outbreak investigation showed that trend analysis of contamination in fat in a slaughter by-product processing plant is an effective way for surveillance of major dioxin contaminations in animal production.

Prevention (integrated food and feed chain control) should require proper evaluation of production processes of feed ingredients. Special emphasis should be on drying processes as this is the second incident in five years with dried bakery waste (Hoogenboom et al. 2004a).

Last but not least a risk assessment is needed about the risk of incidental exposure to high levels of dioxins with special emphasise on potentially elevated dioxin levels in blood and Table 1: Tracing data for the origin of the carcasses deboned at Company B at October 13 th . Table 2: Country of origin of the processed pig skin for the production of fat and gelatine at the Belgian production plant Table 3: Levels of dioxins and PCBs in pooled RoI pork samples and in bread crumbs. Levels for dioxins and dl-PCBs in pg g -1 fat or crumbs, levels of indicator PCBs in ng g -1 fat or crumbs.

Total levels in pg TEQ g -1 fat, based on WHO 1998 -TEFs. Included is also the ratio between the sum of the 6 indicator PCBs and the PCDD/F-TEQ level. 7-8-2008 14-8-2008 21-8-2008 28-8-2008 4-9-2008 11-9-2008 18-9-2008 25-9-2008 2-10-2008 9-10-2008 16-10-2008 23-10-2008 30-10-2008 6-11-2008 13-11-2008 20-11-2008 27-11-2008 4-12-2008 11-12-2008 18-12-2008 25-12-

  efficient control of these contaminants is essential to reduce the exposure to the population.

  different sources, 1 pool sample per pallet was collected. The samples collected on November 25 th were analyzed by a private laboratory for indicator PCBs. A

  tests were initially performed at private laboratories using accredited CALUX ® , and indicator PCB tests. First positive test results were received on Friday

  in pork and fat again indicated that a serious dioxin contamination was taking place somewhere in Europe. The congeners profile showed a comparable distribution of congeners with the meat sample of company A which was a strong argument for one source of contamination.

  number of batches with elevated dioxin levels. This might have given more certainty about the country of origin, probably by mid November.

  and the dioxin TEQ varied between 2000 and 4000, being much lower than

  supply and to sample the incoming carcasses were in this case of crucial importance to identify the source of the contamination. In combination with the negative test results in pigs from Dutch and German origin, there was a high certainty about the origin of contamination after finding a few positive RoI carcasses.

  dioxins. However, in this case this suboptimal detection sensitivity was good enough for detection of highly contaminated samples (tracking and tracing) and could overcome the limited availability of test capacity for dioxin.

  days) at the highest recorded concentration of dioxins (200 pg WHO-TEQ g -1

  contact with the national reference laboratory for their knowledge on dioxins and also for a (very) quick confirmatory analysis of the samples positive in the CALUX-assay and showing elevated PCBs.

  clearly states that the first responsibility for the production of safe food lays at the food and feed business operators.

  of sensitive tissues and the foetus. In new incidents this assessment should be available and should than be regarded with respect to recalls. 2009b. The dioxin crisis in Ireland 2008 -challenges in risk management and risk communication.Organ. Comp. 71: 1169-1171.

FiguresFigure 1 :

 1 Figures

Figure 2 :

 2 Figure 2: Patterns of dioxins (A), dioxin-like PCBs (B) and indicator PCBs (C) in 3 meat

Figure 1 :

 1 Figure 1: Level of PCDD/Fs in fat in the Belgian gelatine production plant at date of sampling (A) in comparison with countries of origin of the pig skins in the batches where the country of origin was traced (B).

Table 1 :

 1 Tracing data for the origin of the carcasses deboned at Company B at October 13 th .

	B

Figure 2: Patterns of dioxins (A), dioxin-like PCBs (B) and indicator PCBs (C) in 3 meat samples, the highest fat sample from the gelatine plant, the bread crumbs and a sample of oil used for the drying process (indicator PCBs only). PCB 123 could not be determined in crumbs due to interfering substances.

Table 2 :

 2 Country of origin of the processed pig skin for the production of fat and gelatine at the Belgian production plant

	Country of origin	#	Avg.% of
		suppliers	supply
	NL	13	40%
	GE	13	40%
	RoI	2	7%
	UK	2	7%
	AU	2	4%
	PL	2	3%
	BE	2	2%

Table 3 .

