Christian Berger-Vachon

Pierre Antoine Cucis

Eric Truy

Hung Thai Van

Stéphane Gallego

Pierre Antoine

H Thai Van

Cochlear Implants: Consequences of Microphone Aging on Speech Recognition

Keywords: Cochlear Implants, Microphone aging, CI & NH listeners, Coding strategies, Cleaning procedures, Syllable recognition in noise

published or not. The documents may come

COCHLEAR IMPLANTS AND AGING

Cochlear implants (CI) have successfully restored partial hearing to over 400,000 deaf persons in the world and the rhythm of implantations is high in the world with a minimal rate of about 30,000 implantations [START_REF] Aguiar | Information theoretic evaluation of a noiseband-based cochlear implant simulator[END_REF].

Economically, CI is a good investment for the society. The machine is expensive (about 25,000 $), the surgery also and the follow up of the patient during the first year (speech therapy, audiology) pushes this figure to 50,000 $. Nevertheless, if we consider the cost of a handicapped persons during his life (education in dedicated centers, perturbed familial life, professional limitations and so on) fitting a deaf person with a CI is worthwhile to be done and many social systems cover the expense mostly when it is for children.

Reliability

Designed to last for more than ten years, CI suffers from failures and some of them come from aging [START_REF] Battmer | A multicenter study of device failure in European cochlear implant centers[END_REF][START_REF] Balkany | Cochlear implant soft failures consensus development conference statement[END_REF] and one of the goal of engineering is to reduce their impact. In this paper we wish to discuss this issue.

Cochlear implants are biomedical prostheses which restore some hearing in profoundly deaf subjects. Basically, they are made with two main components, an external device which is for speech reception and processing, and an implant which decodes the information transmitted by the external device and distributes it to the ends of the auditory nerve in the inner ear. All the elements are subjected to aging and patients are deeply affected when failures occur in the system. There are two kinds of failure, the hard failure easy to detect as the machine breakdown is seen immediately and the soft failure when the failure is intermittent. Another kind of burden, more insidious, is the progressive loss of efficiency on one of the components of the machine. Among them the attack of the microphone is one of the hazards affecting the machine. Several alerts have been raised and some dramatic stories about alterations on the opening in hearing aids have issued warnings [START_REF] Schweitzer | Mind the ports! The Effect of Severe Microphone Inlet Occlusion: port disasters, or how everyday activities can lead to severe microphone occlusion in a directional hearing aid[END_REF] which must not be ignored. A check of the microphone entrance often discovers debris accumulated in undesirable locations and the input sound pathway is more or less obliterated.

Microphone aspect

The microphone repair issue is well ranked into the external failures of a cochlear implant [START_REF] Pereira | Repair issues associated with cochlear implants external components: the influence of age and time of use[END_REF][START_REF] Silverman | Repair Issues Associated with Cochlear Implants[END_REF]. It comes in the fourth or fifth position in the classification after the headpiece (antenna), the battery compartment and the speech processor and it represents nearly 11% of the repairs.

But the progressive drift of the microphone properties is often not noticed and everybody should keep in mind that the microphone works in very adverse environments which can range from very cold and dry to very humid and hot, with a common exposure to sweat, dirt, dust, grease and so on. This is a very aggressive situation and moisture and grease are likely to affect the sensitivity of the microphone and its frequency response.

Consequently this behavior should be studied and this aspect of aging should not be ignored. In a previous approach Razza et al [START_REF] Razza | Acoustical signal check: microphone integrity evaluation through a common hearing aid analyzer. 5th Object[END_REF][START_REF] Razza | An underestimated issue: unsuspected decrease of sound processor microphone sensitivity, technical, and clinical evaluation[END_REF] indicated that speech tracking scores are correlated significantly with the loss of the microphone sensitivity. Responses curves were compared to those of fully working microphones and the consequences on speech recognition need to be studied. This situation is not very simple as the use of cochlear implants occurs often in noise which is part of our usual environment.

Coding strategies

CI allows several coding strategies which can be implanted on almost all the machines, and the choice of a relevant coding strategy is something which must be discussed.

Basically, in CI, the speech input spectrum is divided into frequency bands, and this "simplified" spectrum is distributed to the electrodes distributed along the cochlea which is the hearing organism.

Two main schemes are usually taken to program the speech processors by the CI manufacturers:

 n-of-m: the speech spectrum is divided into m bands, and the n with the highest energy are kept and transmitted to the cochlea. This scheme is popular, because the presentation of selected acoustic features corresponds to an approach based on the phonetic theory. In this circumstance, only a subset of the available electrodes is stimulated. Also, as the number of active electrodes is limited, it is expected that the overlap between two successive stimulations is small, but this may be not true if two consecutive electrodes are selected [START_REF] Wouters | Sound Coding in Cochlear Implants: From electric pulses to hearing[END_REF].  CIS (Continuous Interleaved Sampling): it is a sound processing scheme designed to enhance the delivery of temporal fine structure to the implant, in order to beat the poor intelligibility of speech in a competing noise. All the band pass filters are kept and the number of electrodes is reduced, leading to a wide spacing, between two consecutive electrodes, which is likely to reduce the interaction. Also, the stimulation rate is high and it enhances the delivery of temporal fine structure to the implant, and this is an advantage in a competing noise [START_REF] Wilson | New processing strategies in cochlear implantation[END_REF].

Noise influence

The choice of the noise is also a major issue. The effect of listening conditions determines the performances of the listeners. The signal to noise ratio (SNR) is an important parameter for the assessment of the recognition performances. Lowering the SNR spoils the incoming speech signal and this action is worthy to be measured. Also, with different values of the SNR it is easy to avoid the ceiling and the floor effects, enabling the determination of some characteristic parameters traditionally used in the evaluation of speech recognition [START_REF] Garnham | Speech understanding in noise with a Med-El COMBI 40+ cochlear implant using reduced channel sets[END_REF].

In our work, the choice of the Fournier's lists (a French equivalent to the spondee lists) led to recognition percentages ranging from 0% to 100%, according to the SNR, and it was suitable to the aims of our experiment.

NH versus CI

Two aspects are generally considered when coding strategies are examined in cochlear implants: the recognition seen with patients using the coding schemes, and the behavior of normal hearing (NH) subjects facing the signal (a simulation is done for NH subjects). Both performances are complementary as in one case (CI users) we have the direct results with the group of persons concerned by the prosthesis and in the second group we can test the signal action using a more comprehensive approach [START_REF] Dorman | Recognition of monosyllabic words by cochlear implant patients and by normal-hearing subjects listening to words processed through cochlear implant signal processing strategies[END_REF]. In other words, one matter is more oriented to the patient and the other tests the signal. Both groups receive the same amount of information about speech, but the outcomes may be different or similar [START_REF] Shannon | The number of spectral channels required for speech recognition depends on the difficulty of the listening situation[END_REF][START_REF] Dorman | The identification of consonants and vowels by cochlear implant patients using a 6-channel continuous interleaved sampling processor and by normal-hearing subjects using simulations of processors with two to nine channels[END_REF]. Furthermore, the CI users group is very heterogeneous and the standard deviation in the measures is large making the statistical interpretation of the results more difficult. On the contrary, with the NH listeners, several strategies and microphone drift are evaluated on the same people and intra-subject studies can be done making the evaluation more accurate.

