C. A. Perottoni

Periodic Hartree-Fock linear combination of crystalline orbitals calculation of the structure, equation of state and elastic properties of titanium diboride

We have performed all-electron ab initio calculations for TiB2 in the athermal limit using the CRYSTAL95 code. The lattice parameters of the AlB2-type structure were optimized as a function of pressure. The fitting of a Murnaghan equation of state resulted in values of B0 = 292±1 GPa and B � 0 = 3.34±0.03 for the bulk modulus and its first derivative at zero pressure. The values for the linear bulk modulus along the a-axis and the c-axis are Ba0 = 1031 ± 3G Pa (B � a0 = 10.6 ± 0.2) and Bc0 = 675 ± 3G Pa(B � c0 = 8.8 ± 0.2), respectively. All five independent elastic constants were calculated, and the analysis of the elastic behaviour of titanium diboride indicates that this compound is more isotropic than one would suppose from its crystal structure. The discussion on the nature of the chemical bonds and the electronic charge transfer in titanium diboride gives some insight into its mechanical properties, such as its high hardness, despite an apparent layered structure. In this sense, the analysis of the charge-density distribution shows a non- negligible interaction between graphite-like boron planes along the c-axis, which increases with pressure, and suggests a three-dimensional picture for the TiB2 structure, instead of the traditional planar description.

Compressibility of AlB2-type transition metal diborides

The pressure behaviour of a series of transition metal borides has been studied both experimentally and by means of ab initio calculations. X-ray diffraction patterns measured up to ~50 GPa for VB2 and ZrB2 show no obvious phase transition. Bulk moduli of 322 and 317 GPa, respectively, were obtained using a Murnaghan equation of state. Hartree–Fock LCCO (linear combination of crystal orbitals) calculations performed for TiB2 have allowed its compression behaviour to be studied. The bulk modulus obtained (292 GPa) and the proposed important contribution of the interlayer interaction to the elastic behaviour under high pressure are consistent with the experimental results for the other borides.

The carbon analogues of type-I silicon clathrates

In this paper we present a survey on the structure and equation of state for some silicon clathrates and their carbon analogues, as obtained by means of ab initio calculations within the Hartree–Fock approximation. We restrict our consideration to type-I clathrates, namely Mx(Si, C)46, with M = Na, Ba. The insertion of guest species into the carbon clathrate cages promotes a significant increase in the host volume, thus reducing the bulk modulus for these compounds. In spite of that, the estimated hardness for C46, of about 61 GPa, constitutes an exceptionally large value for a structure with such an open framework. The issue of electronic charge transference from the guest species to the host framework and the stability of carbon and silicon clathrates relative to the diamond phase are also discussed.

Hard-skin development during binder removal from Al2O3-based green ceramic bodies

A paraffin-based binder system was developed for low-pressure injection molding of very fine (0.4 μm) average particle size alumina ceramic bodies. Complex chemical reactions take place, during the process of binder removal, when the ceramic parts are fired in air. The effects of these chemical reactions become more pronounced at about 250°C, giving rise to a hard-skin and promoting cracks in the bodies with large cross-sections. The surface hardening occurs only when the bodies are fired in an oxygen-rich atmosphere. The hardness at the surface of the ceramic bodies fired in air at 250°C varies from about 0.4 to 1.2 GPa, depending on binder composition. Upon increasing temperature, the surface hardness decreases monotonically to more conventional values. Infrared absorption spectroscopy was used to study the debinding process and the chemical mechanism behind the hard-skin development in ceramic bodies fired in air. The experimental evidence suggests that cross-linkage between long carbon chains present in the binder and its interaction with the alumina powder surface is responsible for the high surface hardness observed in parts fired in air at 250°C.

A new partially isostatic method for fast debinding of low-pressure injection molded ceramic parts

Abstract Injection molding has many important advantages over other methods for the production of advanced ceramic bodies with complex shapes. However, the particle size has been a limiting factor both in conventional injection molding as well as in low-pressure injection molding (LPIM). Besides that, both low- and high-pressure processes are very often limited to the production of thin ceramic bodies, due to common problems arising in the debinding stage. In this paper, we present a new method for fast debinding of relatively large cross-section ceramic parts (∼10 mm) with particle size less than 0.5 μm that we call partially isostatic debinding. The green body is embedded in a ceramic powder bed subjected to external pressure. A substantial extraction of the organic additives was achieved in just a few hours. Sintered alumina bodies with large cross section can be obtained free of defects, with little distortion and densities higher than 98% of the theoretical value.