 3 Levels of dioxins and PCBs in pooled RoI pork samples and in bread crumbs. Levels for dioxins and dl-PCBs in pg g -1 fat or crumbs, levels of indicator PCBs in ng g -1 fat or crumbs. Total levels in pg TEQ g -1 fat, based on WHO 1998 -TEFs. Included is also the ratio between the sum of the 6 indicator PCBs and the PCDD/F-TEQ level.

		meat 1	meat 2	meat 3	meat 4	meat 5	meat 6	meat 7 crumbs
	PCDD/Fs (pg g -1 )
	2,3,7,8-TCDF	7.3	2.0	2.0	0.1	1.7	10.2	6.4	12876
	1,2,3,7,8-PeCDF	2.7	0.9	0.7	0.1	0.7	4.4	3.7	3394
	2,3,4,7,8-PeCDF	130.2	58.5	54.6	4.3	52.8	231.3	156.5	12552
	1,2,3,4,7,8-HxCDF	48.0	27.8	21.8	1.6	23.3	91.4	61.8	2875
	1,2,3,6,7,8-HxCDF	15.9	9.8	6.6	0.5	7.9	33.5	25.8	1472
	2,3,4,6,7,8-HxCDF	12.2	6.1	5.4	0.4	5.2	24.6	17.7	2431
	1,2,3,7,8,9-HxCDF	0.1	0.1	0.1	<0.05	0.1	0.4	0.3	402
	1,2,3,4,6,7,8-HpCDF	7.7	5.4	3.8	0.4	5.4	17.3	12.3	695
	1,2,3,4,7,8,9-HpCDF	0.7	0.4	0.4	<0.05	0.4	1.8	0.9	124
	OCDF	0.1	0.2	<0.10	<0.10	*	*	0.1	38
	2,3,7,8-TCDD	0.1	<0.05	<0.05	<0.05	<0.05	0.2	0.1	11
	1,2,3,7,8-PeCDD	0.2	0.1	0.1	<0.05	0.1	0.6	0.3	34
	1,2,3,4,7,8-HxCDD	0.5	0.3	0.2	0.1	0.4	*	0.6	21
	1,2,3,6,7,8-HxCDD	0.2	0.2	0.1	0.1	*	0.5	0.3	22
	1,2,3,7,8,9-HxCDD	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	10
	1,2,3,4,6,7,8-HpCDD	0.6	0.4	0.4	0.2	*	0.7	0.5	67
	OCDD	1.8	1.5	3.3	4.7	4.2	5.0	2.0	50
	dl-PCBs (pg g -1 )
	PCB 81	0.2	0.1	0.1	0.1	0.1	0.3	0.2	164
	PCB 77	1.6	0.6	0.7	0.3	0.6	2.2	1.3	2866
	PCB 126	1.6	0.7	0.7	0.2	0.7	2.8	1.4	294
	PCB 169	3.2	2.3	1.5	0.4	2.5	5.5	3.7	50
	PCB 123	555.0	274.0	271.0	23.8	498.0	1270.0	590.0	*
	PCB 118	1410.0	832.0	597.0	107.0	629.0	2500.0	1810.0	80500
	PCB 114	22.3	13.4	11.2	<10.0	11.6	40.5	29.7	1050
	PCB 105	142.0	80.0	55.6	14.2	58.1	297.0	195.0	17600
	PCB 167	291.9	140.5	140.0	11.5	117.0	455.0	308.0	15500
	PCB 156	3610.0	2600.0	1540.0	143.0	2790.0	6750.0	4420.0	41200
	PCB 157	365.0	265.0	158.0	17.4	295.0	696.0	438.0	3330
	PCB 189	703.0	537.0	296.0	25.6	616.0	1410.0	854.0	6970
	WHO-PCDD/F-TEQ	74.1	34.2	31.2	2.5	30.5	132.9	90.3	8510
	WHO-PCB-TEQ	2.5	1.7	1.1	0.1	1.8	4.6	3.0	64
	WHO-PCDD/F-PCB-TEQ	76.5	35.9	32.2	2.7	32.4	137.5	93.3	8574
	indicator-PCBs (ng g -1 )
	PCB 28	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	33.8
	PCB 52	1.4	0.5	0.5	<0.1	0.9	2.5	1.5	38.9
	PCB 101	1.8	0.7	0.6	<0.1	0.9	3.2	2.1	197
	PCB 153	53.9	41.2	23.6	2.2	47.1	105.0	64.0	458
	PCB 138	36.9	26.7	17.1	1.7	35.0	73.9	45.7	302
	PCB 180	41.4	31.0	17.6	1.6	36.5	81.4	49.3	383
	Sum indicator PCBs	135.5	100.3	59.4	5.7	120.5	266.2	162.7	1413
	ind-PCBs / PCDD/F-TEQ	1829	2932	1908	2257	3946	2003	1802	166

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Acknowledgements

The authors thank Tanja Vervust for the extended tracing suppliers of pig skin and they are grateful to the RoI authorities for releasing the profiling information.

The research leading to these results has received funding from the European community's Seventh Framework Programme FP7/2007-2013 under grant agreement n° 228821