Organization of this study

The aim of this study is to assess the behavior of two coding strategies (n-of-m and CIS) in front of the aging of the CI microphone, and to give some indication to the audiologists in charge of CI settings in the specialized clinical centers.

After an introduction of the CI issues about aging and the main elements which will be manipulated to carry out the study, the second section introduces the tools for the simulation (CI and microphone drift), the subjects who participated to the experiment and the mathematical fitting of the recognition results. Then in the third section, the results (recognition percentages and audiological features) obtained with the CI users and the NH subjects are presented.

The fourth section is a discussion about the different elements which were used in this work, The comparison of the performances of CI users and NH subjects, the choice of the noise and evaluation of the strategy and how to reduce the influence of the microphone drift with aging. Finally, a conclusion sums up the main contributions of this work.

MATERIAL AND METHODS

Stimulation

Cochlear implant principles

We will not expose the details of signal processing in cochlear implants here, but only the essential principles (figure 1). Cochlear implant can be divided into two main parts:  The external block (signal processor) processes the signal and separates the sound in multiple frequency bands (channels).

 The internal block (the implant) receives the information from the processor and distributes the energy of the channels to the electrodes along the cochlea.

The information is transmitted from the processor to the internal part through the skin by a radiofrequency signal.

Sounds in the environment are captured by the microphones that are very impacted by soiling and aging. Then the processor fits the signal received from the microphones to the patient's physiology and separates the signal into multiple frequency bands. The frequency bands correspond to the treatment channels scaled using a logarithmic scale. We chose to use the Bark scale for our CI simulator [START_REF] Traunmüller | Analytical expressions for the tonotopic sensory scale[END_REF].

Most of the existing sound coding strategies are based on two main principles: there are "CIS-like" strategies and "n-of-m" strategies. It is interesting to notice that, for the n-of-m strategies, if n = m it is very similar to CIS. So the value of n is important to be considered. The different steps of the signal processing are:

 The input signal goes through a pre-emphasis filter which is a high-pass filter (cutoff frequency 1.2 kHz and slope 3 dB/octave).  The signal is then sampled (16 kHz sampling frequency, 16-bit quantization), a short term fast Fourier transform (FFT) is applied to the samples, frame length is 128 points (8 ms), leading to 64 spectral beams (amplitude and phase), ranging from 0 to 8 kHz. The step between two beams is 125 Hz.  Then, the spectral beams are grouped into frequency bands which are logarithmically distributed, according to the ear physiology (Bark scale). Considering the usual values taken in cochlear implants we used 20 bands (leading to 20 channels).  In each band, the energy is calculated using the Parseval's formula (the squares of the amplitude of each beam are added). In the "CIS-like" coding all the channels are taken. For the n-of-m coding only the 8 more energetic channels are kept. The value n = 8 is classical in cochlear implants. There is a frame overlap of 6 milliseconds (75% overlap), and a set of pulses is calculated for each frame.  Each channel is represented by a narrow-band spectrum coming from a white noise spectrum. The amplitude of the narrow band follows the energy detected in the corresponding channel. The first two-channels, which are very narrow, were represented by sine waves. The output signal is obtained by summing the selected channels (8 for n-of-m; 20 for "CIS-like").

Evaluation of the frequency drift of a hearing aid microphone

In this study we tested speech recognition in noise with CI users, before and after a microphone cleaning process. For NH subjects we used a CI simulator integrating a low-pass filter mimicking the microphones frequency drift due to soiling. The low-pass filter approximated a measured median attenuation. We based our work on a study about microphone frequency drift due to soiling on conventional hearing-aids. Knowles electronics (Itasca, Illinois, USA) is the manufacturer which leads the market of microphones for hearing-aids and cochlear implants. That is why we considered that this study matched for cochlear implants' microphones too.

The study of the microphones sensitivity loss had been conducted, during a routine maintenance appointment in a hearing care center, on regular hearing aids worn by n hearing impaired patients (n = 129) [START_REF] Serra | Effet de l'entretien des aides auditives sur leurs performances[END_REF]. Two steps were necessary in order to check the microphone frequency drift. First the earphone and the ear mold were deeply cleaned leading to the state "clean earphone and dirty microphone". Secondly, the microphone was carefully cleaned, leading to the state "clean earphone and clean microphone". The subtraction of the two states gave the attenuation due to the microphone soiling.

Then the microphone transfer function was evaluated after a frequency warble sweep ranging from 200 Hz to 8,000 Hz (at 60 dB SPL).

Four degrees of attenuation have been defined:

-Case 1: clean microphone (no loss); the response was not modified, -Case 2: medium loss corresponding to the attenuation seen on 50% of the microphones (percentile 50%) -Case 3: strong loss, corresponding to the 20% worse microphones (percentile 80%), -Case 4: very strong loss, corresponding to the 10% worse microphones (percentile 90%).

The clean signal was modified by the different attenuation curves and then passed through the vocoder.

On figure 3, the three degrees of loss of sensitivity, corresponding to the 50%, 80% and 90% percentiles, are indicated. Each degree was represented by two straight lines for simplification. The signal attenuation started at 800 Hz and then three slopes were introduced, respectively, 2 dB, 7 dB and 9 dB per decade.

Tests

Participants

The work presented in this paper follows a previous pilot study [START_REF] Perreaut | Influence of Microphone Encrusting on the Efficiency of Cochlear Implants Preliminary Study with a Simulation of CIS and "n-of-m" Strategies[END_REF] and it was approved by the French Ethics Committee "Sud Est 2" (August, 27, 2014), under the supervision of the HCL (Civil Hospices of Lyon). All the participants filled an agreement form prior entering the study and all the recordings were made by an audiologist.

CI users

Sixty-one CI patients were included in this study. Their ages ranged from 18 to 60 years (average 37 years old). Nineteen subjects were fitted with "CIS like" strategies and 42 had the n-of-m strategies.

The population of the CI users had been parted into four groups:

-Subpopulation 1 (SP1): Med-El ® and Advanced-Bionics ® subjects (19 patients): "brush and blow" cleaning procedure and CIS-like coding strategies, -Subpopulation 2 (SP2): Neurelec ® subjects (13 patients); "brush and blow" cleaning and n-ofm strategies, -Subpopulation 3 (SP3): Cochlear ® subjects (14 patients): "brush and blow" cleaning and n-ofm strategies, -Subpopulation 4 (SP4): Cochlear ® subjects (15 subjects): replacement of the protective filter and n-of-m strategies.