Colagem de barbotina de aluminas submicrométricas comerciais

O processo de moldagem de pecas cerâmicas conhecido como colagem de barbotina e um processo antigo, mas que ainda e muito utilizado devido ao seu baixo custo, simplicidade e pelas excelentes propriedades mecânicas das pecas sinterizadas. Boas propriedades mecânicas sao obtidas quando o po de partida tem tamanho de particula submicrometrico. Neste trabalho foram moldadas pecas cerâmicas por colagem de barbotina com tres aluminas comerciais com particulas submicrometricas e, apos a sinterizacao, foram medidas algumas de suas propriedades, como dureza, densidade e resistencia ao desgaste. As barbotinas nao flocularam e foram obtidas pecas de excelente qualidade, sendo que as pecas moldadas com a alumina de menor tamanho de particula apresentaram a melhor dureza, elevada densidade (chegando a 99% da densidade teorica) e o menor coeficiente de desgaste. Pecas de alumina de alta qualidade podem ser usadas em inumeras aplicacoes, desde a industria textil ate em aplicacoes biomedicas.

Moldagem por injeção em baixa pressão de peças complexas de cerâmicas avançadas produzidas com pós submicrométricos

A moldagem por injecao em baixa pressao (MIBP) tem constituido, nos ultimos anos, uma excelente alternativa para a producao de pequenos lotes de pecas cerâmicas diversificadas e com formas complexas. Esta tecnica de conformacao e relativamente simples e barata, mas apresenta diversos problemas na etapa de extracao dos ligantes, principalmente quando o po cerâmico e muito fino e as pecas possuem grande secao transversal. Neste trabalho e descrita uma metodologia especifica para a producao de pecas cerâmicas por moldagem por injecao a baixa pressao de alumina submicrometrica (0,4 mm), incluindo a formulacao da mistura para injecao, bem como detalhes do processo de extracao dos ligantes orgânicos utilizados na moldagem. Para o desenvolvimento deste processo fez-se necessario (i) entender melhor o processo de MIBP, (ii) desenvolver a formulacao da mistura de po cerâmico e aditivos orgânicos, (iii) confeccionar moldes adequados para a injecao a baixas pressoes e (iv) otimizar os parâmetros de injecao. A metodologia desenvolvida neste trabalho, possibilita a confeccao de pecas cerâmicas complexas, com secao transversal relativamente grande, permitindo uma producao robusta de pequenos lotes de pecas cerâmicas diversificadas com otima qualidade e excelente reprodutibilidade.

Processing of amorphous carbon films by ultrafast temperature treatment in a confined geometry

A pressure cell with an anvil made of sapphire and the other made of tungsten carbide, was constructed to process thin film samples using a high power Nd:YAG pulsed laser, in a regime of ultrafast quenching rate and confined geometry. The sapphire anvil worked as the optical window to the laser beam and also as a good thermal conductor substrate. Thin films of amorphous carbon deposited over copper substrate were processed under pressure by Nd:YAG laser pulses. This process induces the formation of a high temperature region at the sample surface during a very short time interval of the order of the 8 ns laser pulse duration. To avoid the complete evaporation of the film, an external pressure of about 0.5 to 1.0 GPa was applied, confining the sample. With the aid of the nanosecond pulsed laser, absorbed on a very thin film sample, this specially designed apparatus provides the means to produce ultrafast quenching as the formation of a plume is suppressed and heat dissipation is accelerated by the high ther...

Damage and ageing effects on proton beam irradiated C60 films

Abstract In this paper, we report results on the interaction of 400 keV H + ions with thin C 60 films focusing on post-irradiation ageing effects. Infrared spectroscopy and resonant α-scattering have been used for characterization of the targets. The analysis were made at different times after bombardment during a period of several months. It has been observed that the signal from the characteristic C 60 IR bands decrease up to 50–70% after the first post-irradiation analysis of the targets. Such ageing is correlated to the appearance and evolution of the oxygen signal in the resonant α-scattering spectra, as well as to the presence of carbonyl bands in the IR spectra of aged targets. For a fixed ageing time the intensity of C 60 IR bands decrease exponentially as a function of fluence.

Method for the measurement of powder distribution in green ceramic bodies

J. E. ZORZI, C. A. PEROTTONI, J. A. H. da JORNADA Universidade Federal do Rio Grande do Sul, Instituto de Fisica, 91501-970, Porto Alegre, RS, Brazil Universidade de Caxias do Sul, CCET/DEFQ, R. Francisco Getulio Vargas, 1130, Cidade Universitaria, 95070-560, Caxias do Sul, RS, Brazil. InmetroInstituto Nacional de Metrologia, Normalizacao e Qualidade Industrial, Campus de Xerem, Rodovia Washington Luiz, Km 120.5, BR 040, Duque de Caxias, RJ, Brazil