NH subjects

Twenty NH subjects participated to this experiment. Their age ranged from 18 to 33 years (average 25 years). All the subjects had an ORL examination before entering the study, in order to eliminate previous pathologies or deafness which may corrupt the results. The auditory thresholds were measured; they were always below 20 dB HL for all the frequencies between 250 and 8,000 Hz. According to the BIAP (International Bureau for Audio-Phonology) all the subjects were considered as NH listeners.

Session

Material

In this work we used the French dissyllabic Fournier's lists. These lists were well adapted to the patients' recognition ability in this environment. They were uttered by a male voice and were the vocal part of the signal. They are similar to the spondees lists used in English. Each list contains 10 disyllabic words (for instance "le bouchon" = the cork). Forty lists are available, and the recognition unit was the syllable. So the recognition step was 5%.

A cocktail-party noise has been used in this study. This noise was a mix of voices from 8 speakers, 4 males and 4 females.

Test sessions

The signal delivered to the subjects was made of the Fournier's lists mixed with the cocktail-party noise. The mixing was managed by a Madsen Orbiter 922 audiometer and the signal to noise ratio (SNR) was perfectly adjusted.

*CI users

For CI users, tests were made in free field in an audiometric booth. In all cases only one ear was stimulated. When a subject was fitted with two implants, only the best ear was kept.

For NH subjects, the Fournier's lists and the cocktail-party noise were first processed by the CI simulator and then passed through the audiometer and finally delivered on the right ear by a TDH 39 headset.

For the two groups of subjects the experiment consisted in a speech audiometry in noise. The maximum level delivered was below 65 dB SPL. According to the conditions requested by the ethics committee it did not exceed the 80 dB SPL limitation recommended for professional noises exposure.

The recognition scores of the CI users were collected before and after cleaning the microphone, leading to two different situations ("dirty and clean"). Our experiment took place at the beginning of the periodical clinical check and device setting occurring periodically at the CRIC (Cochlear Implant Setting Center) located in the ORL department of the Edouard-Herriot University hospital of Lyon. This check-up consists of an appointment with a speech therapist, a setting of the implant parameters and a clinical examination. Consequently, in our study the microphone cleaning occurred before the classical check-up. This device check follow-up is carried out at least once a year.

We used two different procedures to clean the microphones ports depending on the type of speech processor: A "Brush and Blow" technique (commonly used in French hearing care centers) and the replacement of the protective filters. The "brush and blow" procedure consists in brushing and blowing a flow of dry air on the microphones ports to remove dirt and dust.

The following tasks were realized in our work:

-Verification of the patient's medical file, -Short training session to help the patient to understand the instructions, -First test with the Fournier's lists, before cleaning the microphone. The lists were presented to the patient with an increasing level of difficulty (SNR decreased from 18 dB to -3 dB; step -3 dB), -Microphone cleaning, -Second test with different Fourier's lists after cleaning the microphone.

The full session lasted about 30 minutes. Sixteen Fourier's lists were used in this experiment (8 before cleaning and 8 after) and the lists were not repeated.

*NH subjects

For NH subjects, stimuli were delivered to only one ear similar to the experiment with CI users. Furthermore, we chose to test the subjects on the right ear considering that the left brain auditory cortex is specialized in processing speech.

Eight conditions have been considered:

-2 coding schemes (CIS-like and n-of-m) -4 degrees of microphone sensitivity loss.

Five levels of SNR were tested for each condition leading to 40 (2*4*5) combinations. Each combination was assigned to a Fournier's list so that the lists were not repeated. Each session started with a short training period to help the listener to understand the instructions. Then the 40 lists were randomly presented to each subject. The session lasted about 45 minutes.

Mathematical representation

The recognition percentages versus the SNR can be represented by a sigmoid regression curve (figure 4).

Three features were considered here:

-the SNR corresponding to 50% of the maximum recognition plateau denoted here x 50% ; -the "slope" (SNR interval, given in dB, between 25% and 75% of the maximum recognition) and denoted here Δ 75%-25% ; -the top asymptote y max shows the maximum recognition score. These analytical values are represented on the sigmoid curve. The minimum recognition was 0% (seen for SNR = -3dB). Thus, the sigmoid equation is:

1

Where:

-y is the recognition percentage -x is the SNR -a is y max -c is x 50% -b is linked to the slope: b= 2.2/ Δ 75%-25% => Δ 75%-25% = 2.

 N-of-m coding scheme; ANOVA and post-hoc analyses

A two-factor repeated measure ANOVA was used to analyze the results. It indicated a significant effect of SNR [p < 10 -4 ] and of the degree of soiling [p < 10 -4 ]. Furthermore a significant interaction was seen between them [p < 10 -4 ].

Table 1 shows the results of Wilcoxon's tests taken for the paired post-hoc comparisons, between soiling conditions, with respect to the SNR; significant differences are highlighted.

Largest differences were located at 9, 3 and 0 dB of SNR (Tab 1). The results indicated that Case 1 was better than Cases 2 and 4 at the highest SNR (9 dB). For the SNR 3dB the intelligibility for the Case 1 was significantly higher than for the Cases 3 and 4 and also Case 2 was better than Case 4. Finally, Case 1 was better than Cases 2, 3 and 4 for the SNR 0 dB.

NH n-of-m p-value The two-factors repeated measures ANOVA indicated a significant effect of SNR [p < 10 -4 ] and of the degree of soiling [p < 10 -4 ]. Furthermore a significant interaction was seen between them [p < 10 -4 ]. Table 2 shows the results of Wilcoxon's tests for the paired post-hoc comparisons with respect to the SNR. The significant differences are highlighted.

C1-C2 C1-C3 C1-C4 C2-C3 C2-C4 C3-C4 SNR9
Largest differences are located between 3 and -3dB of SNR (Tab. 2). The results indicate that Case 1 was better than Cases 3 and 4 and that Case 2 was better than Case 3 at the SNR 3dB. For the SNR 0dB the intelligibility for Case 1 was significantly higher than for the Cases 2, 3 and 4. Finally, Case 1 was better than Case 3 for the SNR -3dB.

CI users

The effect of cleaning the microphone ("Brush and Blow" procedure) was evaluated on the recognition percentages; it is represented on figure 7 according to the strategies. It indicates the efficiency of cleaning. The heterogeneity in the results did not allow definitive conclusions and only indicated clues.

 N-of-m coding schemes; ANOVA analysis

For the n-of-m strategies the improvements (figure 7b) were mostly noticed for the high values of the SNR (12, 15 and 18 dB) and there was an average improvement of 2.7% in syllable recognition after cleaning (corresponding to " ½ syllable" per list).

The two-factors repeated measures ANOVA indicated a significant effect of SNR [p < 10 -4 ] but no difference between before and after cleaning [p = 0.056].

 CIS-like coding schemes; ANOVA analysis

For the CIS-like strategy (figure 7a), the best improvements were observed in the SNR range 0 to 9 dB (0, 3, 6, and 9 dB) and the average improvement after cleaning was 2.9%.

The two-factors repeated measures ANOVA indicated a significant effect of SNR [p < 10 -4 ] but no difference between before and after cleaning [p = 0.12]. In the case of the replacement of the protective filter, there was an average improvement of 4.7% in syllable recognition after replacement (corresponding to one syllable per list); the results are represented on figure 8.

The two-factors repeated measures ANOVA indicated a significant effect of SNR [p < 10 -4 ] but no difference was seen between before and after cleaning [p = 0.43] and no interaction occurred between the two factors [p = 0.91]

Percent syllable correct

Signal to Noise Ratio (dB) b) Cochlear, "Brush and Blow" n = 14

Before After Δ = Af -Bf

Audiological values 3.2.1. NH subjects  N-of-m coding schemes; ANOVA and post-hoc analyses

A one-factor repeated measures ANOVA indicated a significant effect of the soiling degree on x 50% [p < 10 -4 ] but did not reveal a difference for the features Δ 75-25% [p = 0.52] and y max [p = 0.32].

We can see on tab 3 that the feature x 50% was better for Case 1 compared to Cases 2, 3, 4. Also, x 50% for Case 2 was significantly better than for Case 4.

NH n-of-m p-value

C1-C2 C1-C3 C1-C4 C2-C3 C2-C4 C3-C4
x 50% 0.0076 0.0003 0.0008 0.1403 0.0458 0.3411 Tab 3: CIS-like, feature x 50% paired comparisons between the 4 cases  CIS-like coding scheme; ANOVA and post-hoc analyses

Again we used a one-factor repeated measure ANOVA.It indicated a significant effect of the soiling degree on the feature x 50% [p < 10 -4 ] but did not reveal a difference for the features Δ 75-25% [p = 0.48] and y max [p = 0.43].

We can see on tab 4 that the feature x 50% was better for Case 1 compared to Cases 2, 3, 4. Also, x 50% for Case 2 was significantly better than for Case 3.

NH CIS-like p-value C1-C2 C1-C3 C1-C4 C2-C3 C2-C4 C3-C4
x 50% 0.0038 0.0002 0.0012 0.0349 0.1978 0.1851 Tab 4: CIS-like, feature x 50% paired comparisons between the 4 cases

CI users  n-of-m coding schemes; Wilcoxon's tests

The Wilcoxon's tests indicated no significant effect of the cleaning on the curve features x 50% [p = 0.66], Δ 75-25% [p = 0.85] and y max [p = 0.35].

 CIS-like coding schemes; Wilcoxon's tests

The Wilcoxon's tests indicated no significant effect of the cleaning on x 50% [p = 0.63], Δ 75-25% [p = 0.29] and y max [p = 0.41].

Filter replacement procedure  Wilcoxon's tests for the procedure "filter replacement"

The Wilcoxon's tests indicated no significant effect of the cleaning on the curve features x 50% [p = 0.96], Δ 75-25% [p = 0.62] and y max [p = 0.19].

GENERAL COMMENTS About the present study

Several indications came out of this work:

 The higher the SNR, the better the recognition (this is obvious). What is less obvious is the SNR for the ceiling effect. For NH subjects it was 6dB for CIS and n-of-m whatever the microphone drift. Also, the floor effect (0% recognition) was always reached at -3dB of SNR. For CI users, the ceiling was seen for 15dB, before and after cleaning for CIS and n-of-m.  Cleaning improves the recognition (this is sensible). For NH subjects the improvement could not be seen for the extreme values of the SNR range (-3dB and +9dB). The importance of cleaning was maximum for the medium values of the SNR range (0 and 3dB). For CI users, the results were similar to the NH subjects; only the improvement was reduced.  Considering the sound coding strategies, the recognition percentages obtained with CIS are better than those seen with n-of-m, except for the extreme values (floor and ceiling effects). Some exceptions can be pointed out:

For NH subjects, ceilings were not the same for CIS and n-of-m, when the microphone was strongly dirty. For CI users when the microphone was clean, the n-of-m scheme led to better results than CIS at the high values of the SNR. May be due to the heterogeneity of CI users, this result was not significant.

Consequently, the robustness of this finding has to be established and further studies should be done to clarify this aspect. So far we may suggest that the coding scheme could be adapted to the noisy environment. Also, a cleaning protocol should be inserted in the regular check of the machines.

Another aspect which was seen was the cleaning procedure: "Brush and Blow" compared to the microphone protective filter replacement.

Basically it seems that the replacement led to a better improvement of the recognition percentage than the "Brush and Blow" procedure. In our experiment, the "Brush and Blow" procedure was performed by a qualified audiologist; it may be different when audiologists which are not used to this procedure have to accomplish this task, mostly if they are worried about damaging the device. If the filter replacement is cheap and fast, it may be a safer approach to systematically change the head filter protecting the microphone, but it has to be seen in our work that it is not worthy to be done.

Going further, a replacement of the microphone can be suggested but other questions are open by this procedure. Obviously, a new microphone would be the best solution, but this is more costly and the connection can lead to contact problems; the junction can be fragile and could be a source of hard failure. Also, for reasonable and good values of the SNR, the difference of performances seen between a new microphone and a microphone with a small drift (50% of the users) is very light and not significant (result seen with the NH subjects). Also, cleaning the microphone in the case of CIS users, did not lead to a dramatic increase of the performances (possibly, the microphones were only slightly soiled).

About the microphone

Aging occurs for all components but the action is not the same for all. A breakdown is an acute failure, and it will be followed by an immediate repair. When the fault is irregular, it is more difficult to stop it, because no one knows when the incident will occur and this is a very worrying situation. When the trouble comes insidiously, it is often not perceived and the subject loses progressively his efficiency without being aware of it. This is more or less what happens during the life; someone gets older, in good health, and he hardly notices the decrease of this aptitude to perform many tasks in the life, mentally or physically. Slopes get steeper, mental activities get harder, but the subject can still cope, even if it is a little bit more difficult. This is typically what happens with the microphone; it does not break, but it gets less efficient and the user is not aware of it. This is why this evolution must be studied; what are the consequences and how we can beat it with rather inexpensive actions, in time and in money? The community surrounding a patient, a cochlear-implantee in our case, must be aware of this condition and should take the necessary steps to restore the good conditions attached to the machine. Several protective measures are easy to imagine. To get a good protection to moisture, grease, sweat, dust, debris, a head filter is a good shield for the microphone and it should be systematically checked by the audiologist each time the patient come for a follow up with the medical team. The degree of sensitivity reduction has its importance; this reduction can be beaten; we should lower the signal perturbation, with an attenuation and a systematic shift of the frequencies seen with an increase of the stiffness and of the dirt of the membrane [START_REF] Razza | Acoustical signal check: microphone integrity evaluation through a common hearing aid analyzer. 5th Object[END_REF].

A study, in simulation, can bring situations which are not often seen in real life; it emphasizes effects which are attached to extreme conditions and sheds more light on the issue we want to study. The influence of the modifications of the properties of the microphone membrane was tested in classical hearing aids checked by a hearing aid analyzer [START_REF] Serra | Effet de l'entretien des aides auditives sur leurs performances[END_REF]. Then all the situations (coding scheme, microphone drift, signal to noise ratio) were presented to the same NH subjects, a rather homogenous group, and powerful paired student's tests could be conducted. Obviously, the study in simulation cannot ignore the behavior and the performances obtained with CI users as this group is the target for this technology.

About the listening conditions

Three approaches have been done in this work, simulation with NH subjects, simulation of the microphone drift and addition of noise. Noise needs to be introduced in this kind of work for two reasons. First noisy situations are very common in the usual environment, for CI users and for NH subjects. Secondly, to get understandable results, one must avoid the floor and the ceiling effects; the range of the noise, associated with a good choice of the speech signal, allows to fulfill this requirement. Experimental conditions are a real issue, and their choice is far from being easy; pilot studies are more than necessary [START_REF] Perreaut | Influence of Microphone Encrusting on the Efficiency of Cochlear Implants Preliminary Study with a Simulation of CIS and "n-of-m" Strategies[END_REF].

Then, the assessment of a listening condition cannot be done only by the comparison of recognition percentages. Other speech tests are worthy to be considered as they throw another glance on the performance. These parameters help to understand a scatter plot, and we have calculated the SNR for the mean recognition, the slope representing the action of the noise on the percentages, and the ceiling indicating the maximum performance reached in a given situation. In order to do so, a mathematical fitting was necessary, and finally it was worth to be done regarding the data synthesis it provided. Practically, it is necessary for hearing aid management and rehabilitation of CI recipients that their devices are functioning well and are set by an audiologist during the check which is periodically done in all the clinics. The decrease of the microphone sensitivity is part of this clinical evaluation and it should be corrected, as much as possible. Nevertheless the correlation between the decline of the microphone sensitivity and its consequences on speech recognition scores was not found to be dramatic.

Strategy

As indicated above, CIS strategy was less sensitive to the microphone drift than the n-of-m coding, mostly in the middle of the SNR range. This is an indication to the audiologists in charge of CI settings and to the manufacturers.

Is it only due to the stimulation speed, the spacing between the electrodes which lowers the overlapping? Is the spectrum coverage more useful to the brain than a selection of the phonetic features presented to the brain? For many researchers, CIs work, "because there is a brain behind" and we should not forget the efficiency of this premier organ of our body. We give information to the brain, then let it work.

Then other settings on the strategy have their influence: sampling rate, filters settings, length of the time frame (window), and they have been studied by many authors [START_REF] Aguiar | Information theoretic evaluation of a noiseband-based cochlear implant simulator[END_REF][START_REF] Kerber | Sound localization in noise by normal-hearing listeners and cochlear implant users[END_REF][START_REF] Hazrati | Comparison of two channel selection criteria for noise suppression in cochlear implants[END_REF][START_REF] Shannon | Speech recognition with primarily temporal cues[END_REF]. Most of the studies concluded to an advantage of the CIS coding over n-of-m. Then, the Advanced Combination Encoder (ACE) scheme is very efficient; it uses a very fast updating of the data and therefore provides a fine temporal analysis of the signal, indicating that spectrum is not the only component of speech.

Then the results are subjected to the patient's condition and mostly to the degree of nerve survival and to the intellectual background; it has consequences on the appropriate strategy for individual implant patient [START_REF] Wilson | Comparative studies of speech processing strategies for cochlear implants[END_REF]. A good adjustment of a strategy to a subject is a goal to be reached, and it should be adapted throughout the time.

Noise

The robustness of the strategies should be also seen in noise. In a background noise, the performance of CI users is significantly better when using a full filter bank approach ("CIS-like") in comparison to using a peak picking method. Consequently, when the signal is distorted (noise or microphone drift) CIS coding seems to be more robust, but this is not unanimously accepted [START_REF] Nelson | Understanding speech in modulated interference: Cochlear implant users and normal-hearing listeners[END_REF].

With CI users the performance in noise is very difficult to interpret and it is related to the patient's characteristics and to the wide variability in this group of people. The degree of deterioration in noise was similar with each strategy. Also the number of channels needed to understand speech in noise is still an open question [START_REF] Loizou | Recognition of sentences in noise by normal-hearing listeners using simulations of speak-type cochlear implant signal processors[END_REF]. More channels are needed to understand speech in noise than in quiet, both for CI users and NH listeners, even if results are better in simulation with NH subjects. Again, the number of electrodes (channels) must be kept low to reduce the overlap between the electrodes.

A recommendation which is classically delivered when hard of hearing people are concerned is to provide good listening conditions (high SNR) and to speak slowly and distinctively. This situation is not always easy to meet and we should reduce the noise level as much as we can. Consequently many studies have been done to lower the noise, in other words to increase the SNR [START_REF] Jeanvoine | Contribution of Noise Reduction Algorithms: Perception Versus Localization Simulation in the Case of Binaural Cochlear Implant (BCI) Coding[END_REF][START_REF] Kallel | Influence of a shift in frequency distribution and analysis rate on phoneme intelligibility in noisy environments for simulated bilateral cochlear implants[END_REF][START_REF] Hu | A new sound coding strategy for suppressing noise in cochlear implants[END_REF]. This is attractive, but not always easy to implement as hypotheses should be made about the noise.

Aging is an important issue for human, animal, vegetal and machines. "Time flows and we follow". Wisdom is to adapt and to find the best strategies to reduce the consequences; this is also true for the cochlear implants microphones and that was the aim of this dissertation: how can we lower the impact of time?

According to us, to lower that impact on the microphones several suggestions can be made to keep the microphone function:

 "Stop the time flow" (unfortunately this is not possible),  Use the Brush a Blow cleaning procedure,  Make a head filter replacement,  Perform a microphone replacement.

The cheapest is to apply a "Bruch and Blow" cleaning procedure at every routine check and the results on the performance should be satisfactory. So we can recommend this procedure to the specialized teams in charge of the CI patient's care.

CONCLUSION

In this work we have studied the effect of soiling and aging of the microphones of cochlear implants on the recognition of syllables in the French Fournier's disyllabic lists of words.

Several parameters were studied in this work:  The influence of the coding strategies (CIS-like and n-of-m),  The action of the signal to noise ratio,  The effect of cleaning the microphones.

Experiments were conducted with CI users and NH listeners as the two approaches are complementary. Also, two procedures to clean the microphones were considered, the classical "Brush and Blow" and the replacement of the protective filter of the microphone. With CI users we were in the true situation, and with NH subjects we could see more analytically the issue and we could perform a more precise analyses using data taken on the same person.

Results indicated that:

 The CIS-like coding strategies were more robust than the n-of-m strategies regarding the microphone drift; the difference could reach two syllables in a list (10%) and it was seen mainly in the middle of the SNR range,  The SNR effect was very important, the recognition percentages ranging from 0% (SNR = -3dB) to 100% (SNR = 9dB or higher). Results were generally lower with CI users than with NH subjects. In some circumstances, the difference could be very small,  Cleaning the microphones improved the percentages, mostly in the case of NH subjects in simulation as more extreme situations could be considered and they were tested on the same subjects (through paired comparisons).

Figure 1 :

 1 Figure 1: Shematic view of a cochlear implant

Figure 2 :

 2 Figure 2: Block diagram representing the signal processing performed by the n-of-m simulator.

Figure 3 :

 3 Figure 3 : Simulation of the microphone loss of sensitivity.

Figure 4 :

 4 Figure 4: Fitting of the recognition percentages by a sigmoid curve

Figure 5 Figure 5 :

 55 Figure5shows the percentage of correctly repeated syllables, function of the SNR, for the simulated strategies a) CIS-like b) n-of-m. The intelligibility is for the 4 degrees of simulated soiled microphones (no attenuation, 800Hz low pass filtering at -2, -7 and -9 dB per decade)

Figure 6 :

 6 Figure 6: Intelligibility with respect to the SNR in four cases of simulated soiled microphones, comparison of CIS-like and n-of-m.

Figure 7 :

 7 Figure 7: Intelligibility with respect to the SNR before and after cleaning the microphones' port ("Brush and Blow" cleaning procedure), a) CIS-like strategies, b) n-of-m strategies

Figure 8 :

 8 Figure 8: Intelligibility with respect to the SNR before and after cleaning the microphones' port, a) Cochlear processors, protective filters replacement, b)Cochlear processors, "Brush and Blow" cleaning procedure

ACKNOWLEDGMENTS

The authors are grateful to the persons and the bodies who participated to the study; M. Kevin Perreaut who initiated the work, Dr Fabien Seldran and Dr Fabien Millioz for the scientific contribution and Ms. Evelyne Veuillet for the links with the ethic committee. We also wanted to thank the members of the CRIC Lyon and the staff of the Edouard-Herriot hospital for their collaboration, the subjects who listened to the Fourier's lists, the Hospitals of Lyon and the Polytechnic School of the University of Lyon.

 With CI users, it was difficult to get significant results, probably due to the heterogeneity of the group, and this is very general when pathology is considered. Also, the protective filter replacement seemed to be more efficient than the classical "Brush and Blow" cleaning procedure, but this effect was not seen everywhere on the SNR range, and this result was not statistically proven.

Finally, we recommend the use of the CIS-like strategies over the n-of-m coding strategies whenever it is possible. Also, in the regular checkup which is programmed in the follow-up of CI users, we suggest that the audiologists perform a cleaning of the microphone (the "Brush and Blow" procedure is not aggressive) as the drift is very insidious and often not noticed by the subjects. In some cases (very dirty microphones) it would be helpful for the patient.

We cannot stop aging, but we can attenuate its effects and it was the aim of this work on the microphone drift for cochlear implants ("a pin hole in a wide issue").


Paata Kervalishvili

NANOSTRUCTURES, MAGNETIC SEMICONDUCTORS AND SPINELECTRONICS

Keywords:

A short overview is given of recent advances in the field of nanosemiconductors, which are suitable as materials for spin polarized transport of charge carriers. On the basis of last theoretical and experimental achievements it is shown that development of diluted and wide forbidden zone semiconductors with controlled disorders as well as their molecular structures is the very prospective way for magnetic semiconductors preparation.

The main applied instrument of nano science, the interdisciplinary science that draws on physics, chemistry, biology and computational mathematics is a control and manipulation on a nanometre scale, which allows the fabrication of nanostructures with the properties mainly determined by quantum mechanics. Nanostructures constructed from inorganic solids like semiconductors have new electronic and optical properties considerably different from that of the common crystalline state due to their size and quantization effects [1,[START_REF] Kervalishvili | Nanostructures ass Structural Elements for Semiconducting Layers Proceedings of the International Conference on Materials Testing[END_REF]. Quantum dots, for example, have lead to important novel technology for lasers, optical sensors and other electronic devices. The application of nanolayers to data storage, switching, lighting and other devices, can lead to substantially new hardware, for example energy cells, and eventually to the quantum based internet. Nanoscience and nanotechnology encompasses the development of nano spinelectronics, spinelectronics materials production, nano spinelectronic measuring devices and technologies. Nano spinelectronics, based on usage of magnetic semiconductors, represents new area of science and engineering. The reason to that is the perspective of development and creation of principally new materials and devices for information technologies operating as charge, and spin degree of freedom of carriers, free from limitations inherent for metal spinelectronic devices.

SPIN -POLARIZED TRANSPORT IN SEMICONDUCTORS.

The essential effort of the scientists is concentrated on studying of the spin-polarized transport in nanosize multilayer structures, which are including alternating layers of ferromagnetic metals and non-magnetic semiconductors. Operation of a spintronic device requires efficient spin injection into a semiconductor, spin manipulation, control and transport, and spin detection. The relevant role in solution of this problem is shunted to search and investigations of new ferromagnetic materials, which are capable to be reliable and good spin injectors. Among such objects the magnetic discrete alloys are very promising. They are multilayer systems composed of submonolayers of a ferromagnetic material in the matrix of a non-magnetic semiconductor, for example, Mn/GaAs or Mn/GaSb. It is well known, that these alloys have high Curie temperatures and sufficiently high spin polarization. The circumstance is not les s important that it is possible to control and to manage of the "ferromagnetic metalsemiconductor" boundary surface immediately during the synthesis of these materials.

As it was investigated recently they should be prepared only by the methods of the MOS hydride epitaxy and laser epitaxy with usage of pulsed annealing of epitaxial layers. Perspectives of development and creation of new types of a non-volatile memory stipulate the significance of spintronics with random access (MRAM), quantum single-elec tron logical structures, and ultra dense information storage media. Thus, elementary information storage unit will be represented by an electron spin. The realization of the spin-polarized current transfer opens out new possibilities for the solid-state electronics also. For instance, there are observations of the spin-polarized luminescence and creation of the high frequency diodes, output characteristics which ones one can change by an external magnetic field. Another example is the possibility for creation of a new generation of narrow-band devices of the solid-state electronics of millimetre and submillimeter wave ranges like generators, amplifiers, receivers and filters, modulated and frequency tuned by magnetic field and fully current controlled [START_REF] Corbett | Nanotechnology, International Developments and Emerging Products[END_REF]. The discovery of giant magnetoresistance effect (GMR) can be considered as new important step for development of spintronics. This phenomenon is observed during the study of thin films with alternating layers of ferromagnetic and non-magnetic metals. It is found that, depending on the width of a nonmagnetic spacer, there can be a ferromagnetic or antiferromagnetic interaction between magnetic layers, and antiferromagnetic state of magnetic layer can be transformed in ferromagnetic state by an external magnetic field. The spin-dependent scattering of conduction electrons is minimal, causing a small resistance of material, when magnetic moments are aligned in parallel, whereas for antiparallel orientation of magnetic moments the situation is inversed. The effect GMR brightly has demonstrated, that a spinpolarized electrons can carry magnetic moment through non-magnetic materials. Namely the GMR effect was used in a new generation of the magnetic field sensors, which appeared in 1994 as commercial products on market. But present boom in industry producing the information storage devices started a bit later, in 1997, when the IBM Company has presented the first hard drives with the GMR reading heads. The implantation of this technology has allowed more than on the order to increase a density of the information storage on magnetic disks. The sensors operating with the tunnel magnetic junctions (MTJ) fall into the second class spintronics devices. Very thin dielectric layer divides their ferromagnetic electrodes, a nd electrons are tunnelling through a nonconducting barrier under influence of applied voltage. The tunnel conductivity depends on relative orientation of the electrode magnetizations, and tunnel magnetoresistance (TMR) it is small for parallel alignment of magnetizations of electrodes and is high in opposite case. New memory devices include storage units based on the MTJ structures and allow not only to increase essentially the storage density and the access speed to a memory, but also to provide complete saving of data at disconnecting of a power supply. A disadvantage of these devices is the small scale of integration, bound with necessity of usage of additional controlling transistors. Possibility to overcome these limitations is connected nowadays only with the development of the semiconducting spintronics, and, in particular, with the creation of spin transistors. In this case spintronic devices cannot only switch or to detect electrical and optical signals, but also to enhance them, and also to be used as multifunction units. Due to this reason, the third direction of development spintronic devices is based on the development of multilayer nano structures of ferromagnetic semiconductors, which demonstrate properties not available for their metal analogs. One can refer to number of these properties the possibility to control by electric field a magnetic state of material and the giant planar Hall effect, which exceeds on several orders of magnitude the Hall effect in metal ferromagnets. The super -giant TMR effect observed for the first time in epitaxial (Ga,Mn)As/GaAs/(Ga,Mn)As structures is not less promising for applications. There are no effective ways of injection the spin-polarized current in non-magnetic semiconductors at the present moment. The spin injection from magnetic semiconductors in non-magnetic gives good results in a number of cases, but while it has a place only at low temperatures, far from room temperature. So-called magnetic discrete alloys to days are of the most prospective materials for solution of the spin injection problem. These alloys involve a periodic system of sub-monolayers of magnetic ions (for example, Mn), placed between semiconducting layers (GaAs, GaSb, InAs) forming a magnetic superlattice. There are as incidentally distributed Mn ions and 2D magnetic islands of MnAs (or MnSb) as well in manganese containing layers. The discrete alloys have high Curie temperatures (above 300 K for the GaSb-system), demonstrate extraordinary Hall effect at high temperatures and have a relatively high degree of the spin polarization. It is possible in such systems to control not only quality of the border "ferromagnetic metal -non-magnetic semiconductor", but also manage of the current carrier's concentration and change the type of magnet ic ordering. The discrete alloys should be considered as random magnet systems owing to hardly inhomogeneous allocation of a magnetic phase in submonolayers. The interest in so called diluted magnetic semiconductors is given an impetus by the recent demonstration of the ferromagnetic critical temperature T c =110 K in GaMnAs. To date, most theoretical models proposed assume that the holes occupy a Fermi sea in the valence band [START_REF] Meilikhov | Quasi-Two Dimensional Deluted Magnetic Semiconductors[END_REF]. Theoretical models of the virtual crystal approximation have been used to study the influence of disorder on transport and magnetic properties of magnetic semiconductors. The Boltzmann equation with Born approximation scattering rates has provides estimates of Anizotropic Magneto resistance Effect of order up to 12 %. The key of understanding the kinetic and magnetic anisotropy effects is a strong spin-orbit coupling in the basic semiconductor valence band. The most striking feature in off-diagonal conductivity coefficients for example in (GaMn)As and other arsenide and antimonide of diluted magnetic semiconductors occurs is the large anomalous Hall effect, which occurs because of spin-orbit interactions. In the metals standard assumption is the Anomalous Hall arises because of spin-orbit coupling component in the interaction between band quasiparticles and crystal defects, which can lead to skew scattering with Hall resistivity contribution proportional to diagonal resistivity.

For diluted magnetic semiconductors the Anomalous Hall effect is based on spin-orbit coupling in the Hamiltonian of the ideal crystal and implies a final Hall conductivity even without disorder. The Hall fluid effective Hamiltonian theory discussed without free parameters and it is reliably in materials with high critical temperatures where halls are metallic. The effects of the As Ga -As i -V Ga transition to the ferromagnetism of (GaMn)As can be explained by the Mulliken orbital populations of the d-shell for both majority and minority spins and the corresponding spin polarization for the ferromagnetic configuration. In this case the ferromagnetic coupling is strengthening considerably by the distortion, and that all together the energy splitting and Mulliken orbital population of As i V Ga are the very similar to those of defect free (GaMn)As. These suggest that the ferromagnetic order in (GaMn)As is unaffected by the presence of As i V Ga pairs. This result is in agreement with the hole-mediated picture of ferromagnetism, and can understood by noting that Asi VGa defects energy levels show minimal splitting in (GaMn)As [START_REF] Kervalishvili | Semiconducting Nanostructures -Materials for Spinelectronics[END_REF]. More detailed studies of disorders will combine Kondo description of the spin interactions as well as relevant Monte Carlo techniques applied to both metallic and insulating conditions.

NANOSTRUCTURES OF WIDE FORBIDDEN ZONE SEMICONDUCTORS.

High-temperature semiconductors with wide forbidden zones are also the very promisable materials for modern nano electronics. Materials based on carbon and boron provides complicated substances with unique structural properties. The tec hnology for their film preparation is promising with their desirable electric and physical properties such as mechanical hardness and chemical resistance.

Research conducted during the last decades of the 20th century have shown that carbon and boron crystals form clusters, the essential structural elements of which contain 4, 12, 60, or 84 atoms. These nanoelements, due to their thermodynamic properties, transform to amorphous or crystalline films, layers and other deposits, which have some advanced properties.

The clusters having a stable configuration under equilibration conditions take the forms of different geometrical figures -from triangular to dodecahedral and icosahedrical. According to the classical idea of particle formation and growth and in correspondence with the so-called atomistic process of conception, atoms being the germ of the solid phase unite in aggregates (clusters) where their quantity is dependent on their atomic potentials.

Statistical calculations of the thermodynamic properties o f small clusters carried out by means of computer modelling have shown that the potential energy of the atomic cluster components is the main factor determining the chemical potential of the cluster.

The growth in the quantity N of atoms in the cluster results in the increase of the thermodynamic potential P (N), caused by the increase in atoms at the surface. At the same time, the increase of surface energy accompanying the additional atoms is not continuous, but discrete because of the differences between the energetic contributions of the atoms completing the formation of the co-ordinating sphere [START_REF] Vainshtein | Up-to-day Crystallography, v.B -Formation of crystals[END_REF]. Further growth in the aggregate leads an increase in the volume by means of a gradual addition of atoms from the sides to the growing cluster -volume grow th. Using the established and recent approach to the mechanism of cluster formation, it is easy to show that the appearance of small particles analogous to the so-called fractal clusters very often takes place. Following this, the particle growth occurs not by the joining of separate atoms to their existing aggregate, but by a conglomeration of aggregates with stable configuration, which preserves their individual properties. The formation of small particles (clusters) is actually carried out by various methods, among which are supersonic outflow of vapours into the vacuum, thermo-, laserand plasma-chemical modes of substance reduction from their gas-phase compounds, vapour precipitation upon cold substrates, reaction of molecular effusion from a cell, and etc. These techniques are being used to study the process of the small particle formation, volume growth and growth on specially prepared surfaces. The production of elementary boron is presently being developed by various powder and film technologies. The greatest interest is with modes of small particle production to provide high dispersion and purity as well as the study of the processes of cluster conception and growth. Established theory and experiment have shown that the elementary boron atoms group into an aggregate of icosahedrical form consisting of 12 boron atoms (B12). Usually the boron small particles consist of one or more icosahedrons united in a cluster or various configurations depending on the thermodynamic conditions at formation. Designating the chemical potential of the structural element (in boron this is a 12-atom icosahedron, in carbon it is a 4 -atoms tetrahedron) as E, and the chemical potential of the flat particles (cluster) as P, it is apparent that an equilibrium between the longitudinal dimensions and flat cluster thickness will be achieved when µ E -µ P = 2αv/r, where α is the surface specific free energy per one structural element, v is the specific volume of cluster per one structural element (this is analogous to the Gibbs -Thomson expression), r -radius vector . Given that the equilibrium form is subordinated to the second order non-linear differential equation [START_REF] Bernholc | In book Multyscale computational methods in chemistry and physics[END_REF] and the difference µ E -µ P is constant over the whole surface of a particle, the solution of this equation represents the envelope of the cluster:

n r r r = 2α (n r ) ν /( µ E -µ P)
where n r is the vector of the normal to the envelope ( the surface) of the small particle as determined by the radius vector r r = r (the expression is analogous to the Curie-Wulf formula. From this equation it is possible to evaluate the geometric form and longitudinal dimension of the equilibrium state of the cluster of elementary boron in flat form (fig. 1) under given thermodynamic conditions. The analysis of the analogous electron microphotographs shows that the increase of the longitudinal dimensions of the particles under the thermal treatment occurs because of the joining of lammelar clusters formed from boron icosahedrons with the initial dimension 2-5 nm and icosahedron thickness, and because of their stratification. The space between the normal stripes is 0,8 nm, which is very near to the interplane space of planes (111) of β -rhombohedrical boron (d111 =0.7962 nm) in crystalline structure and coincides with values calculated from interreflex space data. Thus, the direct observation of small particles of elementary boron using high-permissible electron microscopy shows that the boron clusters (2-5 nm) are the amorphous plane compounds where the ratio of the thickness to the longitudinal dimension varies from 1:10 to 1:40. These clusters which consist of nonregularly thermally treated small particles of boron proceed to crystallisation, which at first occurs in the centre of a particle without its plane structure, then advances to the stage of a partly crystallized clusters, stratification and finally volume crystallisation. The elementary particles of boron produced by the plasma-chemical method in the freepoured state have shown an unknown effectthe appearance of a plant-shape cluster. In other words, the ultra-dispersive amorphous boron powder clusters consisting of statistically (nonregular) distributed icosahedrons has been found. The thermal treatment of the elementary boron powder, which consists of plane clusters, in a vacuum furnace as well as in the electron microscope's beam resulted at first in a transition of icosahedrons from a statistical into a modulated condition and then their grouping into a volume rhombohedrical configuration of crystalline boron of β -type. At the first stage there has been observed a regulation of the icosahedrons in the flat form (fig. 3), and then at the second stage a unification of these forms accompanied by the crystalline phase formation. Fig. 3 -Electron microphotograph of a small particle of boron at the first stage of regulation Similar results were observed in the case of ultra-dispersive carbon powders. The electron microscope studies show that the carbon powder consists of small structural elements in the forms of tetrahedral disks. On receipt of a very small portion of energy they organized into the bigger clusters, sometimes the socalled Fullerenes. In the electron microscope beam after treatment they become crystalline structures and in suitable thermodynamic conditions, become diamond. The same situation occurs with carbon layers prepared by the laser spraying -laser plasma technologies [START_REF] Kervalishvili | Thin films preparation by Laser -Plasma deposition[END_REF]. The Laser Spraying Method for film and layer preparation and its most attractive and advanced development, Laser Plasma Deposition, was used for boron and carbon thin film and layer production. Investigations have shown that the prepared crystalline layers have a diamond like structure with the lattice parameters close to that of crystalline diamond [START_REF] Gippius | Semiconducting diamond: properties and applications[END_REF]. Further study of small particles, their conception and growth will explain a number of natural phenomenon of the formation of small solid structures. The practical importance of this investigation consists of the possibility to create new technologies for the production of ultradispersive materials with given crystalline or amorphous structure and the requisite properties. Recent experimental studies of small solids -particles, structural elements of some non-organic (carbon, cobalt, etc.) and biological systems (biomolecules, bio -solids) have brought new data regarding the nature of the kinetics of their formation.

Fig. 1 -

 1 Fig.1 -Schematic picture of a cluster of 12atom icosahedrons in plane form During electron-microscopic studies and testing of the structure of elementary boron produced by means of boron three-chloride reduction with hydrogen and laser -chemical multi-photon dissociation of the dichlorboran molecule, the observed structural elementsboron icosahedrons, are statistically disseminated in an amorphous condition and in a crystalline condition with rhombohedric symmetry.During the electron-microscopic study with the electrons acceleration voltage of 100 kV, light-permissible pictures were produced with microscopic magnification 2 × 10 -5 . The electron microscopy pictures show that the particle has an amorphous structure, boron icosahedrons are placed non-regularly in the plane[START_REF] Bernholc | In book Multyscale computational methods in chemistry and physics[END_REF]. The analysis of the mi cro-diffractional picture, consisting of three diffusion halos, as well as calculations of the interplane spaces has given values consistent with those for the planes of the icosahedron B12. A number of boron particles partly overlapping each other is show n in fig.2. The particles form plane structures with longitudinal dimension 20-40 times as large as their thickness. The thickness is approximately equal to the linear size of 12atoms of boron.

Fig. 2 -

 2 Fig. 2 -Electron microphotograph of boron particles which are overlapping each other